From 4c742ffda878607edf081a8d19effac5dcd983cd Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Wed, 23 Oct 2019 12:57:47 -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 --- libringbuffer/frontend_internal.h | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/libringbuffer/frontend_internal.h b/libringbuffer/frontend_internal.h index 1a9c2d4b..7233ed6d 100644 --- a/libringbuffer/frontend_internal.h +++ b/libringbuffer/frontend_internal.h @@ -205,6 +205,15 @@ void lib_ring_buffer_reserve_push_reader(struct lttng_ust_lib_ring_buffer *buf, consumed_new) != consumed_old)); } +/* + * Move consumed position to the beginning of subbuffer in which the + * 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 lttng_ust_lib_ring_buffer *buf, struct lttng_ust_shm_handle *handle) @@ -221,12 +230,10 @@ void lib_ring_buffer_clear_reader(struct lttng_ust_lib_ring_buffer *buf, do { offset = v_read(config, &buf->offset); consumed_old = uatomic_read(&buf->consumed); - if (caa_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 (caa_unlikely(uatomic_cmpxchg(&buf->consumed, consumed_old, consumed_new) != consumed_old)); } -- 2.34.1