virtiofs: schedule blocking async replies in separate worker

In virtiofs (unlike in regular fuse) processing of async replies is
serialized.  This can result in a deadlock in rare corner cases when
there's a circular dependency between the completion of two or more async
replies.

Such a deadlock can be reproduced with xfstests:generic/503 if TEST_DIR ==
SCRATCH_MNT (which is a misconfiguration):

 - Process A is waiting for page lock in worker thread context and blocked
   (virtio_fs_requests_done_work()).
 - Process B is holding page lock and waiting for pending writes to
   finish (fuse_wait_on_page_writeback()).
 - Write requests are waiting in virtqueue and can't complete because
   worker thread is blocked on page lock (process A).

Fix this by creating a unique work_struct for each async reply that can
block (O_DIRECT read).

Fixes: a62a8ef9d97d ("virtio-fs: add virtiofs filesystem")
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 9d67b830fb7a..d400b71b98d5 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -712,6 +712,7 @@ static ssize_t fuse_async_req_send(struct fuse_conn *fc,
 	spin_unlock(&io->lock);
 
 	ia->ap.args.end = fuse_aio_complete_req;
+	ia->ap.args.may_block = io->should_dirty;
 	err = fuse_simple_background(fc, &ia->ap.args, GFP_KERNEL);
 	if (err)
 		fuse_aio_complete_req(fc, &ia->ap.args, err);