[PATCH] fuse: mark DAX inode releases as blocking

[Sergio Lopez <slp@redhat.com>]

Commit 26e5c67deb2e ("fuse: fix livelock in synchronous file put from
fuseblk workers") made fputs on closing files always asynchronous.

As cleaning up DAX inodes may require issuing a number of synchronous
request for releasing the mappings, completing the release request from
the worker thread may lead to it hanging like this:

[   21.386751] Workqueue: events virtio_fs_requests_done_work
[   21.386769] Call trace:
[   21.386770]  __switch_to+0xe4/0x140
[   21.386780]  __schedule+0x294/0x72c
[   21.386787]  schedule+0x24/0x90
[   21.386794]  request_wait_answer+0x184/0x298
[   21.386799]  __fuse_simple_request+0x1f4/0x320
[   21.386805]  fuse_send_removemapping+0x80/0xa0
[   21.386810]  dmap_removemapping_list+0xac/0xfc
[   21.386814]  inode_reclaim_dmap_range.constprop.0+0xd0/0x204
[   21.386820]  fuse_dax_inode_cleanup+0x28/0x5c
[   21.386825]  fuse_evict_inode+0x120/0x190
[   21.386834]  evict+0x188/0x320
[   21.386847]  iput_final+0xb0/0x20c
[   21.386854]  iput+0xa0/0xbc
[   21.386862]  fuse_release_end+0x18/0x2c
[   21.386868]  fuse_request_end+0x9c/0x2c0
[   21.386872]  virtio_fs_request_complete+0x150/0x384
[   21.386879]  virtio_fs_requests_done_work+0x18c/0x37c
[   21.386885]  process_one_work+0x15c/0x2e8
[   21.386891]  worker_thread+0x278/0x480
[   21.386898]  kthread+0xd0/0xdc
[   21.386902]  ret_from_fork+0x10/0x20

Here, the virtio-fs worker_thread is waiting on request_wait_answer()
for a reply from the virtio-fs server that is already in the virtqueue
but will never be processed since it's that same worker thread the one
in charge of consuming the elements from the virtqueue.

To address this issue, when relesing a DAX inode mark the operation as
potentially blocking. Doing this will ensure these release requests are
processed on a different worker thread.

Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 fs/fuse/file.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 3b2a171e652f..a65c5d32a34b 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -117,6 +117,12 @@ static void fuse_file_put(struct fuse_file *ff, bool sync)
 			fuse_simple_request(ff->fm, args);
 			fuse_release_end(ff->fm, args, 0);
 		} else {
+			/*
+			 * DAX inodes may need to issue a number of synchronous
+			 * request for clearing the mappings.
+			 */
+			if (ra && ra->inode && FUSE_IS_DAX(ra->inode))
+				args->may_block = true;
 			args->end = fuse_release_end;
 			if (fuse_simple_background(ff->fm, args,
 						   GFP_KERNEL | __GFP_NOFAIL))
-- 
2.52.0