Patch "tracing: Have branch tracer use recursive field of task struct" has been added to the 3.14-stable tree

Patch "tracing: Have branch tracer use recursive field of task struct" has been added to the 3.14-stable tree

[gregkh]

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 3.14-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-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 6224beb12e190ff11f3c7d4bf50cb2922878f600 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
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)" <rostedt@goodmis.org>

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.

Link: http://lkml.kernel.org/r/20150630141803.GA28071@wfg-t540p.sh.intel.com

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -422,6 +422,7 @@ 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
--- a/kernel/trace/trace_branch.c
+++ b/kernel/trace/trace_branch.c
@@ -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 rostedt@goodmis.org are

queue-3.14/tracing-have-branch-tracer-use-recursive-field-of-task-struct.patch
queue-3.14/tracing-filter-do-not-allow-infix-to-exceed-end-of-string.patch
queue-3.14/tracing-filter-do-not-warn-on-operand-count-going-below-zero.patch