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authorThomas Gleixner <tglx@linutronix.de>2023-04-25 20:49:03 +0200
committerThomas Gleixner <tglx@linutronix.de>2023-06-18 22:41:49 +0200
commitae88967d71f1b4ffb6e48043993d37a106da8109 (patch)
treefb3b4fcae5a77ccda5557f0205594d878eca20f5 /kernel/time
parentposix-timers: Cleanup comments about timer ID tracking (diff)
downloadlinux-ae88967d71f1b4ffb6e48043993d37a106da8109.tar.xz
linux-ae88967d71f1b4ffb6e48043993d37a106da8109.zip
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
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/posix-timers.c39
1 files changed, 32 insertions, 7 deletions
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 79909a2ac3e4..d7890ac703ae 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;