From patchwork Tue Mar 12 13:19:20 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Steven Rostedt X-Patchwork-Id: 13589992 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AFE5F6997E; Tue, 12 Mar 2024 13:17:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710249466; cv=none; b=Gp1/EnvxFFvlREGs/oAyN9IufrVAKCEnkkzk1AqvUtpTu5CG8jeo2uMQXc2j0N1bbX9ppDpOdjhu4X+eIuRAlfflvlRj5nIEiXvYxxLJqNGPVVchnJEdWzIlJfP1TEsGFlQ4elQ2zI4tDt7DVX5jPHgwkXAlN6slDGS90C8I7xg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710249466; c=relaxed/simple; bh=AA3vT42uXJUTEwQhKXY70OTOE0y5XchQ4pdvFf+D2J4=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=ahtqkF8ODKclxngUPIYTsVlOo3jmThWqJPcWVV/3q8a+doMvlt3xuRQCSf5136pia9gQzAQzYAxoIH0NExa+xNYMWO854nUm92dly3DEaH4KTK/rGM0gnpxDiYB3obI6luQd2v99bzWAJRqiYA465/xyTfXtMlas01U6xye6sXM= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4F4C3C43390; Tue, 12 Mar 2024 13:17:46 +0000 (UTC) Received: from rostedt by gandalf with local (Exim 4.97) (envelope-from ) id 1rk22q-00000001vJn-3HFz; Tue, 12 Mar 2024 09:19:52 -0400 Message-ID: <20240312131952.630922155@goodmis.org> User-Agent: quilt/0.67 Date: Tue, 12 Mar 2024 09:19:20 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Cc: Masami Hiramatsu , Mark Rutland , Mathieu Desnoyers , Andrew Morton , stable@vger.kernel.org Subject: [PATCH v2 1/2] ring-buffer: Fix full_waiters_pending in poll References: <20240312131919.314231457@goodmis.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: "Steven Rostedt (Google)" 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) Reviewed-by: Masami Hiramatsu (Google) --- 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;