Re: nfs: infinite loop in fcntl(F_SETLKW)

[Miklos Szeredi]

> > > The behavior of lockd will still depend to some degree on the exact
> > > error returned from the filesystem--e.g. if you return -EAGAIN from
> > > ->lock() without later calling ->fl_grant() it will cause some
> > > unexpected delays.  (Though again clients will eventually give up and
> > > poll for the lock.)
> > 
> > OK, so the semantics of vfs_lock_file() are now:
> 
> Not quite; the original idea was that we didn't care about the blocking
> lock case, since there's already a lock manager callback for that.
> (It's arguably a minor abuse for us to return NLM_LCK_BLOCKED and then
> do a grant later even in the case where there's no conflict, but it
> works.)  So we only changed the nonblocking case.
> 
> Which may be sacrificing elegance for expediency, and I'm not opposed to
> fixing that in whatever way seems best.  As a start, we could document
> this better.  So:
> 
> > 
> > 1) if !FL_SLEEP, then return 0 if granted, -EAGAIN on contention
> > 2) if FL_SLEEP and fl_grant == NULL, then return 0 if granted, block on
> >    contention
> > 3) if FL_SLEEP and fl_grant != NULL, then return 0 if granted, on
> >    contention:
> >    a) either return -EINPROGRESS and call fl_grant when granted
> >    b) or return -EAGAIN  and call fl_notify when the lock needs retrying
> 
> I'd put it this way (after a quick check of the code to convince myself
> I'm remembering this right...):
> 
>   1) If FL_SLEEP, then return 0 if granted, and on contention either:
>      a) block, or
>      b) return -EAGAIN, and call fl_notify when the lock should be
>         retried.

Gfs2 seems to return -EINPROGRESS regardless of the FL_SLEEP flag:

       c) return -EINPROGRESS and call f_grant when the lock is granted

>   2) If !FL_SLEEP, then either
>      a) return a result now, or
>      b) return -EINPROGRESS, then call fl_grant with the result.

OK, I missed that.

> In each case, b) is only of course allowed if the corresponding callback
> is defined.  If you want to support NFS exports, then you should also
> take option b) when possible.  (Though note only 1b is *required* to
> avoid deadlocks; 2b is basically just a performance enhancement.  We
> could perhaps even ditch 2b if we fixed lockd's BKL reliance and made it
> multithreaded.)
> 
> The names can be confusing, since fl_notify generally results in
> granting a previously blocked lock, whereas fl_grant just returns the
> result (positive or negative) of a single lock request.  They have the
> names they do because of the difference in their semantics:
> 
> 	- With fl_grant the filesystem says: I'm giving you the final
> 	  result of the lock operation.  In particular, if I'm telling
> 	  you it succeeded, that means I've already granted you the
> 	  lock; don't ask me about it again.
> 
> 	- With fl_notify the filesystem says: something just happened to
> 	  this lock.  I'm not guaranteeing anything, I'm just telling
> 	  you this would be a good time to try the lock again.  Do it
> 	  and maybe you'll get lucky!
> 
> This difference is why the fl_grant() operation takes a status argument
> (to tell the lock manager what happened), whereas fl_notify() doesn't
> (since it's just a poke in the ribs, and only the subsequent lock call
> will give the real status).
> 
> Is that explanation helpful?

Yes, thanks.

> OK, but I haven't read your patch yet, apologies....

No problem.  Here it is again with some cosmetic fixes and testing.

Miklos
--


Use a special error value FILE_LOCK_DEFERRED to mean that a locking
operation returned asynchronously.  This is returned by

  posix_lock_file() for sleeping locks to mean that the lock has been
  queued on the block list, and will be woken up when it might become
  available and needs to be retried (either fl_lmops->fl_notify() is
  called or fl_wait is woken up).

  f_op->lock() to mean either the above, or that the filesystem will
  call back with fl_lmops->fl_grant() when the result of the locking
  operation is known.  The filesystem can do this for sleeping as well
  as non-sleeping locks.

This is to make sure, that return values of -EAGAIN and -EINPROGRESS
by filesystems are not mistaken to mean an asynchronous locking.

