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authorSteven Rostedt (Red Hat) <rostedt@goodmis.org>2014-10-02 16:51:18 -0400
committerSteven Rostedt <rostedt@goodmis.org>2014-10-02 16:51:18 -0400
commit24607f114fd14f2f37e3e0cb3d47bce96e81e848 (patch)
tree889fca9c84d5972667a8ac18bfef4d7cb96e61c9
parentfe82dcec644244676d55a1384c958d5f67979adb (diff)
ring-buffer: Fix infinite spin in reading buffer
Commit 651e22f2701b "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 651e22f2701b 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: 651e22f2701b "ring-buffer: Always reset iterator to reader page" Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
-rw-r--r--kernel/trace/ring_buffer.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b38fb2b9e237..2d75c94ae87d 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;

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