From 5cbf2fff3bba8d3c6a4d47c1754de1cf57e2b01f Mon Sep 17 00:00:00 2001 From: Kevin Hao Date: Tue, 5 Nov 2019 21:16:57 -0800 Subject: [PATCH] dump_stack: avoid the livelock of the dump_lock 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 Suggested-by: Linus Torvalds Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- lib/dump_stack.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/lib/dump_stack.c b/lib/dump_stack.c index 5cff72f18c4a..33ffbf308853 100644 --- a/lib/dump_stack.c +++ b/lib/dump_stack.c @@ -106,7 +106,12 @@ 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; }