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

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 79909a2..d7890ac 100644 (file)
--- 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;