This also makes error handling in fs/locks.c and lockd/svclock.c
slightly cleaner.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---
 fs/gfs2/locking/dlm/plock.c |    2 +-
 fs/lockd/svclock.c          |   13 ++++---------
 fs/locks.c                  |   28 ++++++++++++++--------------
 include/linux/fs.h          |    6 ++++++
 4 files changed, 25 insertions(+), 24 deletions(-)

Index: linux-2.6/fs/locks.c
===================================================================
--- linux-2.6.orig/fs/locks.c	2008-04-16 17:33:49.000000000 +0200
+++ linux-2.6/fs/locks.c	2008-04-16 17:38:01.000000000 +0200
@@ -784,8 +784,10 @@ find_conflict:
 		if (!flock_locks_conflict(request, fl))
 			continue;
 		error = -EAGAIN;
-		if (request->fl_flags & FL_SLEEP)
-			locks_insert_block(fl, request);
+		if (!(request->fl_flags & FL_SLEEP))
+			goto out;
+		error = FILE_LOCK_DEFERRED;
+		locks_insert_block(fl, request);
 		goto out;
 	}
 	if (request->fl_flags & FL_ACCESS)
@@ -841,7 +843,7 @@ static int __posix_lock_file(struct inod
 			error = -EDEADLK;
 			if (posix_locks_deadlock(request, fl))
 				goto out;
-			error = -EAGAIN;
+			error = FILE_LOCK_DEFERRED;
 			locks_insert_block(fl, request);
 			goto out;
   		}
@@ -1040,7 +1042,7 @@ int posix_lock_file_wait(struct file *fi
 	might_sleep ();
 	for (;;) {
 		error = posix_lock_file(filp, fl, NULL);
-		if ((error != -EAGAIN) || !(fl->fl_flags & FL_SLEEP))
+		if (error != FILE_LOCK_DEFERRED)
 			break;
 		error = wait_event_interruptible(fl->fl_wait, !fl->fl_next);
 		if (!error)
@@ -1112,9 +1114,7 @@ int locks_mandatory_area(int read_write,
 
 	for (;;) {
 		error = __posix_lock_file(inode, &fl, NULL);
-		if (error != -EAGAIN)
-			break;
-		if (!(fl.fl_flags & FL_SLEEP))
+		if (error != FILE_LOCK_DEFERRED)
 			break;
 		error = wait_event_interruptible(fl.fl_wait, !fl.fl_next);
 		if (!error) {
@@ -1534,7 +1534,7 @@ int flock_lock_file_wait(struct file *fi
 	might_sleep();
 	for (;;) {
 		error = flock_lock_file(filp, fl);
-		if ((error != -EAGAIN) || !(fl->fl_flags & FL_SLEEP))
+		if (error != FILE_LOCK_DEFERRED)
 			break;
 		error = wait_event_interruptible(fl->fl_wait, !fl->fl_next);
 		if (!error)
@@ -1719,17 +1719,17 @@ out:
  * fl_grant is set. Callers expecting ->lock() to return asynchronously
  * will only use F_SETLK, not F_SETLKW; they will set FL_SLEEP if (and only if)
  * the request is for a blocking lock. When ->lock() does return asynchronously,
- * it must return -EINPROGRESS, and call ->fl_grant() when the lock
+ * it must return FILE_LOCK_DEFERRED, and call ->fl_grant() when the lock
  * request completes.
  * If the request is for non-blocking lock the file system should return
- * -EINPROGRESS then try to get the lock and call the callback routine with
- * the result. If the request timed out the callback routine will return a
+ * FILE_LOCK_DEFERRED then try to get the lock and call the callback routine
+ * with the result. If the request timed out the callback routine will return a
  * nonzero return code and the file system should release the lock. The file
  * system is also responsible to keep a corresponding posix lock when it
  * grants a lock so the VFS can find out which locks are locally held and do
  * the correct lock cleanup when required.
  * The underlying filesystem must not drop the kernel lock or call
- * ->fl_grant() before returning to the caller with a -EINPROGRESS
+ * ->fl_grant() before returning to the caller with a FILE_LOCK_DEFERRED
  * return code.
  */
 int vfs_lock_file(struct file *filp, unsigned int cmd, struct file_lock *fl, struct file_lock *conf)
@@ -1806,7 +1806,7 @@ again:
 	else {
 		for (;;) {
 			error = posix_lock_file(filp, file_lock, NULL);
-			if (error != -EAGAIN || cmd == F_SETLK)
+			if (error != FILE_LOCK_DEFERRED)
 				break;
 			error = wait_event_interruptible(file_lock->fl_wait,
 					!file_lock->fl_next);
@@ -1934,7 +1934,7 @@ again:
 	else {
 		for (;;) {
 			error = posix_lock_file(filp, file_lock, NULL);
-			if (error != -EAGAIN || cmd == F_SETLK64)
+			if (error != FILE_LOCK_DEFERRED)
 				break;
 			error = wait_event_interruptible(file_lock->fl_wait,
 					!file_lock->fl_next);
Index: linux-2.6/fs/gfs2/locking/dlm/plock.c
===================================================================
--- linux-2.6.orig/fs/gfs2/locking/dlm/plock.c	2008-04-16 17:33:49.000000000 +0200
+++ linux-2.6/fs/gfs2/locking/dlm/plock.c	2008-04-16 17:35:46.000000000 +0200
@@ -108,7 +108,7 @@ int gdlm_plock(void *lockspace, struct l
 	if (xop->callback == NULL)
 		wait_event(recv_wq, (op->done != 0));
 	else
-		return -EINPROGRESS;
+		return FILE_LOCK_DEFERRED;
 
