On Thu, Apr 10, 2008 at 05:20:36PM -0400, Trond Myklebust wrote: > > On Thu, 2008-04-10 at 17:07 -0400, Trond Myklebust wrote: > > On Thu, 2008-04-10 at 17:02 -0400, Trond Myklebust wrote: > > > On Thu, 2008-04-10 at 21:51 +0200, Miklos Szeredi wrote: > > > > Another infinite loop, this one involving both client and server. > > > > > > > > Basically what happens is that on the server nlm_fopen() calls > > > > nfsd_open() which returns -EACCES, to which nlm_fopen() returns > > > > NLM_LCK_DENIED. > > > > > > > > On the client this will turn into a -EAGAIN (nlm_stat_to_errno()), > > > > which in will cause fcntl_setlk() to retry forever. > > > > > > > > I _think_ the solution is to turn NLM_LCK_DENIED into ENOLCK for > > > > blocking locks, as NLM_LCK_BLOCKED is for the contended case. For > > > > testing the lock leave NLM_LCK_DENIED as EAGAIN. That still could be > > > > misleading, but at least there's no infinite loop in that case. > > > > > > > > I've minimally tested this patch to verify that it cures the lockup, > > > > and that simple blocking locks keep working. > > > > > > > > Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx> > > > > --- > > > > fs/lockd/clntproc.c | 3 +++ > > > > 1 file changed, 3 insertions(+) > > > > > > > > Index: linux/fs/lockd/clntproc.c > > > > =================================================================== > > > > --- linux.orig/fs/lockd/clntproc.c 2008-04-02 13:34:57.000000000 +0200 > > > > +++ linux/fs/lockd/clntproc.c 2008-04-10 21:23:46.000000000 +0200 > > > > @@ -536,6 +536,9 @@ again: > > > > up_read(&host->h_rwsem); > > > > } > > > > status = nlm_stat_to_errno(resp->status); > > > > + /* Don't return EAGAIN, as that would make fcntl_setlk() loop */ > > > > + if (status == -EAGAIN) > > > > + status = -ENOLCK; > > > > out_unblock: > > > > nlmclnt_finish_block(block); > > > > /* Cancel the blocked request if it is still pending */ > > > > > > > > > Wait. There is something really weird going on here. > > > > > > According to the spec, LCK_DENIED means 'the request failed' (i.e. > > > ENOLCK is definitely correct) > > > > > > OTOH, LCK_DENIED_NOLOCKS and LCK_DENIED_GRACE_PERIOD are both temporary > > > failures, the first because the server had a resource problem, and the > > > second because the server rebooted and is in the grace period (i.e. > > > EAGAIN would appear to be more appropriate). See > > > > > > http://www.opengroup.org/onlinepubs/9629799/chap10.htm#tagcjh_11_02_02_02 > > > > > > AFAICS, the correct thing to do is to fix nlm_stat_to_errno() by > > > swapping the return values for NLM_LCK_DENIED and > > > NLM_LCK_DENIED_NOLOCKS/NLM_LCK_DENIED_GRACE_PERIOD. > > > > > > The problem is that there appears to be a similar confusion on the Linux > > > server side in nlmsvc_lock(). :-( > > > > Duh... Sorry, EAGAIN is indeed the correct return value for fcntl() when > > the lock attempt failed. I should have reread the manpage/posix spec > > before replying. > > OK. So the correct fix here should really be applied to fcntl_setlk(). > There is absolutely no reason why we should be looping at all if the > filesystem has a ->lock() method. > > In fact, this looping behaviour was introduced recently in commit > 7723ec9777d9832849b76475b1a21a2872a40d20. Apologies, that was indeed a behavioral change introduced in a commit that claimed just to be shuffling code around. > Marc, Bruce, why was this > done, and how are filesystems now supposed to behave? > The assumption must have been that -EAGAIN could only mean that we needed to keep blocking, and hence was a nonsensical return from a filesystem lock method that waited itself for the lock to become available--such a method would return 0, -EINTR (or some more exotic error), or continue waiting. If we agree that EAGAIN is actually a legimate error to return from a blocking lock, then, yes, we need take ->lock() back out of this loop. And I don't think there's any real reason we need the new behavior. So we should probably revert that--I'll take a closer look tomorrow.... --b. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
