netfs, 9p: Fix race between umount and async request completion

There's a problem in 9p's interaction with netfslib whereby a crash occurs
because the 9p_fid structs get forcibly destroyed during client teardown
(without paying attention to their refcounts) before netfslib has finished
with them.  However, it's not a simple case of deferring the clunking that
p9_fid_put() does as that requires the p9_client record to still be
present.

The problem is that netfslib has to unlock pages and clear the IN_PROGRESS
flag before destroying the objects involved - including the fid - and, in
any case, nothing checks to see if writeback completed barring looking at
the page flags.

Fix this by keeping a count of outstanding I/O requests (of any type) and
waiting for it to quiesce during inode eviction.

Reported-by: [email protected]
Link: https://lore.kernel.org/all/[email protected]/
Reported-by: [email protected]
Link: https://lore.kernel.org/all/[email protected]/
Reported-by: [email protected]
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: David Howells <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Tested-by: [email protected]
Reviewed-by: Dominique Martinet <[email protected]>
cc: Eric Van Hensbergen <[email protected]>
cc: Latchesar Ionkov <[email protected]>
cc: Christian Schoenebeck <[email protected]>
cc: Jeff Layton <[email protected]>
cc: Steve French <[email protected]>
cc: Hillf Danton <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
Reported-and-tested-by: [email protected]
Signed-off-by: Christian Brauner <[email protected]>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/netfs/objects.c b/fs/netfs/objects.c
index c90d482b1650..f4a642727479 100644
--- a/fs/netfs/objects.c
+++ b/fs/netfs/objects.c
@@ -72,6 +72,7 @@ struct netfs_io_request *netfs_alloc_request(struct address_space *mapping,
 		}
 	}
 
+	atomic_inc(&ctx->io_count);
 	trace_netfs_rreq_ref(rreq->debug_id, 1, netfs_rreq_trace_new);
 	netfs_proc_add_rreq(rreq);
 	netfs_stat(&netfs_n_rh_rreq);
@@ -124,6 +125,7 @@ static void netfs_free_request(struct work_struct *work)
 {
 	struct netfs_io_request *rreq =
 		container_of(work, struct netfs_io_request, work);
+	struct netfs_inode *ictx = netfs_inode(rreq->inode);
 	unsigned int i;
 
 	trace_netfs_rreq(rreq, netfs_rreq_trace_free);
@@ -142,6 +144,9 @@ static void netfs_free_request(struct work_struct *work)
 		}
 		kvfree(rreq->direct_bv);
 	}
+
+	if (atomic_dec_and_test(&ictx->io_count))
+		wake_up_var(&ictx->io_count);
 	call_rcu(&rreq->rcu, netfs_free_request_rcu);
 }