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

[Paolo Bonzini]

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.

Paolo
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