• Steven Rostedt's avatar
    tracing: Comment the use of event_mutex with trace event flags · 1eaa4787
    Steven Rostedt authored
    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: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Acked-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: Tom Zanussi <tzanussi@gmail.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
ftrace_event.h 7.06 KB