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From: Marcel Moolenaar <marcel@FreeBSD.org>
Date: Sat, 4 Apr 2009 22:23:03 +0000 (UTC)
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Subject: cvs commit: src/sys/powerpc/aim swtch.S
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marcel      2009-04-04 22:23:03 UTC

  FreeBSD src repository

  Modified files:
    sys/powerpc/aim      swtch.S 
  Log:
  SVN rev 190704 on 2009-04-04 22:23:03Z by marcel
  
  Perform a dummy stwcx. when we switch contexts. The context
  being switched out may hold a reservation. The stwcx. will
  clear the reservation. This is architecturally recommended.
  
  The scenario this addresses is as follows:
  1. Thread 1 performs a lwarx and as such holds a reservation.
  2. Thread 1 gets switched out (before doing the matching
     stwcx.) and thread 2 is switched in.
  3. Thread 2 performs a stwcx. to the same reservation granule.
     This will succeed because the processor has the reservation
     even though thread 2 didn't do the lwarx.
  
  Note that on some processors the address given the stwcx. is
  not checked. On these processors the mere condition of having
  a reservation would cause the stwcx. to succeed, irrespective
  of whether the addresses are the same. The dummy stwcx. is
  especially important for those processors.
  
  Revision  Changes    Path
  1.28      +6 -0      src/sys/powerpc/aim/swtch.S