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posix-timers: Add comments about timer lookup

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Document how the timer ID validation in the hash table works.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.091081515@linutronix.de
This commit is contained in:
Thomas Gleixner 2023-04-25 20:49:03 +02:00
parent 8d44b958a1
commit ae88967d71
1 changed files with 32 additions and 7 deletions
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39
kernel/time/posix-timers.c
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@ -506,6 +506,12 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
return -EAGAIN;
spin_lock_init(&new_timer->it_lock);
/*
* Add the timer to the hash table. The timer is not yet valid
* because new_timer::it_signal is still NULL. The timer id is also
* not yet visible to user space.
*/
new_timer_id = posix_timer_add(new_timer);
if (new_timer_id < 0) {
error = new_timer_id;
@ -551,6 +557,7 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
goto out;
spin_lock_irq(&current->sighand->siglock);
/* This makes the timer valid in the hash table */
new_timer->it_signal = current->signal;
list_add(&new_timer->list, &current->signal->posix_timers);
spin_unlock_irq(&current->sighand->siglock);
@ -597,13 +604,6 @@ COMPAT_SYSCALL_DEFINE3(timer_create, clockid_t, which_clock,
}
#endif
/*
* Locking issues: We need to protect the result of the id look up until
* we get the timer locked down so it is not deleted under us. The
* removal is done under the idr spinlock so we use that here to bridge
* the find to the timer lock. To avoid a dead lock, the timer id MUST
* be release with out holding the timer lock.
*/
static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags)
{
struct k_itimer *timr;
@ -615,10 +615,35 @@ static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags)
if ((unsigned long long)timer_id > INT_MAX)
return NULL;
/*
* The hash lookup and the timers are RCU protected.
*
* Timers are added to the hash in invalid state where
* timr::it_signal == NULL. timer::it_signal is only set after the
* rest of the initialization succeeded.
*
* Timer destruction happens in steps:
* 1) Set timr::it_signal to NULL with timr::it_lock held
* 2) Release timr::it_lock
* 3) Remove from the hash under hash_lock
* 4) Call RCU for removal after the grace period
*
* Holding rcu_read_lock() accross the lookup ensures that
* the timer cannot be freed.
*
* The lookup validates locklessly that timr::it_signal ==
* current::it_signal and timr::it_id == @timer_id. timr::it_id
* can't change, but timr::it_signal becomes NULL during
* destruction.
*/
rcu_read_lock();
timr = posix_timer_by_id(timer_id);
if (timr) {
spin_lock_irqsave(&timr->it_lock, *flags);
/*
* Validate under timr::it_lock that timr::it_signal is
* still valid. Pairs with #1 above.
*/
if (timr->it_signal == current->signal) {
rcu_read_unlock();
return timr;