Re: [PATCH] sched: correct testing need_resched in mutex_spin_on_owner()

[Hillf Danton]

On Tue, Jun 7, 2011 at 9:59 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, 2011-06-07 at 15:47 +0200, Peter Zijlstra wrote:
>> On Tue, 2011-06-07 at 21:41 +0800, Hillf Danton wrote:
>> > It is suppose to check the owner task that is not absolutly running on the
>> > local CPU,
>>
>> Oh, why do you think so?
>>
>> > and if NEED_RESCHED is happenly set on the current task of local
>> > CPU, we get incorrect result.
>>
>> Only if your above assumption holds, which it doesn't. It explicitly
>> checks to see if _this_ cpu needs a resched while spinning, if so it
>> bails the spinning and calls schedule in the lock slow path.
>>
>> If the owner cpu reschedules, owner will leave the rq and
>> owner_running() will return false, also breaking the loop.
>>
>> >
>> > Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
>> > ---
>> > Âkernel/sched.c | Â Â2 +-
>> > Â1 files changed, 1 insertions(+), 1 deletions(-)
>> >
>> > diff --git a/kernel/sched.c b/kernel/sched.c
>> > index fd18f39..3ea64fe 100644
>> > --- a/kernel/sched.c
>> > +++ b/kernel/sched.c
>> > @@ -4326,7 +4326,7 @@ int mutex_spin_on_owner(struct mutex *lock,
>> > struct task_struct *owner)
>> > Â Â Â Â Â Â return 0;
>> >
>> > Â Â while (owner_running(lock, owner)) {
>> > - Â Â Â Â Â if (need_resched())
>> > + Â Â Â Â Â if (test_tsk_need_resched(owner))
>> > Â Â Â Â Â Â Â Â Â Â return 0;
>> >
>
> Furthermore, that can crash the machine, as there's no guarantee owner
> is a sane pointer at this point.
>

Well, why is owner_running looped safely?

thanks
           Hillf
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/