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linux-next/kernel/locking/rwsem.c
Waiman Long 990fa7384a locking/rwsem: More optimal RT task handling of null owner
An RT task can do optimistic spinning only if the lock holder is
actually running. If the state of the lock holder isn't known, there
is a possibility that high priority of the RT task may block forward
progress of the lock holder if it happens to reside on the same CPU.
This will lead to deadlock. So we have to make sure that an RT task
will not spin on a reader-owned rwsem.

When the owner is temporarily set to NULL, there are two cases
where we may want to continue spinning:

 1) The lock owner is in the process of releasing the lock, sem->owner
    is cleared but the lock has not been released yet.

 2) The lock was free and owner cleared, but another task just comes
    in and acquire the lock before we try to get it. The new owner may
    be a spinnable writer.

So an RT task is now made to retry one more time to see if it can
acquire the lock or continue spinning on the new owning writer.

When testing on a 8-socket IvyBridge-EX system, the one additional retry
seems to improve locking performance of RT write locking threads under
heavy contentions. The table below shows the locking rates (in kops/s)
with various write locking threads before and after the patch.

    Locking threads     Pre-patch     Post-patch
    ---------------     ---------     -----------
            4             2,753          2,608
            8             2,529          2,520
           16             1,727          1,918
           32             1,263          1,956
           64               889          1,343

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: huang ying <huang.ying.caritas@gmail.com>
Link: https://lkml.kernel.org/r/20190520205918.22251-10-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-06-17 12:28:01 +02:00

1224 lines
33 KiB
C

// SPDX-License-Identifier: GPL-2.0
/* kernel/rwsem.c: R/W semaphores, public implementation
*
* Written by David Howells (dhowells@redhat.com).
* Derived from asm-i386/semaphore.h
*
* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
* and Michel Lespinasse <walken@google.com>
*
* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
*
* Rwsem count bit fields re-definition and rwsem rearchitecture by
* Waiman Long <longman@redhat.com> and
* Peter Zijlstra <peterz@infradead.org>.
*/
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sched/rt.h>
#include <linux/sched/task.h>
#include <linux/sched/debug.h>
#include <linux/sched/wake_q.h>
#include <linux/sched/signal.h>
#include <linux/export.h>
#include <linux/rwsem.h>
#include <linux/atomic.h>
#include "rwsem.h"
#include "lock_events.h"
/*
* The least significant 2 bits of the owner value has the following
* meanings when set.
* - RWSEM_READER_OWNED (bit 0): The rwsem is owned by readers
* - RWSEM_ANONYMOUSLY_OWNED (bit 1): The rwsem is anonymously owned,
* i.e. the owner(s) cannot be readily determined. It can be reader
* owned or the owning writer is indeterminate.
*
* When a writer acquires a rwsem, it puts its task_struct pointer
* into the owner field. It is cleared after an unlock.
*
* When a reader acquires a rwsem, it will also puts its task_struct
* pointer into the owner field with both the RWSEM_READER_OWNED and
* RWSEM_ANONYMOUSLY_OWNED bits set. On unlock, the owner field will
* largely be left untouched. So for a free or reader-owned rwsem,
* the owner value may contain information about the last reader that
* acquires the rwsem. The anonymous bit is set because that particular
* reader may or may not still own the lock.
*
* That information may be helpful in debugging cases where the system
* seems to hang on a reader owned rwsem especially if only one reader
* is involved. Ideally we would like to track all the readers that own
* a rwsem, but the overhead is simply too big.
*/
#define RWSEM_READER_OWNED (1UL << 0)
#define RWSEM_ANONYMOUSLY_OWNED (1UL << 1)
#ifdef CONFIG_DEBUG_RWSEMS
# define DEBUG_RWSEMS_WARN_ON(c, sem) do { \
if (!debug_locks_silent && \
WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
#c, atomic_long_read(&(sem)->count), \
(long)((sem)->owner), (long)current, \
list_empty(&(sem)->wait_list) ? "" : "not ")) \
debug_locks_off(); \
} while (0)
#else
# define DEBUG_RWSEMS_WARN_ON(c, sem)
#endif
/*
* The definition of the atomic counter in the semaphore:
*
* Bit 0 - writer locked bit
* Bit 1 - waiters present bit
* Bit 2 - lock handoff bit
* Bits 3-7 - reserved
* Bits 8-X - 24-bit (32-bit) or 56-bit reader count
*
* atomic_long_fetch_add() is used to obtain reader lock, whereas
* atomic_long_cmpxchg() will be used to obtain writer lock.
