netfs: Fix the handling of stream->front by removing it

The netfs_io_stream::front member is meant to point to the subrequest
currently being collected on a stream, but it isn't actually used this way
by direct write (which mostly ignores it).  However, there's a tracepoint
which looks at it.  Further, stream->front is actually redundant with
stream->subrequests.next.

Fix the potential problem in the direct code by just removing the member
and using stream->subrequests.next instead, thereby also simplifying the
code.

Fixes: a0b4c7a491 ("netfs: Fix unbuffered/DIO writes to dispatch subrequests in strict sequence")
Reported-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/4158599.1774426817@warthog.procyon.org.uk
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/netfs/buffered_read.c

@@ -171,9 +171,8 @@ static void netfs_queue_read(struct netfs_io_request *rreq,
 	spin_lock(&rreq->lock);
 	list_add_tail(&subreq->rreq_link, &stream->subrequests);
 	if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
-		stream->front = subreq;
 		if (!stream->active) {
-			stream->collected_to = stream->front->start;
+			stream->collected_to = subreq->start;
 			/* Store list pointers before active flag */
 			smp_store_release(&stream->active, true);
 		}

fs/netfs/direct_read.c

@@ -71,9 +71,8 @@ static int netfs_dispatch_unbuffered_reads(struct netfs_io_request *rreq)
 		spin_lock(&rreq->lock);
 		list_add_tail(&subreq->rreq_link, &stream->subrequests);
 		if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
-			stream->front = subreq;
 			if (!stream->active) {
-				stream->collected_to = stream->front->start;
+				stream->collected_to = subreq->start;
 				/* Store list pointers before active flag */
 				smp_store_release(&stream->active, true);
 			}

fs/netfs/direct_write.c

@@ -111,7 +111,6 @@ static int netfs_unbuffered_write(struct netfs_io_request *wreq)
 			netfs_prepare_write(wreq, stream, wreq->start + wreq->transferred);
 			subreq = stream->construct;
 			stream->construct = NULL;
-			stream->front = NULL;
 		}
 
 		/* Check if (re-)preparation failed. */

fs/netfs/read_collect.c

@@ -205,7 +205,8 @@ reassess:
 	 * in progress.  The issuer thread may be adding stuff to the tail
 	 * whilst we're doing this.
 	 */
-	front = READ_ONCE(stream->front);
+	front = list_first_entry_or_null(&stream->subrequests,
+					 struct netfs_io_subrequest, rreq_link);
 	while (front) {
 		size_t transferred;
 
@@ -301,7 +302,6 @@ reassess:
 		list_del_init(&front->rreq_link);
 		front = list_first_entry_or_null(&stream->subrequests,
 						 struct netfs_io_subrequest, rreq_link);
-		stream->front = front;
 		spin_unlock(&rreq->lock);
 		netfs_put_subrequest(remove,
 				     notes & ABANDON_SREQ ?

fs/netfs/read_single.c

@@ -107,7 +107,6 @@ static int netfs_single_dispatch_read(struct netfs_io_request *rreq)
 	spin_lock(&rreq->lock);
 	list_add_tail(&subreq->rreq_link, &stream->subrequests);
 	trace_netfs_sreq(subreq, netfs_sreq_trace_added);
-	stream->front = subreq;
 	/* Store list pointers before active flag */
 	smp_store_release(&stream->active, true);
 	spin_unlock(&rreq->lock);

fs/netfs/write_collect.c

@@ -228,7 +228,8 @@ reassess_streams:
 		if (!smp_load_acquire(&stream->active))
 			continue;
 
-		front = stream->front;
+		front = list_first_entry_or_null(&stream->subrequests,
+						 struct netfs_io_subrequest, rreq_link);
 		while (front) {
 			trace_netfs_collect_sreq(wreq, front);
 			//_debug("sreq [%x] %llx %zx/%zx",
@@ -279,7 +280,6 @@ reassess_streams:
 			list_del_init(&front->rreq_link);
 			front = list_first_entry_or_null(&stream->subrequests,
 							 struct netfs_io_subrequest, rreq_link);
-			stream->front = front;
 			spin_unlock(&wreq->lock);
 			netfs_put_subrequest(remove,
 					     notes & SAW_FAILURE ?

fs/netfs/write_issue.c

@@ -206,9 +206,8 @@ void netfs_prepare_write(struct netfs_io_request *wreq,
 	spin_lock(&wreq->lock);
 	list_add_tail(&subreq->rreq_link, &stream->subrequests);
 	if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
-		stream->front = subreq;
 		if (!stream->active) {
-			stream->collected_to = stream->front->start;
+			stream->collected_to = subreq->start;
 			/* Write list pointers before active flag */
 			smp_store_release(&stream->active, true);
 		}

include/linux/netfs.h

@@ -140,7 +140,6 @@ struct netfs_io_stream {
 	void (*issue_write)(struct netfs_io_subrequest *subreq);
 	/* Collection tracking */
 	struct list_head	subrequests;	/* Contributory I/O operations */
-	struct netfs_io_subrequest *front;	/* Op being collected */
 	unsigned long long	collected_to;	/* Position we've collected results to */
 	size_t			transferred;	/* The amount transferred from this stream */
 	unsigned short		error;		/* Aggregate error for the stream */

include/trace/events/netfs.h

@@ -740,19 +740,19 @@ TRACE_EVENT(netfs_collect_stream,
 		    __field(unsigned int,	wreq)
 		    __field(unsigned char,	stream)
 		    __field(unsigned long long,	collected_to)
-		    __field(unsigned long long,	front)
+		    __field(unsigned long long,	issued_to)
 			     ),
 
 	    TP_fast_assign(
 		    __entry->wreq	= wreq->debug_id;
 		    __entry->stream	= stream->stream_nr;
 		    __entry->collected_to = stream->collected_to;
-		    __entry->front	= stream->front ? stream->front->start : UINT_MAX;
+		    __entry->issued_to	= atomic64_read(&wreq->issued_to);
 			   ),
 
-	    TP_printk("R=%08x[%x:] cto=%llx frn=%llx",
+	    TP_printk("R=%08x[%x:] cto=%llx ito=%llx",
 		      __entry->wreq, __entry->stream,
-		      __entry->collected_to, __entry->front)
+		      __entry->collected_to, __entry->issued_to)
 	    );
 
 TRACE_EVENT(netfs_folioq,