On Wed, 4 Sep 2013 14:31:07 +0000 "Myklebust, Trond" <Trond.Myklebust@xxxxxxxxxx> wrote: > On Wed, 2013-09-04 at 17:04 +1000, NeilBrown wrote: > > > > When an NFSv4 client loses contact with the server it can lose any > > locks that it holds. > > > > Currently when it reconnects to the server it simply tries to reclaim > > those locks. This might succeed even though some other client has > > held and released a lock in the mean time. So the first client might > > think the file is unchanged, but it isn't. This isn't good. > > > > If, when recovery happens, the locks cannot be claimed because some > > other client still holds the lock, then we get a message in the kernel > > logs, but the client can still write. So two clients can both think > > they have a lock and can both write at the same time. This is equally > > not good. > > > > There was a patch a while ago > > http://comments.gmane.org/gmane.linux.nfs/41917 > > > > which tried to address some of this, but it didn't seem to go > > anywhere. That patch would also send a signal to the process. That > > might be useful but for now this patch just causes writes to fail. > > > > For NFSv4 (unlike v2/v3) there is a strong link between the lock and > > the write request so we can fairly easily fail any IO of the lock is > > gone. While some applications might not expect this, it is still > > safer than allowing the write to succeed. > > > > Because this is a fairly big change in behaviour a module parameter, > > "recover_locks", is introduced which defaults to true (the current > > behaviour) but can be set to "false" to tell the client not to try to > > recover things that were lost. > > > > Signed-off-by: NeilBrown <neilb@xxxxxxx> > > Thanks! > > > -- > > This alternative uses a module parameter which defaults to the current, > > incorrect, behaviour. > > I suspect we don't want that one.. > > Agreed. We also need to document the module parameter, so I'm adding a > little blurb in Documentation/kernel-parameters.txt (see attachment). > > I'd also like to change the parameter name to "recover_lost_locks" to > make it a little more obvious. > > Finally, I'd like to move the parameter to fs/nfs/super.c so that we can > use the same modprobe.conf 'options' lines for back ports of this patch > (yes I strongly suspect we will want to back port this patch to distro > kernels). > > See attachment. Looks good - thanks. NeilBrown
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