*
* There are three places where the lock handoff bit may be set or cleared.
* 1) rwsem_mark_wake() for readers.
* 2) rwsem_try_write_lock() for writers.
* 3) Error path of rwsem_down_write_slowpath().
*
* For all the above cases, wait_lock will be held. A writer must also
* be the first one in the wait_list to be eligible for setting the handoff
* bit. So concurrent setting/clearing of handoff bit is not possible.
*/
#define RWSEM_WRITER_LOCKED (1UL << 0)
#define RWSEM_FLAG_WAITERS (1UL << 1)
#define RWSEM_FLAG_HANDOFF (1UL << 2)
#define RWSEM_READER_SHIFT 8
#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
RWSEM_FLAG_HANDOFF)
/*
* All writes to owner are protected by WRITE_ONCE() to make sure that
* store tearing can't happen as optimistic spinners may read and use
* the owner value concurrently without lock. Read from owner, however,
* may not need READ_ONCE() as long as the pointer value is only used
* for comparison and isn't being dereferenced.
*/
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
WRITE_ONCE(sem->owner, current);
}
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
WRITE_ONCE(sem->owner, NULL);
}
/*
* The task_struct pointer of the last owning reader will be left in
* the owner field.
*
* Note that the owner value just indicates the task has owned the rwsem
* previously, it may not be the real owner or one of the real owners
* anymore when that field is examined, so take it with a grain of salt.
*/
static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
struct task_struct *owner)
{
unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED
| RWSEM_ANONYMOUSLY_OWNED;
WRITE_ONCE(sem->owner, (struct task_struct *)val);
}
static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
{
__rwsem_set_reader_owned(sem, current);
}
/*
* Return true if the a rwsem waiter can spin on the rwsem's owner
* and steal the lock, i.e. the lock is not anonymously owned.
* N.B. !owner is considered spinnable.
*/
static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
{
return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
}
/*
* Return true if rwsem is owned by an anonymous writer or readers.
*/
static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
{
return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
}
#ifdef CONFIG_DEBUG_RWSEMS
/*
* With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
* is a task pointer in owner of a reader-owned rwsem, it will be the
* real owner or one of the real owners. The only exception is when the
* unlock is done by up_read_non_owner().
*/
static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
{
unsigned long val = (unsigned long)current | RWSEM_READER_OWNED
| RWSEM_ANONYMOUSLY_OWNED;
if (READ_ONCE(sem->owner) == (struct task_struct *)val)
cmpxchg_relaxed((unsigned long *)&sem->owner, val,
RWSEM_READER_OWNED | RWSEM_ANONYMOUSLY_OWNED);
}
#else
static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
{
}
#endif
/*
* Guide to the rw_semaphore's count field.
*
* When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
* by a writer.
*
* The lock is owned by readers when
* (1) the RWSEM_WRITER_LOCKED isn't set in count,
* (2) some of the reader bits are set in count, and
* (3) the owner field has RWSEM_READ_OWNED bit set.
*
* Having some reader bits set is not enough to guarantee a readers owned
* lock as the readers may be in the process of backing out from the count
* and a writer has just released the lock. So another writer may steal
* the lock immediately after that.
*/
/*
* Initialize an rwsem:
*/
void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
* Make sure we are not reinitializing a held semaphore:
*/
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
sem->owner = NULL;
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
osq_lock_init(&sem->osq);
#endif
}
EXPORT_SYMBOL(__init_rwsem);
enum rwsem_waiter_type {
RWSEM_WAITING_FOR_WRITE,
RWSEM_WAITING_FOR_READ
};
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
unsigned long timeout;
};
#define rwsem_first_waiter(sem) \
list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
enum rwsem_wake_type {
RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
RWSEM_WAKE_READERS, /* Wake readers only */
RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
};
enum writer_wait_state {
WRITER_NOT_FIRST, /* Writer is not first in wait list */
WRITER_FIRST, /* Writer is first in wait list */
WRITER_HANDOFF /* Writer is first & handoff needed */
};
/*
* The typical HZ value is either 250 or 1000. So set the minimum waiting
* time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
* queue before initiating the handoff protocol.
*/
#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
* have been set.
