Patch "tracing: Have branch tracer use recursive field of task struct" has been added to the 3.10-stable tree

This is a note to let you know that I've just added the patch titled

tracing: Have branch tracer use recursive field of task struct

to the 3.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tracing-have-branch-tracer-use-recursive-field-of-task-struct.patch
and it can be found in the queue-3.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Steven Rostedt (Red Hat)" <[email protected]>
Date: Tue, 7 Jul 2015 15:05:03 -0400
Subject: tracing: Have branch tracer use recursive field of task struct

From: "Steven Rostedt (Red Hat)" <[email protected]>

commit 6224beb12e190ff11f3c7d4bf50cb2922878f600 upstream.

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.

---
 kernel/trace/trace.h        |    1 +
 kernel/trace/trace_branch.c |   17 ++++++++++-------
 2 files changed, 11 insertions(+), 7 deletions(-)

@@ -428,6 +428,7 @@ 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

@@ -37,9 +37,12 @@ probe_likely_condition(struct ftrace_bra
 	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

@@ -50,10 +53,10 @@ probe_likely_condition(struct ftrace_bra
 	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();

@@ -82,8 +85,8 @@ probe_likely_condition(struct ftrace_bra
 		__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

Patches currently in stable-queue which might be from [email protected] are

queue-3.10/tracing-have-branch-tracer-use-recursive-field-of-task-struct.patch queue-3.10/tracing-filter-do-not-allow-infix-to-exceed-end-of-string.patch queue-3.10/tracing-filter-do-not-warn-on-operand-count-going-below-zero.patch