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Subject[PATCH AUTOSEL for 4.9 140/190] pNFS: Fix a deadlock when coalescing writes and returning the layout
Date
From: Trond Myklebust <trond.myklebust@primarydata.com>

[ 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.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
fs/nfs/pnfs.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
index e9a697e9d292..0e008db16b16 100644
--- a/fs/nfs/pnfs.c
+++ b/fs/nfs/pnfs.c
@@ -1953,8 +1953,6 @@ void pnfs_error_mark_layout_for_return(struct inode *inode,

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()
--
2.14.1
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