Patch "pNFS: Fix a deadlock when coalescing writes and returning the layout" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

pNFS: Fix a deadlock when coalescing writes and returning the layout

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pnfs-fix-a-deadlock-when-coalescing-writes-and-returning-the-layout.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Trond Myklebust <[email protected]>
Date: Mon, 1 May 2017 17:06:56 -0400
Subject: pNFS: Fix a deadlock when coalescing writes and returning the layout

From: Trond Myklebust <[email protected]>

[ Upstream commit 61f454e30c18a28924e96be12592c0d5e24bcc81 ]

Consider the following deadlock:

Process P1 Process P2 Process P3
========== ========== ==========
lock_page(page)

lseg = pnfs_update_layout(inode)

lo = NFS_I(inode)->layout
pnfs_error_mark_layout_for_return(lo)

lock_page(page)

lseg = pnfs_update_layout(inode)

In this scenario,
- P1 has declared the layout to be in error, but P2 holds a reference to a layout segment on that inode, so the layoutreturn is deferred.
- P2 is waiting for a page lock held by P3.
- P3 is asking for a new layout segment, but is blocked waiting for the layoutreturn.

The fix is to ensure that pnfs_error_mark_layout_for_return() does not set the NFS_LAYOUT_RETURN flag, which blocks P3. Instead, we allow the latter to call LAYOUTGET so that it can make progress and unblock P2.

---
 fs/nfs/pnfs.c |    2 --
 1 file changed, 2 deletions(-)

@@ -1953,8 +1953,6 @@ void pnfs_error_mark_layout_for_return(s
 
 	spin_lock(&inode->i_lock);
 	pnfs_set_plh_return_info(lo, range.iomode, 0);
-	/* Block LAYOUTGET */
-	set_bit(NFS_LAYOUT_RETURN, &lo->plh_flags);
 	/*
 	 * mark all matching lsegs so that we are sure to have no live
 	 * segments at hand when sending layoutreturn. See pnfs_put_lseg()

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/pnfs-fix-use-after-free-issues-in-pnfs_do_read.patch queue-4.9/pnfs-fix-a-deadlock-when-coalescing-writes-and-returning-the-layout.patch queue-4.9/nfs-fix-missing-pg_cleanup-after-nfs_pageio_cond_complete.patch