linux-kernel - Re: [PATCH] perf tools: session: avoid infinite loop

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Date:	Fri, 18 Sep 2015 12:29:57 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] perf tools: session: avoid infinite loop

Em Fri, Sep 18, 2015 at 04:18:51PM +0100, Mark Rutland escreveu:
> On Fri, Sep 18, 2015 at 04:00:18PM +0100, Arnaldo Carvalho de Melo wrote:
> > After applying your patch it works, had to tweak the commit log to avoid
> > starting lines with ---, breaks git scripts, also added a commiter log,
> > check it, patch is below, after the one I used to not process any
> > samples.
 
> Sorry for the '---' problem, I'll bear that in mind in future.
 
> Your commit log looks fine, though I'm slightly confused by the
> Reported-by line -- did you mean to add that?

Sorry, I mean Tested-by:, will replace, guess i can replace the one for
Adrian from Cc: to Tested-by too, right?

- Arnaldo
 
> Mark.
> 
> > commit dd486ec4aa33cfca2fd912ef501d49909005de79
> > Author: Mark Rutland <mark.rutland@....com>
> > Date:   Wed Sep 16 18:18:49 2015 +0100
> > 
> >     perf record: Avoid infinite loop at buildid processing with no samples
> >     
> >     If a session contains no events, we can get stuck in an infinite loop in
> >     __perf_session__process_events, with a non-zero file_size and data_offset, but
> >     a zero data_size.
> >     
> >     In this case, we can mmap the entirety of the file (consisting of the file and
> >     attribute headers), and fetch_mmaped_event will correctly refuse to read any
> >     (unmapped and non-existent) event headers. This causes
> >     __perf_session__process_events to unmap the file and retry with the exact same
> >     parameters, getting stuck in an infinite loop.
> >     
> >     This has been observed to result in an exit-time hang when counting
> >     rare/unschedulable events with perf record, and can be triggered artificially
> >     with the script below:
> >     
> >       ----
> >       #!/bin/sh
> >       printf "REPRO: launching perf\n";
> >       ./perf record -e software/config=9/ sleep 1 &
> >       PERF_PID=$!;
> >       sleep 0.002;
> >       kill -2 $PERF_PID;
> >       printf "REPRO: waiting for perf (%d) to exit...\n" "$PERF_PID";
> >       wait $PERF_PID;
> >       printf "REPRO: perf exited\n";
> >       ----
> >     
> >     To avoid this, have __perf_session__process_events bail out early when
> >     the file has no data (i.e. it has no events).
> >     
> >     Commiter note:
> >     
> >     I only managed to reproduce this when setting
> >     /proc/sys/kernel/kptr_restrict to '1' and changing the code to
> >     purposefully not process any samples and no synthesized samples, i.e.
> >     kptr_restrict prevents 'record' from synthesizing the kernel mmaps for
> >     vmlinux + modules and since it is a workload started from perf, we don't
> >     synthesize mmap/comm records for existing threads.
> >     
> >     Adrian Hunter managed to reproduce it in his environment tho.
> >     
> >     Reported-by: Arnaldo Carvalho de Melo <acme@...nel.org>
> >     Signed-off-by: Mark Rutland <mark.rutland@....com>
> >     Cc: Adrian Hunter <adrian.hunter@...el.com>
> >     Cc: Jiri Olsa <jolsa@...hat.com>
> >     Cc: Peter Zijlstra <peterz@...radead.org>
> >     Link: http://lkml.kernel.org/r/1442423929-12253-1-git-send-email-mark.rutland@arm.com
> >     Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> > 
> > diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> > index 8a4537ee9bc3..fc3f7c922f99 100644
> > --- a/tools/perf/util/session.c
> > +++ b/tools/perf/util/session.c
> > @@ -1580,7 +1580,10 @@ static int __perf_session__process_events(struct perf_session *session,
> >  	file_offset = page_offset;
> >  	head = data_offset - page_offset;
> >  
> > -	if (data_size && (data_offset + data_size < file_size))
> > +	if (data_size == 0)
> > +		goto out;
> > +
> > +	if (data_offset + data_size < file_size)
> >  		file_size = data_offset + data_size;
> >  
> >  	ui_progress__init(&prog, file_size, "Processing events...");
> > 
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