After dividing up a proposed write into subrequests, netfslib sets
NETFS_RREQ_ALL_QUEUED to indicate to the collector that it can move on to
the final cleanup once it has emptied the subrequest queues.

Now, whilst the collector will normally end up running at least once after
this bit is set just because it takes a while to process all the write
subrequests before the collector runs out of subrequests, there exists the
possibility that the issuing thread will be forced to sleep and the
collector thread will clean up all the subrequests before ALL_QUEUED gets
set.

In such a case, the collector thread will not get triggered again and will
never clear NETFS_RREQ_IN_PROGRESS thus leaving a request uncompleted and
causing a potential futute hang.

Fix this by scheduling the write collector if all the subrequest queues are
empty (and thus no writes pending issuance).

Note that we'd do this ideally before queuing the subrequest, but in the
case of buffered writeback, at least, we can't find out that we've run out
of folios until after we've called writeback_iter() and it has returned
NULL - at which point we might not actually have any subrequests still
under construction.

Fixes: 288ace2f57c9 ("netfs: New writeback implementation")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/write_issue.c |   42 +++++++++++++++++++++++++++---------------
 1 file changed, 27 insertions(+), 15 deletions(-)

diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index 6293f547e4c3..bf6d507578e5 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -508,6 +508,30 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	return 0;
 }
 
+/*
+ * End the issuing of writes, letting the collector know we're done.
+ */
+static void netfs_end_issue_write(struct netfs_io_request *wreq)
+{
+	bool needs_poke = true;
+
+	smp_wmb(); /* Write subreq lists before ALL_QUEUED. */
+	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
+
+	for (int s = 0; s < NR_IO_STREAMS; s++) {
+		struct netfs_io_stream *stream = &wreq->io_streams[s];
+
+		if (!stream->active)
+			continue;
+		if (!list_empty(&stream->subrequests))
+			needs_poke = false;
+		netfs_issue_write(wreq, stream);
+	}
+
+	if (needs_poke)
+		netfs_wake_write_collector(wreq, false);
+}
+
 /*
  * Write some of the pending data back to the server
  */
@@ -559,10 +583,7 @@ int netfs_writepages(struct address_space *mapping,
 			break;
 	} while ((folio = writeback_iter(mapping, wbc, folio, &error)));
 
-	for (int s = 0; s < NR_IO_STREAMS; s++)
-		netfs_issue_write(wreq, &wreq->io_streams[s]);
-	smp_wmb(); /* Write lists before ALL_QUEUED. */
-	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
+	netfs_end_issue_write(wreq);
 
 	mutex_unlock(&ictx->wb_lock);
 
@@ -650,10 +671,7 @@ int netfs_end_writethrough(struct netfs_io_request *wreq, struct writeback_contr
 	if (writethrough_cache)
 		netfs_write_folio(wreq, wbc, writethrough_cache);
 
-	netfs_issue_write(wreq, &wreq->io_streams[0]);
-	netfs_issue_write(wreq, &wreq->io_streams[1]);
-	smp_wmb(); /* Write lists before ALL_QUEUED. */
-	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
+	netfs_end_issue_write(wreq);
 
 	mutex_unlock(&ictx->wb_lock);
 
@@ -699,13 +717,7 @@ int netfs_unbuffered_write(struct netfs_io_request *wreq, bool may_wait, size_t
 			break;
 	}
 
-	netfs_issue_write(wreq, upload);
-
-	smp_wmb(); /* Write lists before ALL_QUEUED. */
-	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
-	if (list_empty(&upload->subrequests))
-		netfs_wake_write_collector(wreq, false);
-
+	netfs_end_issue_write(wreq);
 	_leave(" = %d", error);
 	return error;
 }

On Wed, 02 Oct 2024 15:45:50 +0100, David Howells wrote:
> After dividing up a proposed write into subrequests, netfslib sets
> NETFS_RREQ_ALL_QUEUED to indicate to the collector that it can move on to
> the final cleanup once it has emptied the subrequest queues.
> 
> Now, whilst the collector will normally end up running at least once after
> this bit is set just because it takes a while to process all the write
> subrequests before the collector runs out of subrequests, there exists the
> possibility that the issuing thread will be forced to sleep and the
> collector thread will clean up all the subrequests before ALL_QUEUED gets
> set.
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] netfs: Fix missing wakeup after issuing writes
      https://git.kernel.org/vfs/vfs/c/1ca4169c391c