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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