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Date: Fri, 20 Dec 2013 05:46:37 +0900
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Subject: Re: [tip:x86/idle] x86, idle: Use static_cpu_has() for CLFLUSH
 workaround, add barriers
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Peter Anvin <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Anvin <hpa@linux.intel.com>, Len Brown <len.brown@intel.com>,
        linux-tip-commits@vger.kernel.org
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On Fri, Dec 20, 2013 at 5:40 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> The first mb() looks superfluous, because see
> current_set_polling_and_test():
>
> static inline bool __must_check current_set_polling_and_test(void)
> {
>         __current_set_polling();
>
>         /*
>          * Polling state must be visible before we test NEED_RESCHED,
>          * paired by resched_task()
>          */
>         smp_mb();
>
>         return unlikely(tif_need_resched());
> }
>
> So it already has a (MFENCE) barrier after ->flags is modified.

So what?

The mb() isn't necessarily against the *write*, it is also against the *read*.

If the cflush is needed before the monitor, it's likely because the
monitor instruction has some bug with already-existing cachelines in
shared state or whatever.

Which means that we want to make sure that the cflush is ordered wrt
*reads* from that cacheline too.

cflush has nothing specifically to do with writes.

             Linus