Re: [PATCH] tracing: Have all levels of checks prevent recursion

On Mon, 18 Oct 2021 12:19:20 +0200
Petr Mladek [off-list ref] wrote:

quoted

quoted

-
 	bit = trace_get_context_bit() + start;
 	if (unlikely(val & (1 << bit))) {
 		/*
 		 * It could be that preempt_count has not been updated during
 		 * a switch between contexts. Allow for a single recursion.
 		 */
-		bit = TRACE_TRANSITION_BIT;
+		bit = TRACE_CTX_TRANSITION + start;  

[..]

quoted

Could we please update the comment? I mean to say if it is a race
or if we trace a function that should not get traced.

What do you think of this change?

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index 1d8cce02c3fb..24f284eb55a7 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h

@@ -168,8 +168,12 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
 	bit = trace_get_context_bit() + start;
 	if (unlikely(val & (1 << bit))) {
 		/*
-		 * It could be that preempt_count has not been updated during
-		 * a switch between contexts. Allow for a single recursion.
+		 * If an interrupt occurs during a trace, and another trace
+		 * happens in that interrupt but before the preempt_count is
+		 * updated to reflect the new interrupt context, then this
+		 * will think a recursion occurred, and the event will be dropped.
+		 * Let a single instance happen via the TRANSITION_BIT to
+		 * not drop those events.
 		 */
 		bit = TRACE_TRANSITION_BIT;
 		if (val & (1 << bit)) {