* - there must be someone on the queue
* - the wait_lock must be held by the caller
* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
* to actually wakeup the blocked task(s) and drop the reference count,
* preferably when the wait_lock is released
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only marked woken if downgrading is false
*/
static void rwsem_mark_wake(struct rw_semaphore *sem,
enum rwsem_wake_type wake_type,
struct wake_q_head *wake_q)
{
struct rwsem_waiter *waiter, *tmp;
long oldcount, woken = 0, adjustment = 0;
struct list_head wlist;
lockdep_assert_held(&sem->wait_lock);
/*
* Take a peek at the queue head waiter such that we can determine
* the wakeup(s) to perform.
*/
waiter = rwsem_first_waiter(sem);
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wake_type == RWSEM_WAKE_ANY) {
/*
* Mark writer at the front of the queue for wakeup.
* Until the task is actually later awoken later by
* the caller, other writers are able to steal it.
* Readers, on the other hand, will block as they
* will notice the queued writer.
*/
wake_q_add(wake_q, waiter->task);
lockevent_inc(rwsem_wake_writer);
}
return;
}
/*
* Writers might steal the lock before we grant it to the next reader.
* We prefer to do the first reader grant before counting readers
* so we can bail out early if a writer stole the lock.
*/
if (wake_type != RWSEM_WAKE_READ_OWNED) {
adjustment = RWSEM_READER_BIAS;
oldcount = atomic_long_fetch_add(adjustment, &sem->count);
if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
/*
* When we've been waiting "too" long (for writers
* to give up the lock), request a HANDOFF to
* force the issue.
*/
if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
time_after(jiffies, waiter->timeout)) {
adjustment -= RWSEM_FLAG_HANDOFF;
lockevent_inc(rwsem_rlock_handoff);
}
atomic_long_add(-adjustment, &sem->count);
return;
}
/*
* Set it to reader-owned to give spinners an early
* indication that readers now have the lock.
*/
__rwsem_set_reader_owned(sem, waiter->task);
}
/*
* Grant an infinite number of read locks to the readers at the front
* of the queue. We know that woken will be at least 1 as we accounted
* for above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up.
*
* We have to do wakeup in 2 passes to prevent the possibility that
* the reader count may be decremented before it is incremented. It
* is because the to-be-woken waiter may not have slept yet. So it
* may see waiter->task got cleared, finish its critical section and
* do an unlock before the reader count increment.
*
* 1) Collect the read-waiters in a separate list, count them and
* fully increment the reader count in rwsem.
* 2) For each waiters in the new list, clear waiter->task and
* put them into wake_q to be woken up later.
*/
list_for_each_entry(waiter, &sem->wait_list, list) {
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
break;
woken++;
}
list_cut_before(&wlist, &sem->wait_list, &waiter->list);
adjustment = woken * RWSEM_READER_BIAS - adjustment;
lockevent_cond_inc(rwsem_wake_reader, woken);
if (list_empty(&sem->wait_list)) {
/* hit end of list above */
adjustment -= RWSEM_FLAG_WAITERS;
}
/*
* When we've woken a reader, we no longer need to force writers
* to give up the lock and we can clear HANDOFF.
*/
if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
adjustment -= RWSEM_FLAG_HANDOFF;
if (adjustment)
atomic_long_add(adjustment, &sem->count);
/* 2nd pass */
list_for_each_entry_safe(waiter, tmp, &wlist, list) {
struct task_struct *tsk;
tsk = waiter->task;
get_task_struct(tsk);
/*
* Ensure calling get_task_struct() before setting the reader
* waiter to nil such that rwsem_down_read_slowpath() cannot
* race with do_exit() by always holding a reference count
* to the task to wakeup.
*/
smp_store_release(&waiter->task, NULL);
/*
* Ensure issuing the wakeup (either by us or someone else)
* after setting the reader waiter to nil.
*/
wake_q_add_safe(wake_q, tsk);
}
}
/*
* This function must be called with the sem->wait_lock held to prevent
* race conditions between checking the rwsem wait list and setting the
* sem->count accordingly.
*
* If wstate is WRITER_HANDOFF, it will make sure that either the handoff
* bit is set or the lock is acquired with handoff bit cleared.
