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charset="UTF-8" When the reader page is also the tail, swap should not be permitted or it would insert the tail behind the head, breaking the integrity of the ring buffer. While __rb_get_reader_page() does correctly prevent this situation, the simple_ring_buffer implementation misses it. Once the reader catches up with the tail, make simple_ring_buffer_swap_reader_page() return 0. The remote reader now can expect an unchanged reader. When it is the case, there's nothing more to read. Also, when the simple_ring_buffer writer moves the tail to a new page, it resets the new tail counters and more specifically page::commit. As moving the tail unblocks page swap, order the page reset before the tail_page update. This ensures the reader can't swap the reader page with the new tail and observe a stale commit value. Cc: stable@vger.kernel.org Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes") Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer") Signed-off-by: Vincent Donnefort --- kernel/trace/ring_buffer.c | 3 ++- kernel/trace/simple_ring_buffer.c | 15 ++++++++++++++- 2 files changed, 16 insertions(+), 2 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 1fd28a72e6ee..3c7aa982d43e 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -5848,7 +5848,8 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer) new_reader = cpu_buffer->subbuf_ids[cpu_buffer->meta_page->reader.id]; - WARN_ON_ONCE(prev_reader == new_reader); + if (prev_reader == new_reader) + return NULL; prev_head = new_reader; /* New reader was also the previous head */ new_head = prev_head; diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c index abfaf9bee12e..d5c84af01925 100644 --- a/kernel/trace/simple_ring_buffer.c +++ b/kernel/trace/simple_ring_buffer.c @@ -139,6 +139,13 @@ int simple_ring_buffer_swap_reader_page(struct simple_rb_per_cpu *cpu_buffer) reader = cpu_buffer->reader_page; + /* + * The reader is already the tail page. See simple_rb_move_tail() for + * memory ordering. + */ + if (reader == smp_load_acquire(&cpu_buffer->tail_page)) + return 0; + do { /* Run after the writer to find the head */ ret = simple_rb_find_head(cpu_buffer); @@ -196,7 +203,13 @@ static struct simple_buffer_page *simple_rb_move_tail(struct simple_rb_per_cpu * } simple_bpage_reset(new_tail); - cpu_buffer->tail_page = new_tail; + + /* + * Setting a new tail may unblock the swap in + * simple_ring_buffer_swap_reader_page(). Ensure the reader observes the + * page::commit reset. + */ + smp_store_release(&cpu_buffer->tail_page, new_tail); simple_rb_meta_inc(cpu_buffer->meta->pages_touched, 1); -- 2.56.0.611.g1030f0b940-goog