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

[Rik van Riel]

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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