fs/nfs/read: fix double-unlock bug in nfs_return_empty_folio()

Sometimes, when a file was read while it was being truncated by
another NFS client, the kernel could deadlock because folio_unlock()
was called twice, and the second call would XOR back the `PG_locked`
flag.

Most of the time (depending on the timing of the truncation), nobody
notices the problem because folio_unlock() gets called three times,
which flips `PG_locked` back off:

 1. vfs_read, nfs_read_folio, ... nfs_read_add_folio,
    nfs_return_empty_folio
 2. vfs_read, nfs_read_folio, ... netfs_read_collection,
    netfs_unlock_abandoned_read_pages
 3. vfs_read, ... nfs_do_read_folio, nfs_read_add_folio,
    nfs_return_empty_folio

The problem is that nfs_read_add_folio() is not supposed to unlock the
folio if fscache is enabled, and a nfs_netfs_folio_unlock() check is
missing in nfs_return_empty_folio().

Rarely this leads to a warning in netfs_read_collection():

 ------------[ cut here ]------------
 R=0000031c: folio 10 is not locked
 WARNING: CPU: 0 PID: 29 at fs/netfs/read_collect.c:133 netfs_read_collection+0x7c0/0xf00
 [...]
 Workqueue: events_unbound netfs_read_collection_worker
 RIP: 0010:netfs_read_collection+0x7c0/0xf00
 [...]
 Call Trace:
  <TASK>
  netfs_read_collection_worker+0x67/0x80
  process_one_work+0x12e/0x2c0
  worker_thread+0x295/0x3a0

Most of the time, however, processes just get stuck forever in
folio_wait_bit_common(), waiting for `PG_locked` to disappear, which
never happens because nobody is really holding the folio lock.

Fixes: 000dbe0 ("NFS: Convert buffered read paths to use netfs when fscache is enabled")
Cc: [email protected]
Signed-off-by: Max Kellermann <[email protected]>
Reviewed-by: Dave Wysochanski <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>

Author: MaxKellermann

fs/nfs/read.c

@@ -56,7 +56,8 @@ static int nfs_return_empty_folio(struct folio *folio)
 {
 	folio_zero_segment(folio, 0, folio_size(folio));
 	folio_mark_uptodate(folio);
-	folio_unlock(folio);
+	if (nfs_netfs_folio_unlock(folio))
+		folio_unlock(folio);
 	return 0;
 }