cvs commit: src/sys/kern subr_sleepqueue.c

[John Baldwin]

jhb         2008-01-25 02:09:38 UTC

  FreeBSD src repository

  Modified files:
    sys/kern             subr_sleepqueue.c 
  Log:
  Fix a race in the sleepqueue timeout code that resulted in sleeps not
  being properly cancelled by a timeout.  In general there is a race
  between a the sleepq timeout handler firing while the thread is still
  in the process of going to sleep.  In 6.x with sched_lock, the race was
  largely protected by sched_lock.  The only place it was "exposed" and had
  to be handled was while checking for any pending signals in
  sleepq_catch_signals().
  
  With the thread lock changes, the thread lock is dropped in between
  sleepq_add() and sleepq_*wait*() opening up a new window for this race.
  Thus, if the timeout fired while the sleeping thread was in between
  sleepq_add() and sleepq_*wait*(), the thread would be marked as timed
  out, but the thread would not be dequeued and sleepq_switch() would
  still block the thread until it was awakened via some other means.  In
  the case of pause(9) where there is no other wakeup, the thread would
  never be awakened.
  
  Fix this by teaching sleepq_switch() to check if the thread has had its
  sleep canceled before blocking by checking the TDF_TIMEOUT flag and
  aborting the sleep and dequeueing the thread if it is set.
  
  MFC after:      3 days
  Reported by:    dwhite, peter
  
  Revision  Changes    Path
  1.42      +25 -4     src/sys/kern/subr_sleepqueue.c