linux-kernel - Re: [PATCH] sched: correct testing need_resched in mutex_spin_on

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Message-ID: <BANLkTimZzpUZe3vFe4aQdQBZBOS1a4=9rQ@mail.gmail.com>
Date:	Tue, 7 Jun 2011 22:14:53 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: correct testing need_resched in mutex_spin_on_owner()

On Tue, Jun 7, 2011 at 9:59 PM, Peter Zijlstra <peterz@...radead.org> 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@...il.com>
>> > ---
>> >  kernel/sched.c |    2 +-
>> >  1 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)
>> >             return 0;
>> >
>> >     while (owner_running(lock, owner)) {
>> > -           if (need_resched())
>> > +           if (test_tsk_need_resched(owner))
>> >                     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
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