Patch "ring-buffer: Fix resetting of shortest_full" has been added to the 6.7-stable tree

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

    ring-buffer: Fix resetting of shortest_full

to the 6.7-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-fix-resetting-of-shortest_full.patch
and it can be found in the queue-6.7 subdirectory.

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



commit 567c9eb438adada02d44eeb22b63d1b9adc2be35
Author: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Date:   Fri Mar 8 15:24:04 2024 -0500

    ring-buffer: Fix resetting of shortest_full
    
    [ Upstream commit 68282dd930ea38b068ce2c109d12405f40df3f93 ]
    
    The "shortest_full" variable is used to keep track of the waiter that is
    waiting for the smallest amount on the ring buffer before being woken up.
    When a tasks waits on the ring buffer, it passes in a "full" value that is
    a percentage. 0 means wake up on any data. 1-100 means wake up from 1% to
    100% full buffer.
    
    As all waiters are on the same wait queue, the wake up happens for the
    waiter with the smallest percentage.
    
    The problem is that the smallest_full on the cpu_buffer that stores the
    smallest amount doesn't get reset when all the waiters are woken up. It
    does get reset when the ring buffer is reset (echo > /sys/kernel/tracing/trace).
    
    This means that tasks may be woken up more often then when they want to
    be. Instead, have the shortest_full field get reset just before waking up
    all the tasks. If the tasks wait again, they will update the shortest_full
    before sleeping.
    
    Also add locking around setting of shortest_full in the poll logic, and
    change "work" to "rbwork" to match the variable name for rb_irq_work
    structures that are used in other places.
    
    Link: https://lore.kernel.org/linux-trace-kernel/20240308202431.948914369@xxxxxxxxxxx
    
    Cc: stable@xxxxxxxxxxxxxxx
    Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
    Cc: Mark Rutland <mark.rutland@xxxxxxx>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
    Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Cc: linke li <lilinke99@xxxxxx>
    Cc: Rabin Vincent <rabin@xxxxxx>
    Fixes: 2c2b0a78b3739 ("ring-buffer: Add percentage of ring buffer full to wake up reader")
    Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Stable-dep-of: 8145f1c35fa6 ("ring-buffer: Fix full_waiters_pending in poll")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 72cebeb0c916f..59f8eb20e5e87 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -902,8 +902,19 @@ static void rb_wake_up_waiters(struct irq_work *work)
 
 	wake_up_all(&rbwork->waiters);
 	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
+		/* Only cpu_buffer sets the above flags */
+		struct ring_buffer_per_cpu *cpu_buffer =
+			container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
+
+		/* Called from interrupt context */
+		raw_spin_lock(&cpu_buffer->reader_lock);
 		rbwork->wakeup_full = false;
 		rbwork->full_waiters_pending = false;
+
+		/* Waking up all waiters, they will reset the shortest full */
+		cpu_buffer->shortest_full = 0;
+		raw_spin_unlock(&cpu_buffer->reader_lock);
+
 		wake_up_all(&rbwork->full_waiters);
 	}
 }
@@ -1082,28 +1093,33 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
 			  struct file *filp, poll_table *poll_table, int full)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
-	struct rb_irq_work *work;
+	struct rb_irq_work *rbwork;
 
 	if (cpu == RING_BUFFER_ALL_CPUS) {
-		work = &buffer->irq_work;
+		rbwork = &buffer->irq_work;
 		full = 0;
 	} else {
 		if (!cpumask_test_cpu(cpu, buffer->cpumask))
 			return EPOLLERR;
 
 		cpu_buffer = buffer->buffers[cpu];
-		work = &cpu_buffer->irq_work;
+		rbwork = &cpu_buffer->irq_work;
 	}
 
 	if (full) {
-		poll_wait(filp, &work->full_waiters, poll_table);
-		work->full_waiters_pending = true;
+		unsigned long flags;
+
+		poll_wait(filp, &rbwork->full_waiters, poll_table);
+
+		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+		rbwork->full_waiters_pending = true;
 		if (!cpu_buffer->shortest_full ||
 		    cpu_buffer->shortest_full > full)
 			cpu_buffer->shortest_full = full;
+		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 	} else {
-		poll_wait(filp, &work->waiters, poll_table);
-		work->waiters_pending = true;
+		poll_wait(filp, &rbwork->waiters, poll_table);
+		rbwork->waiters_pending = true;
 	}
 
 	/*




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