* [PATCH 1/2] ring-buffer: Fix full_waiters_pending in poll
2024-03-12 11:54 [PATCH 0/2] ring-buffer: Fix poll wakeup logic Steven Rostedt
@ 2024-03-12 11:54 ` Steven Rostedt
2024-03-12 11:54 ` [PATCH 2/2] ring-buffer: Reuse rb_watermark_hit() for the poll logic Steven Rostedt
1 sibling, 0 replies; 3+ messages in thread
From: Steven Rostedt @ 2024-03-12 11:54 UTC (permalink / raw)
To: linux-kernel, linux-trace-kernel
Cc: Masami Hiramatsu, Mark Rutland, Mathieu Desnoyers, Andrew Morton,
stable
From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
If a reader of the ring buffer is doing a poll, and waiting for the ring
buffer to hit a specific watermark, there could be a case where it gets
into an infinite ping-pong loop.
The poll code has:
rbwork->full_waiters_pending = true;
if (!cpu_buffer->shortest_full ||
cpu_buffer->shortest_full > full)
cpu_buffer->shortest_full = full;
The writer will see full_waiters_pending and check if the ring buffer is
filled over the percentage of the shortest_full value. If it is, it calls
an irq_work to wake up all the waiters.
But the code could get into a circular loop:
CPU 0 CPU 1
----- -----
[ Poll ]
[ shortest_full = 0 ]
rbwork->full_waiters_pending = true;
if (rbwork->full_waiters_pending &&
[ buffer percent ] > shortest_full) {
rbwork->wakeup_full = true;
[ queue_irqwork ]
cpu_buffer->shortest_full = full;
[ IRQ work ]
if (rbwork->wakeup_full) {
cpu_buffer->shortest_full = 0;
wakeup poll waiters;
[woken]
if ([ buffer percent ] > full)
break;
rbwork->full_waiters_pending = true;
if (rbwork->full_waiters_pending &&
[ buffer percent ] > shortest_full) {
rbwork->wakeup_full = true;
[ queue_irqwork ]
cpu_buffer->shortest_full = full;
[ IRQ work ]
if (rbwork->wakeup_full) {
cpu_buffer->shortest_full = 0;
wakeup poll waiters;
[woken]
[ Wash, rinse, repeat! ]
In the poll, the shortest_full needs to be set before the
full_pending_waiters, as once that is set, the writer will compare the
current shortest_full (which is incorrect) to decide to call the irq_work,
which will reset the shortest_full (expecting the readers to update it).
Also move the setting of full_waiters_pending after the check if the ring
buffer has the required percentage filled. There's no reason to tell the
writer to wake up waiters if there are no waiters.
Cc: stable@vger.kernel.org
Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index aa332ace108b..adfe603a769b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -964,16 +964,32 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
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, &rbwork->waiters, poll_table);
- rbwork->waiters_pending = true;
+ if (full_hit(buffer, cpu, full))
+ return EPOLLIN | EPOLLRDNORM;
+ /*
+ * Only allow full_waiters_pending update to be seen after
+ * the shortest_full is set. If the writer sees the
+ * full_waiters_pending flag set, it will compare the
+ * amount in the ring buffer to shortest_full. If the amount
+ * in the ring buffer is greater than the shortest_full
+ * percent, it will call the irq_work handler to wake up
+ * this list. The irq_handler will reset shortest_full
+ * back to zero. That's done under the reader_lock, but
+ * the below smp_mb() makes sure that the update to
+ * full_waiters_pending doesn't leak up into the above.
+ */
+ smp_mb();
+ rbwork->full_waiters_pending = true;
+ return 0;
}
+ poll_wait(filp, &rbwork->waiters, poll_table);
+ rbwork->waiters_pending = true;
+
/*
* There's a tight race between setting the waiters_pending and
* checking if the ring buffer is empty. Once the waiters_pending bit
@@ -989,9 +1005,6 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
*/
smp_mb();
- if (full)
- return full_hit(buffer, cpu, full) ? EPOLLIN | EPOLLRDNORM : 0;
-
if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) ||
(cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))
return EPOLLIN | EPOLLRDNORM;
--
2.43.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* [PATCH 2/2] ring-buffer: Reuse rb_watermark_hit() for the poll logic
2024-03-12 11:54 [PATCH 0/2] ring-buffer: Fix poll wakeup logic Steven Rostedt
2024-03-12 11:54 ` [PATCH 1/2] ring-buffer: Fix full_waiters_pending in poll Steven Rostedt
@ 2024-03-12 11:54 ` Steven Rostedt
1 sibling, 0 replies; 3+ messages in thread
From: Steven Rostedt @ 2024-03-12 11:54 UTC (permalink / raw)
To: linux-kernel, linux-trace-kernel
Cc: Masami Hiramatsu, Mark Rutland, Mathieu Desnoyers, Andrew Morton
From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
The check for knowing if the poll should wait or not is basically the
exact same logic as rb_watermark_hit(). The only difference is that
rb_watermark_hit() also handles the !full case. But for the full case, the
logic is the same. Just call that instead of duplicating the code in
ring_buffer_poll_wait().
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 19 +++++++------------
1 file changed, 7 insertions(+), 12 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index adfe603a769b..6ef763f57c66 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -963,21 +963,16 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
poll_wait(filp, &rbwork->full_waiters, poll_table);
- raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
- if (!cpu_buffer->shortest_full ||
- cpu_buffer->shortest_full > full)
- cpu_buffer->shortest_full = full;
- raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
- if (full_hit(buffer, cpu, full))
+ if (rb_watermark_hit(buffer, cpu, full))
return EPOLLIN | EPOLLRDNORM;
/*
* Only allow full_waiters_pending update to be seen after
- * the shortest_full is set. If the writer sees the
- * full_waiters_pending flag set, it will compare the
- * amount in the ring buffer to shortest_full. If the amount
- * in the ring buffer is greater than the shortest_full
- * percent, it will call the irq_work handler to wake up
- * this list. The irq_handler will reset shortest_full
+ * the shortest_full is set (in rb_watermark_hit). If the
+ * writer sees the full_waiters_pending flag set, it will
+ * compare the amount in the ring buffer to shortest_full.
+ * If the amount in the ring buffer is greater than the
+ * shortest_full percent, it will call the irq_work handler
+ * to wake up this list. The irq_handler will reset shortest_full
* back to zero. That's done under the reader_lock, but
* the below smp_mb() makes sure that the update to
* full_waiters_pending doesn't leak up into the above.
--
2.43.0
^ permalink raw reply related [flat|nested] 3+ messages in thread