[Devel] [PATCH RHEL7 COMMIT] fuse kio: Fix deadlock at pcs_fuse_submit() error path

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-862.14.4.vz7.72.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-862.14.4.vz7.72.11
------>
commit e95c5e36d1d905e1cdcae309110504c6dfb5d6b9
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Thu Oct 18 15:08:15 2018 +0300

    fuse kio: Fix deadlock at pcs_fuse_submit() error path
    
    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>
    Reviewed-by: Pavel Butsykin <pbutsykin 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 e30463233e8a..3940d6c255ba 100644
--- a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
+++ b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
@@ -905,7 +905,7 @@ 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);
@@ -985,7 +985,11 @@ 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:
@@ -1049,7 +1053,7 @@ 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);
 	}
 }
 
@@ -1196,7 +1200,7 @@ 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);