Andrew Morton wrote:
>
> http://bugme.osdl.org/show_bug.cgi?id=4559
>
> Timer bug, I guess.
Yes, the new timer code is racy. Example:
spinlock_t LOCK;
void void timer_func()
{
spin_lock(&LOCK);
}
timer_list TIMER = TIMER_INITIALIZER(timer_func);
-------------------------------------------------------------------
CPU_0 CPU_1
add_timer(&TIMER);
spin_lock(&LOCK);
__run_timers:
sets ->running_timer = &TIMER;
calls timer_func()
waits for &LOCK
__mod_timer(&TIMER);
/* Ensure the timer is serialized. */
retries while ->running_timer == &TIMER
Many thanks to Maneesh Soni for his excellent analysis in
http://bugme.osdl.org/show_bug.cgi?id=4559.
Note that del_timer_sync has this problem too, but this
situation is forbidden by synchronization rules.
At the moment I don't have a proper solution.
One option is to change __mod_timer() so that it would not
switch ->base when the timer is already running. But this
would be behavioural change: currently __mod_timer() guarantees
that the timer would be armed on the local cpu.
I'll try to find a solution, but perhaps it's better to drop
this patch for now.
Oleg.
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