3.16.80-rc1 review patch. If anyone has any objections, please let me know.
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From: Kevin Hao haokexin@gmail.com
commit 5cbf2fff3bba8d3c6a4d47c1754de1cf57e2b01f upstream.
In the current code, we use the atomic_cmpxchg() to serialize the output of the dump_stack(), but this implementation suffers the thundering herd problem. We have observed such kind of livelock on a Marvell cn96xx board(24 cpus) when heavily using the dump_stack() in a kprobe handler. Actually we can let the competitors to wait for the releasing of the lock before jumping to atomic_cmpxchg(). This will definitely mitigate the thundering herd problem. Thanks Linus for the suggestion.
[akpm@linux-foundation.org: fix comment] Link: http://lkml.kernel.org/r/20191030031637.6025-1-haokexin@gmail.com Fixes: b58d977432c8 ("dump_stack: serialize the output from dump_stack()") Signed-off-by: Kevin Hao haokexin@gmail.com Suggested-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Ben Hutchings ben@decadent.org.uk --- lib/dump_stack.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
--- a/lib/dump_stack.c +++ b/lib/dump_stack.c @@ -44,7 +44,12 @@ retry: was_locked = 1; } else { local_irq_restore(flags); - cpu_relax(); + /* + * Wait for the lock to release before jumping to + * atomic_cmpxchg() in order to mitigate the thundering herd + * problem. + */ + do { cpu_relax(); } while (atomic_read(&dump_lock) != -1); goto retry; }