From: Andrea Righi andrea.righi@canonical.com

In the slow path of __rb_reserve_next() a nested event(s) can happen between evaluating the timestamp delta of the current event and updating write_stamp via local_cmpxchg(); in this case the delta is not valid anymore and it should be set to 0 (same timestamp as the interrupting event), since the event that we are currently processing is not the last event in the buffer.

Link: https://lkml.kernel.org/r/X8IVJcp1gRE+FJCJ@xps-13-7390

Cc: Ingo Molnar mingo@redhat.com Cc: Masami Hiramatsu mhiramat@kernel.org Cc: stable@vger.kernel.org Link: https://lwn.net/Articles/831207 Fixes: a389d86f7fd0 ("ring-buffer: Have nested events still record running time stamp") Signed-off-by: Andrea Righi andrea.righi@canonical.com Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/ring_buffer.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index bccaf88d3706..35d91b20d47a 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3287,11 +3287,11 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, ts = rb_time_stamp(cpu_buffer->buffer); barrier(); /*E*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) && - info->after < ts) { + info->after < ts && + rb_time_cmpxchg(&cpu_buffer->write_stamp, + info->after, ts)) { /* Nothing came after this event between C and E */ info->delta = ts - info->after; - (void)rb_time_cmpxchg(&cpu_buffer->write_stamp, - info->after, ts); info->ts = ts; } else { /*