| Message ID | 20240312131952.630922155@goodmis.org (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | ring-buffer: Fix poll wakeup logic | expand |
On Tue, 12 Mar 2024 09:19:20 -0400 Steven Rostedt <rostedt@goodmis.org> wrote: > 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 ] Oh, so `[ buffer percent ] > shortest_full` does not work because if this happens in this order, shortest_full may be 0. > > 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. > Looks good to me. Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> Thank you, > 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 > >
On Wed, 13 Mar 2024 00:22:10 +0900 Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote: > On Tue, 12 Mar 2024 09:19:20 -0400 > Steven Rostedt <rostedt@goodmis.org> wrote: > > > 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 ] > > Oh, so `[ buffer percent ] > shortest_full` does not work because > if this happens in this order, shortest_full may be 0. Exactly! > > > > > cpu_buffer->shortest_full = full; > > > > [ IRQ work ] > > if (rbwork->wakeup_full) { > > cpu_buffer->shortest_full = 0; And here shortest_full gets set back to zero! (But that's not the bug). > > wakeup poll waiters; > > [woken] > > if ([ buffer percent ] > full) > > break; > > rbwork->full_waiters_pending = true; The bug is setting full_waiters_pending before updating the shortest_full. > > 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. > > > > Looks good to me. > > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> Thanks! I'm running it through my tests and when they finish, I'll be posting the for-linus patches. -- Steve
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;