[PATCH v2] dump_stack: Avoid the livelock of the dump_lock

[Kevin Hao <haokexin@gmail.com>]

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.

Fixes: b58d977432c8 ("dump_stack: serialize the output from dump_stack()")
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Hao <haokexin@gmail.com>
---
v2:
 1. Use the method suggested by Linus.
 2. Since this fix seems pretty simple, maybe it deserve to be
    backported to stable kernel. So CC the stable.

 lib/dump_stack.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/dump_stack.c b/lib/dump_stack.c
index 5cff72f18c4a..0062dfcafd63 100644
--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -106,7 +106,11 @@ asmlinkage __visible void dump_stack(void)
 		was_locked = 1;
 	} else {
 		local_irq_restore(flags);
-		cpu_relax();
+		/*
+		 * Wait 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;
 	}
 
-- 
2.14.4