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

[David Holland <dholland-bugs%netbsd.org@localhost>]

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

From: David Holland <dholland-bugs%netbsd.org@localhost>
To: Simon Burge <simonb%NetBSD.org@localhost>, gnats-bugs%netbsd.org@localhost
Cc: kern-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
        netbsd-bugs%netbsd.org@localhost
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
 dholland%netbsd.org@localhost