From ac4a87e53febd0dfc438ce75948e57d649d3b113 Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Wed, 23 Oct 2019 12:56:59 -0400 Subject: [PATCH] lttng-clear: stop tracing required Require that tracing is stopped when buffers are cleared. Update comments and warning checks to that effect. Signed-off-by: Mathieu Desnoyers --- lib/ringbuffer/frontend_internal.h | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/lib/ringbuffer/frontend_internal.h b/lib/ringbuffer/frontend_internal.h index a1bfe558..04838614 100644 --- a/lib/ringbuffer/frontend_internal.h +++ b/lib/ringbuffer/frontend_internal.h @@ -189,7 +189,12 @@ void lib_ring_buffer_reserve_push_reader(struct lib_ring_buffer *buf, /* * Move consumed position to the beginning of subbuffer in which the - * write offset is. + * write offset is. Should only be used on ring buffers that are not + * actively being written into, because clear_reader does not take into + * account the commit counters when moving the consumed position, which + * can make concurrent trace producers or consumers observe consumed + * position further than the write offset, which breaks ring buffer + * algorithm guarantees. */ static inline void lib_ring_buffer_clear_reader(struct lib_ring_buffer *buf, @@ -201,12 +206,10 @@ void lib_ring_buffer_clear_reader(struct lib_ring_buffer *buf, do { offset = v_read(config, &buf->offset); consumed_old = atomic_long_read(&buf->consumed); - if (unlikely(subbuf_trunc(offset, chan) - - subbuf_trunc(consumed_old, chan) - > 0)) - consumed_new = subbuf_trunc(offset, chan); - else - return; + CHAN_WARN_ON(chan, (long) (subbuf_trunc(offset, chan) + - subbuf_trunc(consumed_old, chan)) + < 0); + consumed_new = subbuf_trunc(offset, chan); } while (unlikely(atomic_long_cmpxchg(&buf->consumed, consumed_old, consumed_new) != consumed_old)); } -- 2.34.1