cvs commit: src/usr.bin/lockf lockf.c

[Christian S.J. Peron]

csjp        2005-10-05 17:39:15 UTC

  FreeBSD src repository

  Modified files:
    usr.bin/lockf        lockf.c 
  Log:
  Fix long standing race condition associated with how lockf uses open(2)
  for mutual exclusion:
  
  A brief description of the problem:
  
          1) Proc A picks up non-blocking lock on file X
          2) Proc B attempts to pickup lock, fails then waits
          3) Proc C attempts to pickup lock, fails then waits
          4) Proc A releases lock
          5) Proc B acquires lock, release it to pickup a non-blocking version
          6) Proc C acquires lock, release it to pickup a non-blocking version
          7) Both process B and C race each other to pickup lock again
  
  This occurs mainly because the processes do not keep the lock after they have
  been waiting on it. They drop it, attempt to re-acquire it. (They use the wait
  to notify when the lock has become available then race to pick it up). This
  results in additional CPU utilization during the race, and can also result
  in processes picking locks up out of order.
  
  This change attempts to correct this problem by eliminating the test/acquire
  race and having the operating system handle it.
  
  Reported by:    kris
  Tested by:      kris
  MFC after:      1 week
  
  Revision  Changes    Path
  1.12      +8 -30     src/usr.bin/lockf/lockf.c