Messages in this thread | | | Date | Wed, 28 Oct 2015 12:49:43 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb() |
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On Tue, 27 Oct 2015, Peter Zijlstra wrote:
>On Wed, Oct 28, 2015 at 06:33:56AM +0900, Linus Torvalds wrote: >> On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso <dave@stgolabs.net> wrote: >> > >> > Note that this might affect callers that could/would rely on the >> > atomicity semantics, but there are no guarantees of that for >> > smp_store_mb() mentioned anywhere, plus most archs use this anyway. >> > Thus we continue to be consistent with the memory-barriers.txt file, >> > and more importantly, maintain the semantics of the smp_ nature. >> > >> So with this patch, the whole thing becomes pointless, I feel. (Ok, so >> it may have been pointless before too, but at least before this patch >> it generated special code, now it doesn't). So why carry it along at >> all? > >So I suppose this boils down to if: XCHG ends up being cheaper than >MOV+FENCE.
If so, could this be the reasoning behind the mix and match of xchg and MOV+FENCE? for different archs? This is from the days when set_mb() was introduced. I wonder if it still even matters... I at least haven't seen much difference in general workloads (I guess any difference would be neglictible for practical matters). But could obviously be missing something.
>PeterA, any idea?
I suppose you're referring to hpa, Cc'ing him.
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