From 1eaa4787a774c4896518c81f24e8bccaa2244924 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Fri, 14 May 2010 10:19:13 -0400 Subject: tracing: Comment the use of event_mutex with trace event flags The flags variable is protected by the event_mutex when modifying, but the event_mutex is not held when reading the variable. This is due to the fact that the reads occur in critical sections where taking a mutex (or even a spinlock) is not wanted. But the two flags that exist (enable and filter_active) have the code written as such to handle the reads to not need a lock. The enable flag is used just to know if the event is enabled or not and its use is always under the event_mutex. Whether or not the event is actually enabled is really determined by the tracepoint being registered. The flag is just a way to let the code know if the tracepoint is registered. The filter_active is different. It is read without the lock. If it is set, then the event probes jump to the filter code. There can be a slight mismatch between filters available and filter_active. If the flag is set but no filters are available, the code safely jumps to a filter nop. If the flag is not set and the filters are available, then the filters are skipped. This is acceptable since filters are usually set before tracing or they are set by humans, which would not notice the slight delay that this causes. v2: Fixed typo: "cacheing" -> "caching" Reported-by: Mathieu Desnoyers Acked-by: Mathieu Desnoyers Cc: Tom Zanussi Signed-off-by: Steven Rostedt --- include/linux/ftrace_event.h | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'include/linux/ftrace_event.h') diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h index 5ac97a42950d..dc7fc646fa2e 100644 --- a/include/linux/ftrace_event.h +++ b/include/linux/ftrace_event.h @@ -169,7 +169,14 @@ struct ftrace_event_call { * bit 1: enabled * bit 2: filter_active * - * Must hold event_mutex to change. + * Changes to flags must hold the event_mutex. + * + * Note: Reads of flags do not hold the event_mutex since + * they occur in critical sections. But the way flags + * is currently used, these changes do no affect the code + * except that when a change is made, it may have a slight + * delay in propagating the changes to other CPUs due to + * caching and such. */ unsigned int flags; -- cgit v1.2.3