linux-kernel - [tip:perf/core] perf c2c report: Fix crash for empty browser

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Date:   Thu, 2 Aug 2018 01:12:02 -0700
From:   tip-bot for Jiri Olsa <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, acme@...hat.com, hpa@...or.com,
        namhyung@...nel.org, alexander.shishkin@...ux.intel.com,
        jmario@...hat.com, dzickus@...hat.com, peterz@...radead.org,
        mingo@...nel.org, dsahern@...il.com, tglx@...utronix.de,
        jolsa@...nel.org
Subject: [tip:perf/core] perf c2c report: Fix crash for empty browser

Commit-ID:  73978332572ccf5e364c31e9a70ba953f8202b46
Gitweb:     https://git.kernel.org/tip/73978332572ccf5e364c31e9a70ba953f8202b46
Author:     Jiri Olsa <jolsa@...nel.org>
AuthorDate: Tue, 24 Jul 2018 08:20:08 +0200
Committer:  Arnaldo Carvalho de Melo <acme@...hat.com>
CommitDate: Tue, 31 Jul 2018 10:53:20 -0300

perf c2c report: Fix crash for empty browser

'perf c2c' scans read/write accesses and tries to find false sharing
cases, so when the events it wants were not asked for or ended up not
taking place, we get no histograms.

So do not try to display entry details if there's not any. Currently
this ends up in crash:

  $ perf c2c report # then press 'd'
  perf: Segmentation fault
  $

Committer testing:

Before:

Record a perf.data file without events of interest to 'perf c2c report',
then call it and press 'd':

  # perf record sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.001 MB perf.data (6 samples) ]
  # perf c2c report
  perf: Segmentation fault
  -------- backtrace --------
  perf[0x5b1d2a]
  /lib64/libc.so.6(+0x346df)[0x7fcb566e36df]
  perf[0x46fcae]
  perf[0x4a9f1e]
  perf[0x4aa220]
  perf(main+0x301)[0x42c561]
  /lib64/libc.so.6(__libc_start_main+0xe9)[0x7fcb566cff29]
  perf(_start+0x29)[0x42c999]
  #

After the patch the segfault doesn't take place, a follow up patch to
tell the user why nothing changes when 'd' is pressed would be good.

Reported-by: rodia@...istici.org
Signed-off-by: Jiri Olsa <jolsa@...nel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: David Ahern <dsahern@...il.com>
Cc: Don Zickus <dzickus@...hat.com>
Cc: Joe Mario <jmario@...hat.com>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Fixes: f1c5fd4d0bb9 ("perf c2c report: Add TUI cacheline browser")
Link: http://lkml.kernel.org/r/20180724062008.26126-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 tools/perf/builtin-c2c.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/builtin-c2c.c b/tools/perf/builtin-c2c.c
index f2ea85ee573f..f3aa9d02a5ab 100644
--- a/tools/perf/builtin-c2c.c
+++ b/tools/perf/builtin-c2c.c
@@ -2349,6 +2349,9 @@ static int perf_c2c__browse_cacheline(struct hist_entry *he)
 	" s             Toggle full length of symbol and source line columns \n"
 	" q             Return back to cacheline list \n";
 
+	if (!he)
+		return 0;
+
 	/* Display compact version first. */
 	c2c.symbol_full = false;