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Subject[PATCH 4.9 34/54] ftrace: Move RCU is watching check after recursion check
Date
From: Steven Rostedt (VMware) <rostedt@goodmis.org>

commit b40341fad6cc2daa195f8090fd3348f18fff640a upstream.

The first thing that the ftrace function callback helper functions should do
is to check for recursion. Peter Zijlstra found that when
"rcu_is_watching()" had its notrace removed, it caused perf function tracing
to crash. This is because the call of rcu_is_watching() is tested before
function recursion is checked and and if it is traced, it will cause an
infinite recursion loop.

rcu_is_watching() should still stay notrace, but to prevent this should
never had crashed in the first place. The recursion prevention must be the
first thing done in callback functions.

Link: https://lore.kernel.org/r/20200929112541.GM2628@hirez.programming.kicks-ass.net

Cc: stable@vger.kernel.org
Cc: Paul McKenney <paulmck@kernel.org>
Fixes: c68c0fa293417 ("ftrace: Have ftrace_ops_get_func() handle RCU and PER_CPU flags too")
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/trace/ftrace.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5334,17 +5334,15 @@ static void ftrace_ops_assist_func(unsig
{
int bit;

- if ((op->flags & FTRACE_OPS_FL_RCU) && !rcu_is_watching())
- return;
-
bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
if (bit < 0)
return;

preempt_disable_notrace();

- if (!(op->flags & FTRACE_OPS_FL_PER_CPU) ||
- !ftrace_function_local_disabled(op)) {
+ if ((!(op->flags & FTRACE_OPS_FL_RCU) || rcu_is_watching()) &&
+ (!(op->flags & FTRACE_OPS_FL_PER_CPU) ||
+ !ftrace_function_local_disabled(op))) {
op->func(ip, parent_ip, op, regs);
}


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