public inbox for linux-kernel@vger.kernel.org
 help / color / mirror / Atom feed
* 0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94 (posix-timers: Fix division by zero bug).  Problems with glibc.
@ 2019-02-01 21:47 Alan Mackenzie
  2019-02-01 22:04 ` Thomas Gleixner
  0 siblings, 1 reply; 8+ messages in thread
From: Alan Mackenzie @ 2019-02-01 21:47 UTC (permalink / raw)
  To: Thomas Gleixner, linux-kernel; +Cc: 34235, Eli Zaretskii, Alex Branham

Hello, Thomas, Hello Linux.

0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94
posix-timers: Fix division by zero bug
Committed: 2018-12-17 17:35:45 +0100

With this patch in place I am seeing problems with glibc's function
timer_create.  I am an Emacs maintainer, and saw these problems whilst
investigating Emacs bug #34235 "27.0.50; lisp profiler does not work".
Full details of this bug are at
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=34235.

Emacs's profiler fails in kernel 4.19.13, but works in a version of
4.19.13 with the patch reversed, otherwise unchanged.  My current version
of glibc is 2.27-r6 (I think the "-r6" comes from Gentoo, my distro).

The Emacs profiler works by a signal handler being repeatedly triggered
by the SIGPROF signal every 1 millisecond.  In the bug scenario, this
signal gets triggered precisely once each time the Emacs profiler is
started, rather than continually.

The core of the code in Emacs which initialises the glibc timer is:

      int i;
      struct sigevent sigev;
      sigev.sigev_value.sival_ptr = &profiler_timer;
      sigev.sigev_signo = SIGPROF;
      sigev.sigev_notify = SIGEV_SIGNAL;

      for (i = 0; i < ARRAYELTS (system_clock); i++)
        if (timer_create (system_clock[i], &sigev, &profiler_timer) == 0)
          {
            profiler_timer_ok = 1;
            break;
          }
    }

  if (profiler_timer_ok)
    {
      struct itimerspec ispec;
      ispec.it_value = ispec.it_interval = interval;
      if (timer_settime (profiler_timer, 0, &ispec, 0) == 0)
        return TIMER_SETTIME_RUNNING;
    }

The variable `interval' has been checked as non-zero.  This code is in
.../emacs/src/profiler.c

It seems either that the patch has uncovered some invalid call between
Emacs and glibc, or between glibc and Linux, or that there is some
intrinsic problem with the patch.

I have very little familiarity with glibc and Linux source code, so I
would be greatful if you could help me investigate the bug scenario.
Naturally, I will help as I can in this process.

Thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).

^ permalink raw reply	[flat|nested] 8+ messages in thread
* Re: 0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94 (posix-timers: Fix, division by zero bug). Problems with glibc.
@ 2019-02-02  2:07 Paul Eggert
  2019-02-02  9:21 ` Thomas Gleixner
  0 siblings, 1 reply; 8+ messages in thread
From: Paul Eggert @ 2019-02-02  2:07 UTC (permalink / raw)
  To: Thomas Gleixner
  Cc: Alex Branham, Alan Mackenzie, Eli Zaretskii, linux-kernel, 34235

[-- Attachment #1: Type: text/plain, Size: 381 bytes --]

Thanks for helping to track down this bug. Since the problem occurs only 
with a few Linux kernel versions and affects Emacs only when doing CPU 
profiling, it doesn't seem worth spending time to try to patch Emacs to 
work around the bug. So I installed the attached patch into emacs-26's 
etc/PROBLEMS file to warn users about the problem, and am closing the 
Emacs bug report.


[-- Attachment #2: 0001-etc-PROBLEMS-Mention-profiler-report-bug-Bug-34235.patch --]
[-- Type: text/x-patch, Size: 826 bytes --]

From 1243188bc4f722abf16518bf73924ce5f17750cf Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 1 Feb 2019 17:58:05 -0800
Subject: [PATCH] * etc/PROBLEMS: Mention profiler-report bug (Bug#34235).

---
 etc/PROBLEMS | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index cab087631c..00583f016d 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -1850,6 +1850,12 @@ term/xterm.el) for more details.
 
 ** GNU/Linux
 
+*** GNU/Linux: profiler-report outputs nothing.
+
+A few versions of the Linux kernel have timer bugs that break CPU
+profiling; see Bug#34235.  To fix the problem, upgrade to kernel
+versions 4.19.19 or 4.20.6, or later.
+
 *** GNU/Linux: Process output is corrupted.
 
 There is a bug in Linux kernel 2.6.10 PTYs that can cause emacs to
-- 
2.20.1


^ permalink raw reply related	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2019-02-05 13:56 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2019-02-01 21:47 0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94 (posix-timers: Fix division by zero bug). Problems with glibc Alan Mackenzie
2019-02-01 22:04 ` Thomas Gleixner
2019-02-02 10:44   ` Alan Mackenzie
2019-02-04 17:25     ` Thomas Gleixner
2019-02-05 13:54       ` Alan Mackenzie
  -- strict thread matches above, loose matches on Subject: below --
2019-02-02  2:07 0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94 (posix-timers: Fix, " Paul Eggert
2019-02-02  9:21 ` Thomas Gleixner
2019-02-03  6:28   ` Paul Eggert

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox