tracing: Disable buffer switching when starting or stopping trace
When the trace iterator is read, tracing_start() and tracing_stop() is called to stop tracing while the iterator is processing the trace output. These functions disable both the standard buffer and the max latency buffer. But if the wakeup tracer is running, it can switch these buffers between the two disables: buffer = global_trace.buffer; if (buffer) ring_buffer_record_disable(buffer); <<<--------- swap happens here buffer = max_tr.buffer; if (buffer) ring_buffer_record_disable(buffer); What happens is that we disabled the same buffer twice. On tracing_start() we can enable the same buffer twice. All ring_buffer_record_disable() must be matched with a ring_buffer_record_enable() or the buffer can be disable permanently, or enable prematurely, and cause a bug where a reset happens while a trace is commiting. This patch protects these two by taking the ftrace_max_lock to prevent a switch from occurring. Found with Li Zefan's ftrace_stress_test. Cc: stable@kernel.org Reported-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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283740c619
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a2f8071428
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@ -950,6 +950,8 @@ void tracing_start(void)
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goto out;
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}
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/* Prevent the buffers from switching */
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arch_spin_lock(&ftrace_max_lock);
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buffer = global_trace.buffer;
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if (buffer)
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@ -959,6 +961,8 @@ void tracing_start(void)
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if (buffer)
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ring_buffer_record_enable(buffer);
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arch_spin_unlock(&ftrace_max_lock);
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ftrace_start();
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out:
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spin_unlock_irqrestore(&tracing_start_lock, flags);
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@ -980,6 +984,9 @@ void tracing_stop(void)
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if (trace_stop_count++)
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goto out;
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/* Prevent the buffers from switching */
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arch_spin_lock(&ftrace_max_lock);
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buffer = global_trace.buffer;
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if (buffer)
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ring_buffer_record_disable(buffer);
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@ -988,6 +995,8 @@ void tracing_stop(void)
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if (buffer)
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ring_buffer_record_disable(buffer);
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arch_spin_unlock(&ftrace_max_lock);
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out:
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spin_unlock_irqrestore(&tracing_start_lock, flags);
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}
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