linux-kernel - Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT

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Message-ID: <CA+55aFysegA0nYxu4DYugz6R+wP6c1oDOemHpMT7av645S+cVg@mail.gmail.com>
Date:	Tue, 9 Apr 2013 07:32:42 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <srostedt@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christian Ruppert <christian.ruppert@...lis.com>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT_COUNT

On Mon, Apr 8, 2013 at 8:07 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Apr 8, 2013 at 7:59 AM, Steven Rostedt <srostedt@...hat.com> wrote:
>>>     +/* This is only a barrier to other asms. Notably get_user/put_user */
>>
>> Probably should add in the comment:
>>
>>         " or anything else that can cause a hidden schedule. "
>>
>
> Fair enough. And I just remembered why I thought UP was special - we
> need to do the same thing about spinlocks, for the same reasons.
>
> So that "asm_barrier()" should probably be in <linux/compiler.h> along
> with the "normal" barrier() definition.
>

I'm a moron.

Yes, "asm_barrier()" is a valid barrier for asms. But without the
"memory" clobber, it doesn't actually end up being a barrier to any
normal C loads and stores from memory, so it doesn't actually help.

So I suspect we need to just make UP spinlocks and preemption
enable/disable be full compiler barriers after all.

Something like the attached (still untested, although it seems to at
least compile) patch. Comments?

                    Linus

Download attachment "patch.diff" of type "application/octet-stream" (4275 bytes)