From de5ad22dc230f6f9856bbfb60c8ad2ce569e5054 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (Red Hat)" Date: Thu, 2 Oct 2014 16:51:18 -0400 Subject: [PATCH] ring-buffer: Fix infinite spin in reading buffer Commit f12b4e5b3d65 "ring-buffer: Always reset iterator to reader page" fixed one bug but in the process caused another one. The reset is to update the header page, but that fix also changed the way the cached reads were updated. The cache reads are used to test if an iterator needs to be updated or not. A ring buffer iterator, when created, disables writes to the ring buffer but does not stop other readers or consuming reads from happening. Although all readers are synchronized via a lock, they are only synchronized when in the ring buffer functions. Those functions may be called by any number of readers. The iterator continues down when its not interrupted by a consuming reader. If a consuming read occurs, the iterator starts from the beginning of the buffer. The way the iterator sees that a consuming read has happened since its last read is by checking the reader "cache". The cache holds the last counts of the read and the reader page itself. Commit f12b4e5b3d65 changed what was saved by the cache_read when the rb_iter_reset() occurred, making the iterator never match the cache. Then if the iterator calls rb_iter_reset(), it will go into an infinite loop by checking if the cache doesn't match, doing the reset and retrying, just to see that the cache still doesn't match! Which should never happen as the reset is suppose to set the cache to the current value and there's locks that keep a consuming reader from having access to the data. Fixes: f12b4e5b3d65 "ring-buffer: Always reset iterator to reader page" Signed-off-by: Steven Rostedt --- kernel/trace/ring_buffer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index b38fb2b9e2377..2d75c94ae87d8 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3359,7 +3359,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter) iter->head = cpu_buffer->reader_page->read; iter->cache_reader_page = iter->head_page; - iter->cache_read = iter->head; + iter->cache_read = cpu_buffer->read; if (iter->head) iter->read_stamp = cpu_buffer->read_stamp; -- 2.39.5