linux-kernel - Re: [PATCH v3] perf record: Use an eventfd to wakeup when done

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Message-ID: <20200513113941.GK3158213@krava>
Date:   Wed, 13 May 2020 13:39:41 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Anand K Mistry <amistry@...gle.com>
Cc:     linux-perf-users@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] perf record: Use an eventfd to wakeup when done

On Wed, May 13, 2020 at 12:20:23PM +1000, Anand K Mistry wrote:
> The setting and checking of 'done' contains a rare race where the signal
> handler setting 'done' is run after checking to break the loop, but
> before waiting in evlist__poll(). In this case, the main loop won't wake
> up until either another signal is sent, or the perf data fd causes a
> wake up.
> 
> The following simple script can trigger this condition (but you might
> need to run it for several hours):
> for ((i = 0; i >= 0; i++)) ; do
>   echo "Loop $i"
>   delay=$(echo "scale=4; 0.1 * $RANDOM/32768" | bc)
>   ./perf record -- sleep 30000000 >/dev/null&
>   pid=$!
>   sleep $delay
>   kill -TERM $pid
>   echo "PID $pid"
>   wait $pid
> done
> 
> At some point, the loop will stall. Adding logging, even though perf has
> received the SIGTERM and set 'done = 1', perf will remain sleeping until
> a second signal is sent.
> 
> Signed-off-by: Anand K Mistry <amistry@...gle.com>
> 
> ---
> 
> Changes in v3:
> - Move done_fd creation to below session initialisation
> - Close done_fd on exit
> - Log errno when write(done_fd) fails

Acked-by: Jiri Olsa <jolsa@...hat.com>

thanks,
jirka