netfs: fix reference leak

Commit 20d72b00ca81 ("netfs: Fix the request's work item to not
require a ref") modified netfs_alloc_request() to initialize the
reference counter to 2 instead of 1.  The rationale was that the
requet's "work" would release the second reference after completion
(via netfs_{read,write}_collection_worker()).  That works most of the
time if all goes well.

However, it leaks this additional reference if the request is released
before the I/O operation has been submitted: the error code path only
decrements the reference counter once and the work item will never be
queued because there will never be a completion.

This has caused outages of our whole server cluster today because
tasks were blocked in netfs_wait_for_outstanding_io(), leading to
deadlocks in Ceph (another bug that I will address soon in another
patch).  This was caused by a netfs_pgpriv2_begin_copy_to_cache() call
which failed in fscache_begin_write_operation().  The leaked
netfs_io_request was never completed, leaving `netfs_inode.io_count`
with a positive value forever.

All of this is super-fragile code.  Finding out which code paths will
lead to an eventual completion and which do not is hard to see:

- Some functions like netfs_create_write_req() allocate a request, but
  will never submit any I/O.

- netfs_unbuffered_read_iter_locked() calls netfs_unbuffered_read()
  and then netfs_put_request(); however, netfs_unbuffered_read() can
  also fail early before submitting the I/O request, therefore another
  netfs_put_request() call must be added there.

A rule of thumb is that functions that return a `netfs_io_request` do
not submit I/O, and all of their callers must be checked.

For my taste, the whole netfs code needs an overhaul to make reference
counting easier to understand and less fragile & obscure.  But to fix
this bug here and now and produce a patch that is adequate for a
stable backport, I tried a minimal approach that quickly frees the
request object upon early failure.

I decided against adding a second netfs_put_request() each time
because that would cause code duplication which obscures the code
further.  Instead, I added the function netfs_put_failed_request()
which frees such a failed request synchronously under the assumption
that the reference count is exactly 2 (as initially set by
netfs_alloc_request() and never touched), verified by a
WARN_ON_ONCE().  It then deinitializes the request object (without
going through the "cleanup_work" indirection) and frees the allocation
(with RCU protection to protect against concurrent access by
netfs_requests_seq_start()).

All code paths that fail early have been changed to call
netfs_put_failed_request() instead of netfs_put_request().
Additionally, I have added a netfs_put_request() call to
netfs_unbuffered_read() as explained above because the
netfs_put_failed_request() approach does not work there.

Fixes: 20d72b00ca81 ("netfs: Fix the request's work item to not require a ref")
Signed-off-by: Max Kellermann <[email protected]>
Signed-off-by: David Howells <[email protected]>
cc: Paulo Alcantara <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
Signed-off-by: Christian Brauner <[email protected]>

1 files changed, 27 insertions, 3 deletions

diff --git a/fs/netfs/objects.c b/fs/netfs/objects.c
index e8c99738b5bb..40a1c7d6f6e0 100644
--- a/fs/netfs/objects.c
+++ b/fs/netfs/objects.c
@@ -116,10 +116,8 @@ static void netfs_free_request_rcu(struct rcu_head *rcu)
 	netfs_stat_d(&netfs_n_rh_rreq);
 }
 
-static void netfs_free_request(struct work_struct *work)
+static void netfs_deinit_request(struct netfs_io_request *rreq)
 {
-	struct netfs_io_request *rreq =
-		container_of(work, struct netfs_io_request, cleanup_work);
 	struct netfs_inode *ictx = netfs_inode(rreq->inode);
 	unsigned int i;
 
@@ -149,6 +147,14 @@ static void netfs_free_request(struct work_struct *work)
 
 	if (atomic_dec_and_test(&ictx->io_count))
 		wake_up_var(&ictx->io_count);
+}
+
+static void netfs_free_request(struct work_struct *work)
+{
+	struct netfs_io_request *rreq =
+		container_of(work, struct netfs_io_request, cleanup_work);
+
+	netfs_deinit_request(rreq);
 	call_rcu(&rreq->rcu, netfs_free_request_rcu);
 }
 
@@ -168,6 +174,24 @@ void netfs_put_request(struct netfs_io_request *rreq, enum netfs_rreq_ref_trace
 }
 
 /*
+ * Free a request (synchronously) that was just allocated but has
+ * failed before it could be submitted.
+ */
+void netfs_put_failed_request(struct netfs_io_request *rreq)
+{
+	int r = refcount_read(&rreq->ref);
+
+	/* new requests have two references (see
+	 * netfs_alloc_request(), and this function is only allowed on
+	 * new request objects
+	 */
+	WARN_ON_ONCE(r != 2);
+
+	trace_netfs_rreq_ref(rreq->debug_id, r, netfs_rreq_trace_put_failed);
+	netfs_free_request(&rreq->cleanup_work);
+}
+
+/*
  * Allocate and partially initialise an I/O request structure.
  */
 struct netfs_io_subrequest *netfs_alloc_subrequest(struct netfs_io_request *rreq)