Tom Talpey wrote:
At 10:59 PM 5/28/2009, Rob Gardner wrote:
J. Bruce Fields wrote:
Looking at the code.... This is all under the BKL, and as far as I can
tell there aren't any blocking operations anywhere there, so I don't
think this should happen if the filesystem is careful. Have you seen it
happen?
Aha, I just figured it out and you were right. The filesystem in this
case was not careful. It broke the rules and actually made the fl_grant
call *before* even returning to nlmsvc_lock's call to vfs_lock_file, and
it did it in the lockd thread! So the BKL was of no use, and I saw
nlmsvc_grant_deferred print "grant for unknown block". So I think
everything is ok, no huge race in lockd for async lock requests. Thank
you for clearing this up.
Gack! I'm surprised it worked at all. The fact that the BKL allows itself to
be taken recursively really masked your filesystem bug. If the BKL had
blocked, or asserted, the bug would never have happened.
Yeah, recall that I'm using a very old kernel (circa 2.6.18) which I
think must still allow the BKL to be acquired recursively.
This is as good a time as any to point out that the BKL's use in the lockd
code is insidious and needs some serious attention.
No disagreement here! I think I almost understand enough about lockd to
remove the BKL, but the operative word there is "almost".
Rob Gardner
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