cvs commit: src/sys/kern kern_timeout.c

Alfred Perlstein alfred at
Sat Mar 22 00:29:45 PDT 2008

alfred      2008-03-22 07:29:45 UTC

  FreeBSD src repository

  Modified files:
    sys/kern             kern_timeout.c 
  Fix a race where timeout/untimeout could cause crashes for Giant locked
  The bug:
  There exists a race condition for timeout/untimeout(9) due to the
  way that the softclock thread dequeues timeouts.
  The softclock thread sets the c_func and c_arg of the callout to
  NULL while holding the callout lock but not Giant.  It then drops
  the callout lock and acquires Giant.
  It is at this point where untimeout(9) on another cpu/thread could
  be called.
  Since c_arg and c_func are cleared, untimeout(9) does not touch the
  callout and returns as if the callout is canceled.
  The softclock then tries to acquire Giant and likely blocks due to
  the other cpu/thread holding it.
  The other cpu/thread then likely deallocates the backing store that
  c_arg points to and finishes working and hence drops Giant.
  Softclock resumes and acquires giant and calls the function with
  the now free'd c_arg and we have corruption/crash.
  The fix:
  We need to track curr_callout even for timeout(9) (LOCAL_ALLOC)
  callouts.  We need to free the callout after the softclock processes
  it to deal with the race here.
  Obtained from: Juniper Networks, iedowse
  Reviewed by: jhb, iedowse
  MFC After: 2 weeks.
  Revision  Changes    Path
  1.111     +19 -4     src/sys/kern/kern_timeout.c

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