[PATCH V2 4/4] posix-timers: Use RCU in posix_timer_add()

[Eric Dumazet <edumazet@google.com>]

If many posix timers are hashed in posix_timers_hashtable,
hash_lock can be held for long durations.

This can be really bad in some cases as Thomas
explained in https://lore.kernel.org/all/87ednpyyeo.ffs@tglx/

We can perform all searches under RCU, then acquire
the lock only when there is a good chance to need it,
and after cpu caches were populated.

Also add a cond_resched() in the possible long loop.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 kernel/time/posix-timers.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index ed27c7eab456..bd73bc4707c1 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -125,7 +125,19 @@ static int posix_timer_add(struct k_itimer *timer)
 
 		head = &posix_timers_hashtable[hash(sig, id)];
 
+		rcu_read_lock();
+		if (posix_timers_find(head, sig, id)) {
+			rcu_read_unlock();
+			cond_resched();
+			continue;
+		}
+		rcu_read_unlock();
 		spin_lock(&hash_lock);
+		/*
+		 * We must perform the lookup under hash_lock protection
+		 * because another thread could have used the same id.
+		 * This is very unlikely, but possible.
+		 */
 		if (!posix_timers_find(head, sig, id)) {
 			timer->it_id = (timer_t)id;
 			timer->it_signal = (struct signal_struct *)((unsigned long)sig | 1UL);
-- 
2.48.1.601.g30ceb7b040-goog