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Merge branch 'for-4.16-console-waiter-logic' into for-4.16
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commit
bb4f552a59
@ -1547,6 +1547,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
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return do_syslog(type, buf, len, SYSLOG_FROM_READER);
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}
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/*
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* Special console_lock variants that help to reduce the risk of soft-lockups.
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* They allow to pass console_lock to another printk() call using a busy wait.
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*/
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#ifdef CONFIG_LOCKDEP
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static struct lockdep_map console_owner_dep_map = {
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.name = "console_owner"
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};
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#endif
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static DEFINE_RAW_SPINLOCK(console_owner_lock);
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static struct task_struct *console_owner;
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static bool console_waiter;
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/**
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* console_lock_spinning_enable - mark beginning of code where another
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* thread might safely busy wait
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*
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* This basically converts console_lock into a spinlock. This marks
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* the section where the console_lock owner can not sleep, because
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* there may be a waiter spinning (like a spinlock). Also it must be
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* ready to hand over the lock at the end of the section.
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*/
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static void console_lock_spinning_enable(void)
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{
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raw_spin_lock(&console_owner_lock);
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console_owner = current;
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raw_spin_unlock(&console_owner_lock);
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/* The waiter may spin on us after setting console_owner */
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spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
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}
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/**
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* console_lock_spinning_disable_and_check - mark end of code where another
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* thread was able to busy wait and check if there is a waiter
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*
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* This is called at the end of the section where spinning is allowed.
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* It has two functions. First, it is a signal that it is no longer
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* safe to start busy waiting for the lock. Second, it checks if
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* there is a busy waiter and passes the lock rights to her.
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*
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* Important: Callers lose the lock if there was a busy waiter.
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* They must not touch items synchronized by console_lock
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* in this case.
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*
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* Return: 1 if the lock rights were passed, 0 otherwise.
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*/
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static int console_lock_spinning_disable_and_check(void)
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{
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int waiter;
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raw_spin_lock(&console_owner_lock);
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waiter = READ_ONCE(console_waiter);
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console_owner = NULL;
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raw_spin_unlock(&console_owner_lock);
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if (!waiter) {
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spin_release(&console_owner_dep_map, 1, _THIS_IP_);
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return 0;
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}
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/* The waiter is now free to continue */
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WRITE_ONCE(console_waiter, false);
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spin_release(&console_owner_dep_map, 1, _THIS_IP_);
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/*
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* Hand off console_lock to waiter. The waiter will perform
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* the up(). After this, the waiter is the console_lock owner.
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*/
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mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
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return 1;
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}
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/**
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* console_trylock_spinning - try to get console_lock by busy waiting
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*
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* This allows to busy wait for the console_lock when the current
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* owner is running in specially marked sections. It means that
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* the current owner is running and cannot reschedule until it
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* is ready to lose the lock.
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*
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* Return: 1 if we got the lock, 0 othrewise
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*/
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static int console_trylock_spinning(void)
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{
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struct task_struct *owner = NULL;
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bool waiter;
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bool spin = false;
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unsigned long flags;
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if (console_trylock())
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return 1;
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printk_safe_enter_irqsave(flags);
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raw_spin_lock(&console_owner_lock);
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owner = READ_ONCE(console_owner);
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waiter = READ_ONCE(console_waiter);
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if (!waiter && owner && owner != current) {
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WRITE_ONCE(console_waiter, true);
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spin = true;
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}
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raw_spin_unlock(&console_owner_lock);
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/*
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* If there is an active printk() writing to the
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* consoles, instead of having it write our data too,
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* see if we can offload that load from the active
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* printer, and do some printing ourselves.
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* Go into a spin only if there isn't already a waiter
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* spinning, and there is an active printer, and
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* that active printer isn't us (recursive printk?).
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*/
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if (!spin) {
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printk_safe_exit_irqrestore(flags);
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return 0;
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}
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/* We spin waiting for the owner to release us */
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spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
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/* Owner will clear console_waiter on hand off */
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while (READ_ONCE(console_waiter))
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cpu_relax();
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spin_release(&console_owner_dep_map, 1, _THIS_IP_);
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printk_safe_exit_irqrestore(flags);
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/*
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* The owner passed the console lock to us.
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* Since we did not spin on console lock, annotate
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* this as a trylock. Otherwise lockdep will
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* complain.
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*/
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mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
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return 1;
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}
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/*
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* Call the console drivers, asking them to write out
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* log_buf[start] to log_buf[end - 1].
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@ -1752,13 +1892,20 @@ asmlinkage int vprintk_emit(int facility, int level,
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/* If called from the scheduler, we can not call up(). */
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if (!in_sched) {
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/*
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* Disable preemption to avoid being preempted while holding
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* console_sem which would prevent anyone from printing to
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* console
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*/
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preempt_disable();
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/*
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* Try to acquire and then immediately release the console
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* semaphore. The release will print out buffers and wake up
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* /dev/kmsg and syslog() users.
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*/
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if (console_trylock())
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if (console_trylock_spinning())
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console_unlock();
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preempt_enable();
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}
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return printed_len;
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@ -1859,6 +2006,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
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static ssize_t msg_print_ext_body(char *buf, size_t size,
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char *dict, size_t dict_len,
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char *text, size_t text_len) { return 0; }
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static void console_lock_spinning_enable(void) { }
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static int console_lock_spinning_disable_and_check(void) { return 0; }
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static void call_console_drivers(const char *ext_text, size_t ext_len,
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const char *text, size_t len) {}
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static size_t msg_print_text(const struct printk_log *msg,
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@ -2084,20 +2233,7 @@ int console_trylock(void)
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return 0;
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}
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console_locked = 1;
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/*
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* When PREEMPT_COUNT disabled we can't reliably detect if it's
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* safe to schedule (e.g. calling printk while holding a spin_lock),
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* because preempt_disable()/preempt_enable() are just barriers there
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* and preempt_count() is always 0.
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*
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* RCU read sections have a separate preemption counter when
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* PREEMPT_RCU enabled thus we must take extra care and check
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* rcu_preempt_depth(), otherwise RCU read sections modify
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* preempt_count().
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*/
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console_may_schedule = !oops_in_progress &&
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preemptible() &&
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!rcu_preempt_depth();
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console_may_schedule = 0;
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return 1;
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}
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EXPORT_SYMBOL(console_trylock);
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@ -2247,14 +2383,29 @@ skip:
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console_seq++;
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raw_spin_unlock(&logbuf_lock);
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/*
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* While actively printing out messages, if another printk()
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* were to occur on another CPU, it may wait for this one to
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* finish. This task can not be preempted if there is a
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* waiter waiting to take over.
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*/
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console_lock_spinning_enable();
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stop_critical_timings(); /* don't trace print latency */
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call_console_drivers(ext_text, ext_len, text, len);
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start_critical_timings();
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if (console_lock_spinning_disable_and_check()) {
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printk_safe_exit_irqrestore(flags);
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return;
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}
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printk_safe_exit_irqrestore(flags);
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if (do_cond_resched)
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cond_resched();
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}
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console_locked = 0;
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/* Release the exclusive_console once it is used */
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