Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

From: Rik van Riel
Date: Mon May 04 2015 - 09:31:10 EST


On 05/04/2015 05:26 AM, Paolo Bonzini wrote:
>
>
> On 02/05/2015 07:27, Ingo Molnar wrote:
>>
>> - synchronize_rcu() avoids having to send an IPI by taking a
>> peak at rq->curr's pt_regs::flag, and if:
>>
>> - the flag is 0 then it has observed a quiescent state.
>>
>> - the flag is 1, then it would set TIF_NOHZ and wait for a
>> completion from a TIF_NOHZ callback.
>
> Isn't this racy?
>
> synchronize_rcu CPU nohz CPU
> ---------------------------------------------------------
> set flag = 0
> read flag = 0
> return to userspace
> set TIF_NOHZ
>
> and there's no guarantee that TIF_NOHZ is ever processed by the nohz CPU.

If the flag and TIF_NOHZ live in the same word, we can cmpxchg
and be guaranteed atomicity.

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