tracing: Have branch tracer use recursive field of task struct (6224beb1) · Commits · 戴 / test

Fengguang Wu's tests triggered a bug in the branch tracer's start up test when CONFIG_DEBUG_PREEMPT set. This was because that config adds some debug logic in the per cpu field, which calls back into the branch tracer. The branch tracer has its own recursive checks, but uses a per cpu variable to implement it. If retrieving the per cpu variable calls back into the branch tracer, you can see how things will break. Instead of using a per cpu variable, use the trace_recursion field of the current task struct. Simply set a bit when entering the branch tracing and clear it when leaving. If the bit is set on entry, just don't do the tracing. There's also the case with lockdep, as the local_irq_save() called before the recursion can also trigger code that can call back into the function. Changing that to a raw_local_irq_save() will protect that as well. This prevents the recursion and the inevitable crash that follows. Link: http://lkml.kernel.org/r/20150630141803.GA28071@wfg-t540p.sh.intel.com Cc: stable@vger.kernel.org # 3.10+ Reported-by:

Fengguang Wu <fengguang.wu@intel.com> Tested-by:

Fengguang Wu <fengguang.wu@intel.com> Signed-off-by:

Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -444,6 +444,7 @@ enum {

		TRACE_CONTROL_BIT,

		TRACE_BRANCH_BIT,
		/*
		* Abuse of the trace_recursion.
		* As we need a way to maintain state if we are tracing the function

kernel/trace/trace_branch.c

+10 −7

Original line number	Diff line number	Diff line
		@@ -36,9 +36,12 @@ probe_likely_condition(struct ftrace_branch_data *f, int val, int expect)
		struct trace_branch *entry;
		struct ring_buffer *buffer;
		unsigned long flags;
		int cpu, pc;
		int pc;
		const char *p;

		if (current->trace_recursion & TRACE_BRANCH_BIT)
		return;

		/*
		* I would love to save just the ftrace_likely_data pointer, but
		* this code can also be used by modules. Ugly things can happen
		@@ -49,10 +52,10 @@ probe_likely_condition(struct ftrace_branch_data *f, int val, int expect)
		if (unlikely(!tr))
		return;

		local_irq_save(flags);
		cpu = raw_smp_processor_id();
		data = per_cpu_ptr(tr->trace_buffer.data, cpu);
		if (atomic_inc_return(&data->disabled) != 1)
		raw_local_irq_save(flags);
		current->trace_recursion \|= TRACE_BRANCH_BIT;
		data = this_cpu_ptr(tr->trace_buffer.data);
		if (atomic_read(&data->disabled))
		goto out;

		pc = preempt_count();
		@@ -81,8 +84,8 @@ probe_likely_condition(struct ftrace_branch_data *f, int val, int expect)
		__buffer_unlock_commit(buffer, event);

		out:
		atomic_dec(&data->disabled);
		local_irq_restore(flags);
		current->trace_recursion &= ~TRACE_BRANCH_BIT;
		raw_local_irq_restore(flags);
		}

		static inline

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