Global timers could be expired remotely when the target CPU is idle. After
a remote timer expiry, the remote timer_base->next_expiry value is updated
while holding the timer_base->lock. When the formerly idle CPU becomes
active at the same time and checks whether timers need to expire, this
check is done lockless as it is on the local CPU. This could lead to a data
race, which was reported by sysbot:
https://lore.kernel.org/r/000000000000916e55061f969e14@google.com
When the value is read lockless but changed by the remote CPU, only two non
critical scenarios could happen:
1) The already update value is read -> everything is perfect
2) The old value is read -> a superfluous timer soft interrupt is raised
The same situation could happen when enqueueing a new first pinned timer by
a remote CPU also with non critical scenarios:
1) The already update value is read -> everything is perfect
2) The old value is read -> when the CPU is idle, an IPI is executed
nevertheless and when the CPU isn't idle, the updated value will be visible
on the next tick and the timer might be late one jiffie.
As this is very unlikely to happen, the overhead of doing the check under
the lock is a way more effort, than a superfluous timer soft interrupt or a
possible 1 jiffie delay of the timer.
Document and annotate this non critical behavior in the code by using
READ/WRITE_ONCE() pair when accessing timer_base->next_expiry.
Reported-by: syzbot+bf285fcc0a048e028118@syzkaller.appspotmail.com
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20240829154305.19259-1-anna-maria@linutronix.de
Closes: https://lore.kernel.org/lkml/000000000000916e55061f969e14@google.com
79f8b28e85