Commit 80fc7314 authored by Sage Weil's avatar Sage Weil
Browse files

ceph: fix mds sync() race with completing requests 



The wait_unsafe_requests() helper dropped the mdsc mutex to wait
for each request to complete, and then examined r_node to get the
next request after retaking the lock.  But the request completion
removes the request from the tree, so r_node was always undefined
at this point.  Since it's a small race, it usually led to a
valid request, but not always.  The result was an occasional
crash in rb_next() while dereferencing node->rb_left.

Fix this by clearing the rb_node when removing the request from
the request tree, and not walking off into the weeds when we
are done waiting for a request.  Since the request we waited on
will _always_ be out of the request tree, take a ref on the next
request, in the hopes that it won't be.  But if it is, it's ok:
we can start over from the beginning (and traverse over older read
requests again).

Signed-off-by: default avatarSage Weil <sage@newdream.net>
parent 916623da

fs/ceph/mds_client.c

+20 −7
Original line number Diff line number Diff line
@@ -529,6 +529,7 @@ static void __unregister_request(struct ceph_mds_client *mdsc, 
{

	dout("__unregister_request %p tid %lld\n", req, req->r_tid);

	rb_erase(&req->r_node, &mdsc->request_tree);

	RB_CLEAR_NODE(&req->r_node);

	ceph_mdsc_put_request(req);



	if (req->r_unsafe_dir) {
@@ -2682,29 +2683,41 @@ void ceph_mdsc_pre_umount(struct ceph_mds_client *mdsc) 
 */

static void wait_unsafe_requests(struct ceph_mds_client *mdsc, u64 want_tid)

{

	struct ceph_mds_request *req = NULL;

	struct ceph_mds_request *req = NULL, *nextreq;

	struct rb_node *n;



	mutex_lock(&mdsc->mutex);

	dout("wait_unsafe_requests want %lld\n", want_tid);

restart:

	req = __get_oldest_req(mdsc);

	while (req && req->r_tid <= want_tid) {

		/* find next request */

		n = rb_next(&req->r_node);

		if (n)

			nextreq = rb_entry(n, struct ceph_mds_request, r_node);

		else

			nextreq = NULL;

		if ((req->r_op & CEPH_MDS_OP_WRITE)) {

			/* write op */

			ceph_mdsc_get_request(req);

			if (nextreq)

				ceph_mdsc_get_request(nextreq);

			mutex_unlock(&mdsc->mutex);

			dout("wait_unsafe_requests  wait on %llu (want %llu)\n",

			     req->r_tid, want_tid);

			wait_for_completion(&req->r_safe_completion);

			mutex_lock(&mdsc->mutex);

			n = rb_next(&req->r_node);

			ceph_mdsc_put_request(req);

		} else {

			n = rb_next(&req->r_node);

			if (!nextreq)

				break;  /* next dne before, so we're done! */

			if (RB_EMPTY_NODE(&nextreq->r_node)) {

				/* next request was removed from tree */

				ceph_mdsc_put_request(nextreq);

				goto restart;

			}

		if (!n)

			break;

		req = rb_entry(n, struct ceph_mds_request, r_node);

			ceph_mdsc_put_request(nextreq);  /* won't go away */

		}

		req = nextreq;

	}

	mutex_unlock(&mdsc->mutex);

	dout("wait_unsafe_requests done\n");

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