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Re: kern/39206: ffs um_lock handling isn't great

The following reply was made to PR kern/39206; it has been noted by GNATS.

From: David Holland <>
To: Simon Burge <>,
Subject: Re: kern/39206: ffs um_lock handling isn't great
Date: Fri, 25 Jul 2008 15:24:44 +0000

 On Fri, Jul 25, 2008 at 02:15:00PM +0000, Simon Burge wrote:
  >         One example pointed out by pooka@ is at the top of
  >         ffs_alloccg().  It appears that once the free block check at
  >         the top of this function succeeds, this function isn't allowed
  >         to fail.  This is noted in the "XXX fvdl mapsearch ..." comment
  >         further down.  This function is entered with um_lock held, and
  >         once the free block check has passed um_lock is dropped.  This
  >         then allows another thread to reach the same point, and could
  >         lead to problems if there was only one block free in the CG
  >         before the first thread get there.
  >         This PR is entered as priority "medium" and not "high" since no
  >         actual problems have been observed in practice yet.
 Unless this is the source of those occasional "ffs_alloccg: map
 corrupted" panics...
 David A. Holland

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