[Devel] [PATCH] fuse kio: Fix deadlock at pcs_fuse_submit() error path

[Kirill Tkhai]

request_end() takes fc->lock, so we in case of error we bump
into deadlock:

Call Trace:
  [<ffffffffb3bb63f5>] _raw_spin_lock+0x75/0xc0
  [<ffffffffc170871b>] spin_lock+0x18/0x1b [fuse]
  [<ffffffffc170ba63>] request_end+0x265/0x72b [fuse]
  [<ffffffffc18a1b8d>] pcs_fuse_submit+0x9fb/0xaa3 [fuse_kio_pcs]
  [<ffffffffc18a35c4>] kpcs_req_send+0x793/0xa60 [fuse_kio_pcs]
  [<ffffffffc170b6ca>] flush_bg_queue+0x14f/0x283 [fuse]
  [<ffffffffc170d4d4>] fuse_request_send_background_locked+0x50b/0x512 [fuse]
  [<ffffffffc170d844>] fuse_request_send_background+0x369/0x43f [fuse]
  [<ffffffffc173028b>] fuse_send_readpages+0x372/0x3b5 [fuse]
  [<ffffffffc1730c3c>] fuse_readpages+0x28c/0x2f0 [fuse]
  [<ffffffffb296ba58>] __do_page_cache_readahead+0x518/0x6d0

Fix this by unlocking fc->lock before request_end() call. Note,
that it may look strange to have two same lk parameters in
pcs_fuse_submit(pfc, req, lk, lk), but the current design
interprets requests submitted with locked lk as async and
we keep this logic.

Generally, I feel we need to improve design in a thing
of queueing requests and locking, but we need more
inverstigation and thinking here, so let's delay this
to next VZ update.

https://pmc.acronis.com/browse/VSTOR-16246

Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 fs/fuse/kio/pcs/pcs_fuse_kdirect.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
index b286a956a751..61415e029c45 100644
--- a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
+++ b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
@@ -883,7 +883,7 @@ static int pcs_fuse_prep_rw(struct pcs_fuse_req *r)
 	return ret;
 }
 
-static void pcs_fuse_submit(struct pcs_fuse_cluster *pfc, struct fuse_req *req, int async)
+static void pcs_fuse_submit(struct pcs_fuse_cluster *pfc, struct fuse_req *req, bool async, bool lk)
 {
 	struct pcs_fuse_req *r = pcs_req_from_fuse(req);
 	struct fuse_inode *fi = get_fuse_inode(req->io_inode);
@@ -963,7 +963,11 @@ static void pcs_fuse_submit(struct pcs_fuse_cluster *pfc, struct fuse_req *req,
 error:
 	DTRACE("do fuse_request_end req:%p op:%d err:%d\n", &r->req, r->req.in.h.opcode, r->req.out.h.error);
 
+	if (lk)
+		spin_unlock(&pfc->fc->lock);
 	request_end(pfc->fc, &r->req);
+	if (lk)
+		spin_lock(&pfc->fc->lock);
 	return;
 
 submit:
@@ -1027,7 +1031,7 @@ static void _pcs_shrink_end(struct fuse_conn *fc, struct fuse_req *req)
 
 		TRACE("resubmit %p\n", &r->req);
 		list_del_init(&ireq->list);
-		pcs_fuse_submit(pfc, &r->req, 1);
+		pcs_fuse_submit(pfc, &r->req, true, false);
 	}
 }
 
@@ -1174,7 +1178,7 @@ static int kpcs_req_send(struct fuse_conn* fc, struct fuse_req *req, bool bg, bo
 	}
 	__clear_bit(FR_PENDING, &req->flags);
 
-	pcs_fuse_submit(pfc, req, lk);
+	pcs_fuse_submit(pfc, req, lk, lk);
 	if (!bg)
 		wait_event(req->waitq,
 			   test_bit(FR_FINISHED, &req->flags) && !req->end);