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

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Date:   Mon, 31 May 2021 17:48:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        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 Mon, May 31, 2021 at 10:53:02PM +0800, Leo Yan wrote:
> Hi Peter, Adrian,
> 
> On Thu, May 27, 2021 at 11:57:37AM +0200, Peter Zijlstra wrote:
> > On Thu, May 27, 2021 at 12:24:15PM +0300, Adrian Hunter wrote:
> > 
> > > > If all we want is a compiler barrier, then shouldn't that be what we use?
> > > > i.e. barrier()
> 
> Sorry for a bit late.  Just bring up one question before I respin
> this patch set.
> 
> > > I guess you are saying we still need to stop potential re-ordering across
> > > CPUs, so please ignore my comments.
> > 
> > Right; so the ordering issue is real, consider:
> > 
> > 	CPU0 (kernel)		CPU1 (user)
> > 
> > 	write data		read head
> > 	smp_wmb()		smp_rmb()
> > 	write head		read data
> 
> One thing should be mentioned is the Linux kernel has _not_ used an
> explict "smb_wmb()" between writing AUX trace data and updating header
> "aux_head".  Please see the function perf_aux_output_end():

I think we pushed that into the driver. There is nothing the generic
code can do here.

It is the drivers responsibility of ensuring the data is stable before
calling perf_aux_output_end() or something along those lines.