tracing: Have branch tracer use recursive field of task struct

[ Upstream commit 6224beb12e190ff11f3c7d4bf50cb2922878f600 ]

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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 385391fb1d3bc508270fbda17233fdef1c8e36b4..5642436bf97ed3721e27025fbbc94ef04263f76a 100644 (file)
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -442,6 +442,7 @@ 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

diff --git a/kernel/trace/trace_branch.c b/kernel/trace/trace_branch.c
index 697fb9bac8f0d69d8e5a5023d5e5d10fdd3ed09c..60850b4fcb04c9796f1a2d7bfe648a9b24efe12d 100644 (file)
--- a/kernel/trace/trace_branch.c
+++ b/kernel/trace/trace_branch.c
@@ -37,9 +37,12 @@ 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
@@ -50,10 +53,10 @@ 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();
@@ -82,8 +85,8 @@ 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