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From: Ryan Stone <rstone@FreeBSD.org>
Date: Tue, 7 Sep 2010 00:23:45 +0000 (UTC)
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Subject: cvs commit: src/sys/vm vm_mmap.c
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rstone      2010-09-07 00:23:45 UTC

  FreeBSD src repository

  Modified files:
    sys/vm               vm_mmap.c 
  Log:
  SVN rev 212281 on 2010-09-07 00:23:45Z by rstone
  
  In munmap() downgrade the vm_map_lock to a read lock before taking a read
  lock on the pmc-sx lock.  This prevents a deadlock with
  pmc_log_process_mappings, which has an exclusive lock on pmc-sx and tries
  to get a read lock on a vm_map.  Downgrading the vm_map_lock in munmap
  allows pmc_log_process_mappings to continue, preventing the deadlock.
  
  Without this change I could cause a deadlock on a multicore 8.1-RELEASE
  system by having one thread constantly mmap'ing and then munmap'ing a
  PROT_EXEC mapping in a loop while I repeatedly invoked and stopped pmcstat
  in system-wide sampling mode.
  
  Reviewed by:    fabient
  Approved by:    emaste (mentor)
  MFC after:      2 weeks
  
  Revision  Changes    Path
  1.245     +11 -3     src/sys/vm/vm_mmap.c