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Date: Tue, 6 Oct 2015 14:39:02 -0700 From: "H. Peter Anvin" <hpa@...or.com> To: Pranith Kumar <bobby.prani@...il.com> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] x86: cmpxchg_double: Add missing memory clobber On 10/06/2015 01:29 PM, Pranith Kumar wrote: > On Tue, Oct 6, 2015 at 4:16 PM, H. Peter Anvin <hpa@...or.com> wrote: >> >> NAK. We already have the "+m" for exactly this reason; adding an >> explicit memory clobber should only be used to prevent movement of >> *other* memory operations around this one (i.e. a barrier). >> > > OK. If that is so, can you please explain why we need it in the > __raw_cmpxchg() case? I think it is a good idea to make cmpxchg() and > cmpxchg_double() have similar barrier semantics. > OK, it is a bit of a mess. We use the same macros for locked operations (__cmpxchg and __sync_cmpxchg) and unlocked operations (__cmpxchg_local). For locked operations we generally want a compiler barrier, although there are exceptions. I'm wondering if it would be better to add an explicit barrier(); to the locked versions. However, I think one of the major uses for cmpxchg_double() is for page table manipulation, and for that it isn't clear that a compiler barrier is needed nor desired. On the other hand, perhaps all of this is false optimization and we should just add the memory clobber. The real issue is the impact on the _local variants. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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