[patch 12/17] dump_stack: avoid the livelock of the dump_lock

[patch 12/17] dump_stack: avoid the livelock of the dump_lock

[akpm]

From: Kevin Hao <haokexin@gmail.com>
Subject: 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 <haokexin@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/dump_stack.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/lib/dump_stack.c~dump_stack-avoid-the-livelock-of-the-dump_lock
+++ a/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;
 	}
 
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