*/
static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
enum writer_wait_state wstate)
{
long count, new;
lockdep_assert_held(&sem->wait_lock);
count = atomic_long_read(&sem->count);
do {
bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
if (has_handoff && wstate == WRITER_NOT_FIRST)
return false;
new = count;
if (count & RWSEM_LOCK_MASK) {
if (has_handoff || (wstate != WRITER_HANDOFF))
return false;
new |= RWSEM_FLAG_HANDOFF;
} else {
new |= RWSEM_WRITER_LOCKED;
new &= ~RWSEM_FLAG_HANDOFF;
if (list_is_singular(&sem->wait_list))
new &= ~RWSEM_FLAG_WAITERS;
}
} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
/*
* We have either acquired the lock with handoff bit cleared or
* set the handoff bit.
*/
if (new & RWSEM_FLAG_HANDOFF)
return false;
rwsem_set_owner(sem);
return true;
}
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
* Try to acquire write lock before the writer has been put on wait queue.
*/
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
long count = atomic_long_read(&sem->count);
while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
count | RWSEM_WRITER_LOCKED)) {
rwsem_set_owner(sem);
lockevent_inc(rwsem_opt_wlock);
return true;
}
}
return false;
}
static inline bool owner_on_cpu(struct task_struct *owner)
{
/*
* As lock holder preemption issue, we both skip spinning if
* task is not on cpu or its cpu is preempted
*/
return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
}
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner;
bool ret = true;
BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
if (need_resched())
return false;
rcu_read_lock();
owner = READ_ONCE(sem->owner);
if (owner) {
ret = is_rwsem_owner_spinnable(owner) &&
owner_on_cpu(owner);
}
rcu_read_unlock();
return ret;
}
/*
* The rwsem_spin_on_owner() function returns the folowing 4 values
* depending on the lock owner state.
* OWNER_NULL : owner is currently NULL
* OWNER_WRITER: when owner changes and is a writer
* OWNER_READER: when owner changes and the new owner may be a reader.
* OWNER_NONSPINNABLE:
* when optimistic spinning has to stop because either the
* owner stops running, is unknown, or its timeslice has
* been used up.
*/
enum owner_state {
OWNER_NULL = 1 << 0,
OWNER_WRITER = 1 << 1,
OWNER_READER = 1 << 2,
OWNER_NONSPINNABLE = 1 << 3,
};
#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER)
static inline enum owner_state rwsem_owner_state(unsigned long owner)
{
if (!owner)
return OWNER_NULL;
if (owner & RWSEM_ANONYMOUSLY_OWNED)
return OWNER_NONSPINNABLE;
if (owner & RWSEM_READER_OWNED)
return OWNER_READER;
return OWNER_WRITER;
}
static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *tmp, *owner = READ_ONCE(sem->owner);
enum owner_state state = rwsem_owner_state((unsigned long)owner);
if (state != OWNER_WRITER)
return state;
rcu_read_lock();
for (;;) {
if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
state = OWNER_NONSPINNABLE;
break;
}
tmp = READ_ONCE(sem->owner);
if (tmp != owner) {
state = rwsem_owner_state((unsigned long)tmp);
break;
}
/*
* Ensure we emit the owner->on_cpu, dereference _after_
* checking sem->owner still matches owner, if that fails,
* owner might point to free()d memory, if it still matches,
* the rcu_read_lock() ensures the memory stays valid.
*/
barrier();
if (need_resched() || !owner_on_cpu(owner)) {
state = OWNER_NONSPINNABLE;
break;
}
cpu_relax();
}
rcu_read_unlock();
return state;
}
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
bool taken = false;
int prev_owner_state = OWNER_NULL;
preempt_disable();
/* sem->wait_lock should not be held when doing optimistic spinning */
if (!rwsem_can_spin_on_owner(sem))
goto done;
if (!osq_lock(&sem->osq))
goto done;
/*
* Optimistically spin on the owner field and attempt to acquire the
* lock whenever the owner changes. Spinning will be stopped when:
* 1) the owning writer isn't running; or
* 2) readers own the lock as we can't determine if they are
* actively running or not.
*/
for (;;) {
enum owner_state owner_state = rwsem_spin_on_owner(sem);
if (!(owner_state & OWNER_SPINNABLE))
break;
/*
* Try to acquire the lock
*/
if (rwsem_try_write_lock_unqueued(sem)) {
taken = true;
break;
}
/*
* An RT task cannot do optimistic spinning if it cannot
* be sure the lock holder is running or live-lock may
* happen if the current task and the lock holder happen
* to run in the same CPU. However, aborting optimistic
* spinning while a NULL owner is detected may miss some
* opportunity where spinning can continue without causing
* problem.
