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ring-buffer: prevent false positive warning 

Impact: eliminate false WARN_ON message

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>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Steven Rostedt 2008-12-23 11:32:25 -05:00 committed by Ingo Molnar
parent a8ccf1d6f6
commit 98db8df777
1 changed files with 5 additions and 2 deletions
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7
kernel/trace/ring_buffer.c
View File

@ -962,12 +962,15 @@ 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;
@ -993,7 +996,7 @@ __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;
}