linux-kernel - [PATCH 1/2] tracing: Fix cpu buffers unavailable due to 'record

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Date:   Fri, 4 Aug 2023 20:45:48 +0800
From:   Zheng Yejian <zhengyejian1@...wei.com>
To:     <rostedt@...dmis.org>, <mhiramat@...nel.org>,
        <vnagarnaik@...gle.com>, <shuah@...nel.org>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>
Subject: [PATCH 1/2] tracing: Fix cpu buffers unavailable due to 'record_disabled' messed

Trace ring buffer can no longer record anything after executing like:
  # cd /sys/kernel/tracing
  # cat tracing_cpumask
  fff
  # echo 0 > tracing_cpumask
  # echo 1 > snapshot
  # echo fff > tracing_cpumask
  # echo "hello world" > trace_marker
  -bash: echo: write error: Bad file descriptor

The root cause is that:
  1. After `echo 0 > tracing_cpumask`, 'record_disabled' of cpu buffers
     in 'tr->array_buffer.buffer' became 1 (see tracing_set_cpumask());
  2. After `echo 1 > snapshot`, 'tr->array_buffer.buffer' is swapped
     with 'tr->max_buffer.buffer', then the 'record_disabled' became 0
     (see update_max_tr());
  3. After `echo fff > tracing_cpumask`, the 'record_disabled' become -1;
Then array_buffer and max_buffer are both unavailable due to value of
'record_disabled' is not 0.

To fix it, enable or disable both array_buffer and max_buffer at the same
time in tracing_set_cpumask().

Fixes: 71babb2705e2 ("tracing: change CPU ring buffer state from tracing_cpumask")
Signed-off-by: Zheng Yejian <zhengyejian1@...wei.com>
---
 kernel/trace/trace.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b8870078ef58..691ec4d4cd19 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5277,11 +5277,13 @@ int tracing_set_cpumask(struct trace_array *tr,
 				!cpumask_test_cpu(cpu, tracing_cpumask_new)) {
 			atomic_inc(&per_cpu_ptr(tr->array_buffer.data, cpu)->disabled);
 			ring_buffer_record_disable_cpu(tr->array_buffer.buffer, cpu);
+			ring_buffer_record_disable_cpu(tr->max_buffer.buffer, cpu);
 		}
 		if (!cpumask_test_cpu(cpu, tr->tracing_cpumask) &&
 				cpumask_test_cpu(cpu, tracing_cpumask_new)) {
 			atomic_dec(&per_cpu_ptr(tr->array_buffer.data, cpu)->disabled);
 			ring_buffer_record_enable_cpu(tr->array_buffer.buffer, cpu);
+			ring_buffer_record_enable_cpu(tr->max_buffer.buffer, cpu);
 		}
 	}
 	arch_spin_unlock(&tr->max_lock);
-- 
2.25.1