Messages in this thread Patch in this message | | | Date | Tue, 23 Dec 2008 11:32:25 -0500 | From | Steven Rostedt <> | Subject | [PATCH 2/2] ring-buffer: prevent false positive warning |
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From: Steven Rostedt <srostedt@redhat.com>
Impact: fix to false WARN_ON
If an interrupt goes off after the setting of the local variable tail_page and before incrementing the write index of that page, the interrupt could push the commit forward to the next page.
Later a check is made to see if interrupts pushed the buffer around the entire ring buffer by comparing the next page to the last commited page. This can produce a false positive if the interrupt had pushed the commit page forward as stated above.
Thanks to Jiaying Zhang for finding this race.
Reported-by: Jiaying Zhang <jiayingz@google.com> Signed-off-by: Steven Rostedt <srostedt@redhat.com> --- kernel/trace/ring_buffer.c | 7 +++++-- 1 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index f64aee5..71202ac 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -892,12 +892,15 @@ static struct ring_buffer_event * __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, unsigned type, unsigned long length, u64 *ts) { - struct buffer_page *tail_page, *head_page, *reader_page; + struct buffer_page *tail_page, *head_page, *reader_page, *commit_page; unsigned long tail, write; struct ring_buffer *buffer = cpu_buffer->buffer; struct ring_buffer_event *event; unsigned long flags; + commit_page = cpu_buffer->commit_page; + /* we just need to protect against interrupts */ + barrier(); tail_page = cpu_buffer->tail_page; write = local_add_return(length, &tail_page->write); tail = write - length; @@ -921,7 +924,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, * it all the way around the buffer, bail, and warn * about it. */ - if (unlikely(next_page == cpu_buffer->commit_page)) { + if (unlikely(next_page == commit_page)) { WARN_ON_ONCE(1); goto out_unlock; } -- 1.5.6.5 --
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