linux-kernel - [PATCH 3.16 122/136] dump_stack: avoid the livelock of the dump

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Message-ID: <lsq.1576543535.283326510@decadent.org.uk>
Date:   Tue, 17 Dec 2019 00:47:36 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Kevin Hao" <haokexin@...il.com>
Subject: [PATCH 3.16 122/136] dump_stack: avoid the livelock of the dump_lock

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Kevin Hao <haokexin@...il.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@...ux-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@...il.com>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.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;
 	}