*
* There are 2 possible cases where an RT task may be able
* to continue spinning.
*
* 1) The lock owner is in the process of releasing the
* lock, sem->owner is cleared but the lock has not
* been released yet.
* 2) The lock was free and owner cleared, but another
* task just comes in and acquire the lock before
* we try to get it. The new owner may be a spinnable
* writer.
*
* To take advantage of two scenarios listed agove, the RT
* task is made to retry one more time to see if it can
* acquire the lock or continue spinning on the new owning
* writer. Of course, if the time lag is long enough or the
* new owner is not a writer or spinnable, the RT task will
* quit spinning.
*
* If the owner is a writer, the need_resched() check is
* done inside rwsem_spin_on_owner(). If the owner is not
* a writer, need_resched() check needs to be done here.
*/
if (owner_state != OWNER_WRITER) {
if (need_resched())
break;
if (rt_task(current) &&
(prev_owner_state != OWNER_WRITER))
break;
}
prev_owner_state = owner_state;
/*
* The cpu_relax() call is a compiler barrier which forces
* everything in this loop to be re-loaded. We don't need
* memory barriers as we'll eventually observe the right
* values at the cost of a few extra spins.
*/
cpu_relax();
}
osq_unlock(&sem->osq);
done:
preempt_enable();
lockevent_cond_inc(rwsem_opt_fail, !taken);
return taken;
}
#else
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
return false;
}
#endif
/*
* Wait for the read lock to be granted
*/
static struct rw_semaphore __sched *
rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
{
long count, adjustment = -RWSEM_READER_BIAS;
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list)) {
/*
* In case the wait queue is empty and the lock isn't owned
* by a writer or has the handoff bit set, this reader can
* exit the slowpath and return immediately as its
* RWSEM_READER_BIAS has already been set in the count.
*/
if (!(atomic_long_read(&sem->count) &
(RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
raw_spin_unlock_irq(&sem->wait_lock);
rwsem_set_reader_owned(sem);
lockevent_inc(rwsem_rlock_fast);
return sem;
}
adjustment += RWSEM_FLAG_WAITERS;
}
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
count = atomic_long_add_return(adjustment, &sem->count);
/*
* If there are no active locks, wake the front queued process(es).
*
* If there are no writers and we are first in the queue,
* wake our own waiter to join the existing active readers !
*/
if (!(count & RWSEM_LOCK_MASK) ||
(!(count & RWSEM_WRITER_MASK) && (adjustment & RWSEM_FLAG_WAITERS)))
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
/* wait to be given the lock */
while (true) {
set_current_state(state);
if (!waiter.task)
break;
if (signal_pending_state(state, current)) {
raw_spin_lock_irq(&sem->wait_lock);
if (waiter.task)
goto out_nolock;
raw_spin_unlock_irq(&sem->wait_lock);
break;
}
schedule();
lockevent_inc(rwsem_sleep_reader);
}
__set_current_state(TASK_RUNNING);
lockevent_inc(rwsem_rlock);
return sem;
out_nolock:
list_del(&waiter.list);
if (list_empty(&sem->wait_list)) {
atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
&sem->count);
}
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
lockevent_inc(rwsem_rlock_fail);
return ERR_PTR(-EINTR);
}
/*
* Wait until we successfully acquire the write lock
*/
static struct rw_semaphore *
rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
{
long count;
enum writer_wait_state wstate;
struct rwsem_waiter waiter;
struct rw_semaphore *ret = sem;
DEFINE_WAKE_Q(wake_q);
/* do optimistic spinning and steal lock if possible */
if (rwsem_optimistic_spin(sem))
return sem;
/*
* Optimistic spinning failed, proceed to the slowpath
* and block until we can acquire the sem.
*/
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
raw_spin_lock_irq(&sem->wait_lock);
/* account for this before adding a new element to the list */
wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock */
if (wstate == WRITER_NOT_FIRST) {
count = atomic_long_read(&sem->count);
/*
* If there were already threads queued before us and:
* 1) there are no no active locks, wake the front
* queued process(es) as the handoff bit might be set.
* 2) there are no active writers and some readers, the lock
* must be read owned; so we try to wake any read lock
* waiters that were queued ahead of us.
*/
if (count & RWSEM_WRITER_MASK)
goto wait;
rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
? RWSEM_WAKE_READERS
: RWSEM_WAKE_ANY, &wake_q);
if (!wake_q_empty(&wake_q)) {
/*
* We want to minimize wait_lock hold time especially
* when a large number of readers are to be woken up.
