[PATCH v2 2/2] ring-buffer: Stop remote reader update when page swap fails

[Ivan Immanuel Shaji via B4 Relay <devnull+ivanimmanuel1234.gmail.com@kernel.org>]

From: Ivan Immanuel Shaji <ivanimmanuel1234@gmail.com>

The remote swap_reader_page callback can return -EBUSY when the writer
moves the head before the remote catches it, particularly during an event
storm on a small buffer. __rb_get_reader_page_from_remote() currently
warns about that failure but continues with the unchanged reader ID and
rearranges the local page list as though the swap succeeded.

Handle the callback failure as a recoverable error. Report it with
pr_warn_ratelimited() and return NULL. Callers already handle a NULL reader
page as a failed attempt. This avoids splicing the same page as both the
previous and new reader without flooding the log under contention.

Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
Cc: stable@vger.kernel.org
Assisted-by: LLM sparse
Signed-off-by: Ivan Immanuel Shaji <ivanimmanuel1234@gmail.com>
---
 kernel/trace/ring_buffer.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 3c3ed639923d..b0963ac6fd16 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5805,8 +5805,11 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
 
 	prev_reader = cpu_buffer->subbuf_ids[cpu_buffer->meta_page->reader.id];
 
-	WARN_ON_ONCE(cpu_buffer->remote->swap_reader_page(cpu_buffer->cpu,
-							  cpu_buffer->remote->priv));
+	if (cpu_buffer->remote->swap_reader_page(cpu_buffer->cpu,
+						 cpu_buffer->remote->priv)) {
+		pr_warn_ratelimited("Remote reader page swap failed\n");
+		return NULL;
+	}
 	/* nr_pages doesn't include the reader page */
 	if (WARN_ON_ONCE(cpu_buffer->meta_page->reader.id > cpu_buffer->nr_pages))
 		return NULL;

-- 
2.53.0