On Wed, 2015-08-26 at 16:32 -0700, Jason Low wrote:
> Perhaps to be safer, we use something like load_acquire() and
> store_release() for accessing both the ->running and ->checking_timer
> fields?
Regarding using barriers, one option could be to pair them between
sig->cputime_expires and the sig->cputimer.checking_timer accesses.
fastpath_timer_check()
{
...
if (READ_ONCE(sig->cputimer.running))
struct task_cputime group_sample;
sample_cputime_atomic(&group_sample,
&sig->cputimer.cputime_atomic);
if (task_cputime_expired(&group_sample, &sig->cputime_expires))
{
/*
* Comments
*/
mb();
if (!READ_ONCE(sig->cputimer.checking_timer))
return 1;
}
}
}
check_process_timers()
{
...
WRITE_ONCE(sig->cputimer.checking_timer, 0);
/*
* Comments
*/
mb();
sig->cputime_expires.prof_exp = expires_to_cputime(prof_expires);
sig->cputime_expires.virt_exp = expires_to_cputime(virt_expires);
sig->cputime_expires.sched_exp = sched_expires;
...
}
By the time the cputime_expires fields get updated at the end of
check_process_timers(), other threads in the fastpath_timer_check()
should observe the value 0 for READ_ONCE(sig->cputimer.checking_timer).
In the case where threads in the fastpath don't observe the
WRITE_ONCE(checking_timer, 1) early enough, that's fine, since it will
just (unnecessarily) go through the slowpath which is what we also do in
the current code.
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