*/
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
wake_q_init(&wake_q); /* Used again, reinit */
raw_spin_lock_irq(&sem->wait_lock);
}
} else {
atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
}
wait:
/* wait until we successfully acquire the lock */
set_current_state(state);
while (true) {
if (rwsem_try_write_lock(sem, wstate))
break;
raw_spin_unlock_irq(&sem->wait_lock);
/* Block until there are no active lockers. */
for (;;) {
if (signal_pending_state(state, current))
goto out_nolock;
schedule();
lockevent_inc(rwsem_sleep_writer);
set_current_state(state);
/*
* If HANDOFF bit is set, unconditionally do
* a trylock.
*/
if (wstate == WRITER_HANDOFF)
break;
if ((wstate == WRITER_NOT_FIRST) &&
(rwsem_first_waiter(sem) == &waiter))
wstate = WRITER_FIRST;
count = atomic_long_read(&sem->count);
if (!(count & RWSEM_LOCK_MASK))
break;
/*
* The setting of the handoff bit is deferred
* until rwsem_try_write_lock() is called.
*/
if ((wstate == WRITER_FIRST) && (rt_task(current) ||
time_after(jiffies, waiter.timeout))) {
wstate = WRITER_HANDOFF;
lockevent_inc(rwsem_wlock_handoff);
break;
}
}
raw_spin_lock_irq(&sem->wait_lock);
}
__set_current_state(TASK_RUNNING);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
lockevent_inc(rwsem_wlock);
return ret;
out_nolock:
__set_current_state(TASK_RUNNING);
raw_spin_lock_irq(&sem->wait_lock);
list_del(&waiter.list);
if (unlikely(wstate == WRITER_HANDOFF))
atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
if (list_empty(&sem->wait_list))
atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
else
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
lockevent_inc(rwsem_wlock_fail);
return ERR_PTR(-EINTR);
}
/*
* handle waking up a waiter on the semaphore
* - up_read/up_write has decremented the active part of count if we come here
*/
static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
{
unsigned long flags;
DEFINE_WAKE_Q(wake_q);
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (!list_empty(&sem->wait_list))
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
wake_up_q(&wake_q);
return sem;
}
/*
* downgrade a write lock into a read lock
* - caller incremented waiting part of count and discovered it still negative
* - just wake up any readers at the front of the queue
*/
static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
unsigned long flags;
DEFINE_WAKE_Q(wake_q);
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (!list_empty(&sem->wait_list))
rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
wake_up_q(&wake_q);
return sem;
}
/*
* lock for reading
*/
inline void __down_read(struct rw_semaphore *sem)
{
if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
&sem->count) & RWSEM_READ_FAILED_MASK)) {
rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
RWSEM_READER_OWNED), sem);
} else {
rwsem_set_reader_owned(sem);
}
}
static inline int __down_read_killable(struct rw_semaphore *sem)
{
if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
&sem->count) & RWSEM_READ_FAILED_MASK)) {
if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
return -EINTR;
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
RWSEM_READER_OWNED), sem);
} else {
rwsem_set_reader_owned(sem);
}
return 0;
}
static inline int __down_read_trylock(struct rw_semaphore *sem)
{
/*
* Optimize for the case when the rwsem is not locked at all.
*/
long tmp = RWSEM_UNLOCKED_VALUE;
do {
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
tmp + RWSEM_READER_BIAS)) {
rwsem_set_reader_owned(sem);
return 1;
}
} while (!(tmp & RWSEM_READ_FAILED_MASK));
return 0;
}
/*
* lock for writing
*/
static inline void __down_write(struct rw_semaphore *sem)
{
long tmp = RWSEM_UNLOCKED_VALUE;
if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
RWSEM_WRITER_LOCKED)))
rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
rwsem_set_owner(sem);
}
static inline int __down_write_killable(struct rw_semaphore *sem)
{
long tmp = RWSEM_UNLOCKED_VALUE;
if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
RWSEM_WRITER_LOCKED))) {
if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
return -EINTR;
}
rwsem_set_owner(sem);
return 0;
}
static inline int __down_write_trylock(struct rw_semaphore *sem)
{
long tmp = RWSEM_UNLOCKED_VALUE;
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
RWSEM_WRITER_LOCKED)) {
rwsem_set_owner(sem);
return true;
}
return false;
}
/*
* unlock after reading
*/
inline void __up_read(struct rw_semaphore *sem)
{
long tmp;
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED), sem);
rwsem_clear_reader_owned(sem);
tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
RWSEM_FLAG_WAITERS))
rwsem_wake(sem, tmp);
}
/*
* unlock after writing
*/
static inline void __up_write(struct rw_semaphore *sem)
{
long tmp;
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
rwsem_clear_owner(sem);
tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
if (unlikely(tmp & RWSEM_FLAG_WAITERS))
rwsem_wake(sem, tmp);
}
/*
* downgrade write lock to read lock
*/
static inline void __downgrade_write(struct rw_semaphore *sem)
{
long tmp;
/*
* When downgrading from exclusive to shared ownership,
* anything inside the write-locked region cannot leak
* into the read side. In contrast, anything in the
* read-locked region is ok to be re-ordered into the
* write side. As such, rely on RELEASE semantics.
