linux-kernel - Re: [PATCH v1 1/2] perf auxtrace: Change to use SMP memory barriers

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Message-ID: <YK9qMWMRWIMfzcPh@hirez.programming.kicks-ass.net>
Date:   Thu, 27 May 2021 11:45:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] perf auxtrace: Change to use SMP memory barriers

On Thu, May 27, 2021 at 11:25:40AM +0300, Adrian Hunter wrote:
> On 27/05/21 11:11 am, Peter Zijlstra wrote:
> > On Thu, May 27, 2021 at 10:54:56AM +0300, Adrian Hunter wrote:
> >> On 19/05/21 5:03 pm, Leo Yan wrote:
> >>> The AUX ring buffer's head and tail can be accessed from multiple CPUs
> >>> on SMP system, so changes to use SMP memory barriers to replace the
> >>> uniprocessor barriers.
> >>
> >> I don't think user space should attempt to be SMP-aware.
> > 
> > Uhh, what? It pretty much has to. Since userspace cannot assume UP, it
> > must assume SMP.
> 
> Yeah that is what I meant, but consequently we generally shouldn't be
> using functions called smp_<anything>

Of course we should; they're the SMP class of barriers.

> > So ACK on the patch, it's sane and an optimization for both x86 and ARM.
> > Just the Changelog needs work.
> 
> If all we want is a compiler barrier, then shouldn't that be what we use?
> i.e. barrier()

No, we want the SMP barriers, smp_rmb() happens to be a compiler barrier
on x86, but it will be dmb(ishld) on Aarrgh64 for example.