Patch "ring-buffer: Do not set shortest_full when full target is hit" has been added to the 6.8-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    ring-buffer: Do not set shortest_full when full target is hit

to the 6.8-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-do-not-set-shortest_full-when-full-targe.patch
and it can be found in the queue-6.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 2fbd6b14cc7e1a3e180158c5f10d9776a2105d17
Author: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Date:   Tue Mar 12 11:56:41 2024 -0400

    ring-buffer: Do not set shortest_full when full target is hit
    
    [ Upstream commit 761d9473e27f0c8782895013a3e7b52a37c8bcfc ]
    
    The rb_watermark_hit() checks if the amount of data in the ring buffer is
    above the percentage level passed in by the "full" variable. If it is, it
    returns true.
    
    But it also sets the "shortest_full" field of the cpu_buffer that informs
    writers that it needs to call the irq_work if the amount of data on the
    ring buffer is above the requested amount.
    
    The rb_watermark_hit() always sets the shortest_full even if the amount in
    the ring buffer is what it wants. As it is not going to wait, because it
    has what it wants, there's no reason to set shortest_full.
    
    Link: https://lore.kernel.org/linux-trace-kernel/20240312115641.6aa8ba08@xxxxxxxxxxxxxxxxxx
    
    Cc: stable@xxxxxxxxxxxxxxx
    Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
    Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
    Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
    Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index aa332ace108b1..6ffbccb9bcf00 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -834,9 +834,10 @@ static bool rb_watermark_hit(struct trace_buffer *buffer, int cpu, int full)
 		pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page;
 		ret = !pagebusy && full_hit(buffer, cpu, full);
 
-		if (!cpu_buffer->shortest_full ||
-		    cpu_buffer->shortest_full > full)
-			cpu_buffer->shortest_full = full;
+		if (!ret && (!cpu_buffer->shortest_full ||
+			     cpu_buffer->shortest_full > full)) {
+		    cpu_buffer->shortest_full = full;
+		}
 		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 	}
 	return ret;