*/
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
tmp = atomic_long_fetch_add_release(
-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
rwsem_set_reader_owned(sem);
if (tmp & RWSEM_FLAG_WAITERS)
rwsem_downgrade_wake(sem);
}
/*
* lock for reading
*/
void __sched down_read(struct rw_semaphore *sem)
{
might_sleep();
rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
}
EXPORT_SYMBOL(down_read);
int __sched down_read_killable(struct rw_semaphore *sem)
{
might_sleep();
rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
rwsem_release(&sem->dep_map, 1, _RET_IP_);
return -EINTR;
}
return 0;
}
EXPORT_SYMBOL(down_read_killable);
/*
* trylock for reading -- returns 1 if successful, 0 if contention
*/
int down_read_trylock(struct rw_semaphore *sem)
{
int ret = __down_read_trylock(sem);
if (ret == 1)
rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
return ret;
}
EXPORT_SYMBOL(down_read_trylock);
/*
* lock for writing
*/
void __sched down_write(struct rw_semaphore *sem)
{
might_sleep();
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
}
EXPORT_SYMBOL(down_write);
/*
* lock for writing
*/
int __sched down_write_killable(struct rw_semaphore *sem)
{
might_sleep();
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
__down_write_killable)) {
rwsem_release(&sem->dep_map, 1, _RET_IP_);
return -EINTR;
}
return 0;
}
EXPORT_SYMBOL(down_write_killable);
/*
* trylock for writing -- returns 1 if successful, 0 if contention
*/
int down_write_trylock(struct rw_semaphore *sem)
{
int ret = __down_write_trylock(sem);
if (ret == 1)
rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
return ret;
}
EXPORT_SYMBOL(down_write_trylock);
/*
* release a read lock
*/
void up_read(struct rw_semaphore *sem)
{
rwsem_release(&sem->dep_map, 1, _RET_IP_);
__up_read(sem);
}
EXPORT_SYMBOL(up_read);
/*
* release a write lock
*/
void up_write(struct rw_semaphore *sem)
{
rwsem_release(&sem->dep_map, 1, _RET_IP_);
__up_write(sem);
}
EXPORT_SYMBOL(up_write);
/*
* downgrade write lock to read lock
*/
void downgrade_write(struct rw_semaphore *sem)
{
lock_downgrade(&sem->dep_map, _RET_IP_);
__downgrade_write(sem);
}
EXPORT_SYMBOL(downgrade_write);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
void down_read_nested(struct rw_semaphore *sem, int subclass)
{
might_sleep();
rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
}
EXPORT_SYMBOL(down_read_nested);
void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
{
might_sleep();
rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
}
EXPORT_SYMBOL(_down_write_nest_lock);
void down_read_non_owner(struct rw_semaphore *sem)
{
might_sleep();
__down_read(sem);
__rwsem_set_reader_owned(sem, NULL);
}
EXPORT_SYMBOL(down_read_non_owner);
void down_write_nested(struct rw_semaphore *sem, int subclass)
{
might_sleep();
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
}
EXPORT_SYMBOL(down_write_nested);
int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
{
might_sleep();
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
__down_write_killable)) {
rwsem_release(&sem->dep_map, 1, _RET_IP_);
return -EINTR;
}
return 0;
}
EXPORT_SYMBOL(down_write_killable_nested);
void up_read_non_owner(struct rw_semaphore *sem)
{
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
sem);
__up_read(sem);
}
EXPORT_SYMBOL(up_read_non_owner);
#endif