 	spin_lock(&ops_lock);
 	if (!list_empty(&op->list)) {
Index: linux-2.6/fs/lockd/svclock.c
===================================================================
--- linux-2.6.orig/fs/lockd/svclock.c	2008-04-16 17:33:49.000000000 +0200
+++ linux-2.6/fs/lockd/svclock.c	2008-04-16 17:35:46.000000000 +0200
@@ -423,8 +423,8 @@ nlmsvc_lock(struct svc_rqst *rqstp, stru
 			goto out;
 		case -EAGAIN:
 			ret = nlm_lck_denied;
-			break;
-		case -EINPROGRESS:
+			goto out;
+		case FILE_LOCK_DEFERRED:
 			if (wait)
 				break;
 			/* Filesystem lock operation is in progress
@@ -439,10 +439,6 @@ nlmsvc_lock(struct svc_rqst *rqstp, stru
 			goto out;
 	}
 
-	ret = nlm_lck_denied;
-	if (!wait)
-		goto out;
-
 	ret = nlm_lck_blocked;
 
 	/* Append to list of blocked */
@@ -520,7 +516,7 @@ nlmsvc_testlock(struct svc_rqst *rqstp, 
 	}
 
 	error = vfs_test_lock(file->f_file, &lock->fl);
-	if (error == -EINPROGRESS) {
+	if (error == FILE_LOCK_DEFERRED) {
 		ret = nlmsvc_defer_lock_rqst(rqstp, block);
 		goto out;
 	}
@@ -744,8 +740,7 @@ nlmsvc_grant_blocked(struct nlm_block *b
 	switch (error) {
 	case 0:
 		break;
-	case -EAGAIN:
-	case -EINPROGRESS:
+	case FILE_LOCK_DEFERRED:
 		dprintk("lockd: lock still blocked error %d\n", error);
 		nlmsvc_insert_block(block, NLM_NEVER);
 		nlmsvc_release_block(block);
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h	2008-04-16 17:33:49.000000000 +0200
+++ linux-2.6/include/linux/fs.h	2008-04-16 18:17:12.000000000 +0200
@@ -837,6 +837,12 @@ extern spinlock_t files_lock;
 #define FL_SLEEP	128	/* A blocking lock */
 
 /*
+ * Special return value from posix_lock_file() and vfs_lock_file() for
+ * asynchronous locking.
+ */
+#define FILE_LOCK_DEFERRED 1
+
+/*
  * The POSIX file lock owner is determined by
  * the "struct files_struct" in the thread group
  * (or NULL for no owner - BSD locks).



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