From: Mathieu Desnoyers Date: Mon, 1 Jul 2013 21:10:52 +0000 (-0400) Subject: Fix: ring buffer: handle concurrent update in nested buffer wrap around check X-Git-Tag: v2.2.1~5 X-Git-Url: http://git.liburcu.org/?a=commitdiff_plain;h=1793b0b5fd18ce7ccb5bdab19745cffdf55756e3;hp=1793b0b5fd18ce7ccb5bdab19745cffdf55756e3;p=lttng-ust.git Fix: ring buffer: handle concurrent update in nested buffer wrap around check With stress-test loads that trigger sub-buffer switch very frequently (small 4kB sub-buffers, frequent flush) in lttng-modules, we currently observe this kind of warnings once every few minutes: [65335.896208] ring buffer relay-overwrite-mmap, cpu 5: records were lost. Caused by: [65335.896208] [ 0 buffer full, 1 nest buffer wrap-around, 0 event too big ] It appears that the check for nested buffer wrap-around does not take into account that a concurrent execution contexts (either nested for per-cpu buffers, or from another CPU or nested for global buffers) can update the commit_count value concurrently. What we really want to do with this check is to ensure that if we enter a sub-buffer that had an unbalanced reserve/commit count, assuming there is no hope that this gets rebalanced promptly, we detect this and drop the current event. However, in the case where the commit counter has been concurrently updated by another reserve or a switch, we want to retry the entire reserve operation. One way to detect this is to sample the reserve offset twice, around the commit counter read, along with the appropriate memory barriers. Therefore, we can detect if the mismatch between reserve and commit counter is actually caused by a concurrent update, which necessarily has updated the reserve counter. Signed-off-by: Mathieu Desnoyers ---