[Bug 838975] [NEW] weird pthread/fork race/deadlock

[Ryan Lortie]

Public bug reported:

There appears to be a strange bug in glibc that causes deadlocks when
calling fork() from threads.  We had a testcase in GLib failing from
time to time because of this.

I've attached a minimal testcase that uses only pure pthreads + libc.
Compile it with -pthread and run it.  It should fill your screen with
dots for a while, then hang when it hits the bug (which happens randomly
anywhere between 1 dot and hundreds).  I've already received independent
verification that this testcase hangs on several people's computers.

I believe this to be an upstream issue since this bug is visible on
Fedora 15 and 16, but the glibc website says I should file bugs against
distributions first.  I also believe the issue to be a regression since
Lucid is fine but Oneiric is not.  The problem appears to affect both 32
and 64bits.

Some notes:

 - compiling the testcase with -static has the side-effect of causing
the bug to go away

 - compiling the testcase with -DFORK_DIRECTLY also appears to solve the
problem

 - replacing the execv() with a direct exit(0) doesn't solve the problem
but causes the frequency to change


The fact that both static linking and making the fork() syscall directly cause the problem to disappear leads me to believe that this is a libc bug rather than a kernel bug (which is the only other possibility).  I'm not 100% sure of that, though, since libc actually uses the clone() syscall to implement fork(), so there could be a different inside the kernel because of that.

** Affects: eglibc (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/838975

Title:
  weird pthread/fork race/deadlock

Status in “eglibc” package in Ubuntu:
  New

Bug description:
  There appears to be a strange bug in glibc that causes deadlocks when
  calling fork() from threads.  We had a testcase in GLib failing from
  time to time because of this.

  I've attached a minimal testcase that uses only pure pthreads + libc.
  Compile it with -pthread and run it.  It should fill your screen with
  dots for a while, then hang when it hits the bug (which happens
  randomly anywhere between 1 dot and hundreds).  I've already received
  independent verification that this testcase hangs on several people's
  computers.

  I believe this to be an upstream issue since this bug is visible on
  Fedora 15 and 16, but the glibc website says I should file bugs
  against distributions first.  I also believe the issue to be a
  regression since Lucid is fine but Oneiric is not.  The problem
  appears to affect both 32 and 64bits.

  Some notes:

   - compiling the testcase with -static has the side-effect of causing
  the bug to go away

   - compiling the testcase with -DFORK_DIRECTLY also appears to solve
  the problem

   - replacing the execv() with a direct exit(0) doesn't solve the
  problem but causes the frequency to change

  
  The fact that both static linking and making the fork() syscall directly cause the problem to disappear leads me to believe that this is a libc bug rather than a kernel bug (which is the only other possibility).  I'm not 100% sure of that, though, since libc actually uses the clone() syscall to implement fork(), so there could be a different inside the kernel because of that.

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