[PATCH v1] trace: Fix race in trace_open and buffer resize call

[Gaurav Kohli]

Below race can come, if trace_open and resize of
cpu buffer is running parallely on different cpus
CPUX                                CPUY
                                    ring_buffer_resize
                                    atomic_read(&buffer->resize_disabled)
tracing_open
tracing_reset_online_cpus
ring_buffer_reset_cpu
rb_reset_cpu
                                    rb_update_pages
                                    remove/insert pages
resetting pointer

This race can cause data abort or some times infinte loop in
rb_remove_pages and rb_insert_pages while checking pages
for sanity.

Take buffer lock to fix this.

Signed-off-by: Gaurav Kohli <gko...@codeaurora.org>
Cc: sta...@vger.kernel.org
---
Changes since v0:
  -Addressed Steven's review comments.

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 93ef0ab..15bf28b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4866,6 +4866,9 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, 
int cpu)
        if (!cpumask_test_cpu(cpu, buffer->cpumask))
                return;
 
+       /* prevent another thread from changing buffer sizes */
+       mutex_lock(&buffer->mutex);
+
        atomic_inc(&cpu_buffer->resize_disabled);
        atomic_inc(&cpu_buffer->record_disabled);
 
@@ -4876,6 +4879,8 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, 
int cpu)
 
        atomic_dec(&cpu_buffer->record_disabled);
        atomic_dec(&cpu_buffer->resize_disabled);
+
+       mutex_unlock(&buffer->mutex);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
 
@@ -4889,6 +4894,9 @@ void ring_buffer_reset_online_cpus(struct trace_buffer 
*buffer)
        struct ring_buffer_per_cpu *cpu_buffer;
        int cpu;
 
+       /* prevent another thread from changing buffer sizes */
+       mutex_lock(&buffer->mutex);
+
        for_each_online_buffer_cpu(buffer, cpu) {
                cpu_buffer = buffer->buffers[cpu];
 
@@ -4907,6 +4915,8 @@ void ring_buffer_reset_online_cpus(struct trace_buffer 
*buffer)
                atomic_dec(&cpu_buffer->record_disabled);
                atomic_dec(&cpu_buffer->resize_disabled);
        }
+
+       mutex_unlock(&buffer->mutex);
 }
 
 /**
-- 
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