linux-kernel - Re: [tip:x86/idle] x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers

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Date:	Fri, 20 Dec 2013 05:46:37 +0900
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Anvin <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Anvin <hpa@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/idle] x86, idle: Use static_cpu_has() for CLFLUSH
 workaround, add barriers

On Fri, Dec 20, 2013 at 5:40 AM, Ingo Molnar <mingo@...nel.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
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