Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 24 Oct 2022 08:30:13 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Ian Rogers <irogers@google.com>, Greg Thelen <gthelen@google.com>,
        Anand K Mistry <amistry@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v1] perf record: Fix event fd races
Message-ID: <Y1Z3RfP7sVltTfyF@kernel.org>
References: <20221024011024.462518-1-irogers@google.com>
 <Y1X+7FCemionnepj@leoy-huanghe.lan>
 <CAP-5=fWUrwQ187O7d0incvRFQ7RT+1viG30k+F1ZEEU3BEFeFA@mail.gmail.com>
 <Y1ZYCD1LjC5OhWsq@leoy-huanghe.lan>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y1ZYCD1LjC5OhWsq@leoy-huanghe.lan>
Precedence: bulk

Em Mon, Oct 24, 2022 at 05:16:56PM +0800, Leo Yan escreveu:
> On Sun, Oct 23, 2022 at 10:33:30PM -0700, Ian Rogers wrote:
> 
> [...]
> 
> > > > +static volatile int done_fd = -1;
> > >
> > > Here is a bit suspecious for adding volatile qualifier.  See the
> > > document: process/volatile-considered-harmful.rst.
> > >
> > > I know the document is mainly for kernel programming, but seems to me
> > > it's also valid for C programming in userspace.
> > >
> > > I not sure what's the purpose for adding volatile for done_fd, if we
> > > really have concern for reading any stale value for done_fd, should we
> > > use WRITE_ONCE/READ_ONCE?
> > 
> > We could just switch to C11 and stdatomic. The volatile is consistent
> > with the code above and more consistent with the expectation of
> > writing to a variable that is read in a signal handler.
> 
> Thanks for the info for C11 and stdatomic.h.  The documentation [1] says
> the safe way is for accessing shared data in signal handler is:
> 
>   static volatile sig_atomic_t done_fd = -1;
> 
> It's fine if you want to use another patch to address this issue, this
> patch for fixing errno is fine for me:
> 
> Reviewed-by: Leo Yan <leo.yan@linaro.org>


Thanks, applied.

- Arnaldo

 
> [1] https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers

-- 

- Arnaldo