[tip: timers/core] posix-timers: Add comments about timer lookup

tip-bot2 for Thomas Gleixner
2023-06-05T15:09:42Z

The following commit has been merged into the timers/core branch of tip:

Commit-ID: 46c7863e061f64c96745003f699912667c2a1bd6
Gitweb: https://git.kernel.org/tip/46c7863e061f64c96745003f699912667c2a1bd6
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 25 Apr 2023 20:49:03 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 05 Jun 2023 17:03:37 +02:00

posix-timers: Add comments about timer lookup

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

---
kernel/time/posix-timers.c | 39 ++++++++++++++++++++++++++++++-------
1 file changed, 32 insertions(+), 7 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 355c6f4..f6650c1 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -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;