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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@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;
}

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