pbzip2 1.1.5 hangs during compression

Bug #876686 reported by David James on 2011-10-17
This bug affects 1 person
Affects Status Importance Assigned to Milestone
Yavor Nikolov

Bug Description

pbzip2 1.1.5 occasionally hangs during compression. This hang occurs pretty rarely.

Here's the strace output from a hang:
[pid 6756] futex(0x60f4e4, FUTEX_WAIT_PRIVATE, 1, NULL <unfinished ...>
[pid 6891] futex(0x961564, FUTEX_WAIT_PRIVATE, 39, NULL <unfinished ...>
[pid 6755] rt_sigtimedwait([HUP INT ABRT USR1 TERM], NULL, NULL, 8 <unfinished ...>
[pid 6549] futex(0x7f4c9a86d9d0, FUTEX_WAIT, 6891, NULL

File is completely written out, and stdin is closed.

Pid 6549 is in pthread_join, likely waiting on 6891 to exit.
Pid 6755 is the signal thread
Pid 6756 is the terminator thread
Pid 6891 is in pthread_cond_wait

I investigated the pbzip2 source code and found a situation where this hang can occur:
  1. Consumer thread grabs fifo->mut lock, checks that the fifo is empty, and checks that producer is not done.
  2. Producer thread sets done state and broadcasts FifoQueue->notEmpty
  3. fileWriter thread finishes, broadcasts FifoQueue->notEmpty
  4. consumer blocks waiting for FifoQueue->notEmpty signal
  5. Main thread blocks in pthread_join, waiting for consumer thread to exit.

This hang normally occurs very rarely, because the above situation is quite unlikely to happen. So I attached a patch to make the hang more likely (just so you can reproduce the hang more easily.)

This is tracked on Chromium OS bug tracker at http://crosbug.com/21726

David James (james82) wrote :
David James (james82) wrote :

Here's a patch that fixes the issue. This patch ensures that we always grab the associated mutex before broadcasting a signal. That in turn ensures that any threads that have checked the condition associated with the mutex have made it to the 'wait' stage and won't miss the signal.

Yavor Nikolov (yavor-nikolov) wrote :

David, thanks for reporting that bug and providing a patch.
I'll try to merge a fix and push a new pbzip2 release (I hope soon).

Changed in pbzip2:
importance: Undecided → High
Changed in pbzip2:
assignee: nobody → Yavor Nikolov (yavor-nikolov)
Changed in pbzip2:
status: New → Confirmed
Changed in pbzip2:
status: Confirmed → Fix Committed
milestone: none → 1.1.6
Changed in pbzip2:
status: Fix Committed → Fix Released
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