This is a note to let you know that I've just added the patch titled
ring-buffer: Allow splice to read previous partially read pages
to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ring-buffer-allow-splice-to-read-previous-partially-read-pages.patch
and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From fa8f4a89736b654125fb254b0db753ac68a5fced Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
Date: Tue, 27 Sep 2022 14:43:17 -0400
Subject: ring-buffer: Allow splice to read previous partially read pages
From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
commit fa8f4a89736b654125fb254b0db753ac68a5fced upstream.
If a page is partially read, and then the splice system call is run
against the ring buffer, it will always fail to read, no matter how much
is in the ring buffer. That's because the code path for a partial read of
the page does will fail if the "full" flag is set.
The splice system call wants full pages, so if the read of the ring buffer
is not yet full, it should return zero, and the splice will block. But if
a previous read was done, where the beginning has been consumed, it should
still be given to the splice caller if the rest of the page has been
written to.
This caused the splice command to never consume data in this scenario, and
let the ring buffer just fill up and lose events.
Link: https://lkml.kernel.org/r/20220927144317.46be6b80@xxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8789a9e7df6bf ("ring-buffer: read page interface")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/ring_buffer.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4623,7 +4623,15 @@ int ring_buffer_read_page(struct ring_bu
unsigned int pos = 0;
unsigned int size;
- if (full)
+ /*
+ * If a full page is expected, this can still be returned
+ * if there's been a previous partial read and the
+ * rest of the page can be read and the commit page is off
+ * the reader page.
+ */
+ if (full &&
+ (!read || (len < (commit - read)) ||
+ cpu_buffer->reader_page == cpu_buffer->commit_page))
goto out_unlock;
if (len > (commit - read))
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-4.9/ring-buffer-fix-race-between-reset-page-and-reading-page.patch
queue-4.9/ring-buffer-allow-splice-to-read-previous-partially-read-pages.patch
queue-4.9/ring-buffer-check-pending-waiters-when-doing-wake-ups-as-well.patch
