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Date:	Sat, 24 Aug 2013 06:50:45 -0400
From:	Peter Hurley <>
To:	Andy Lutomirski <>
CC:	"" <>
Subject: Re: Finding out who's holding a lock?

On 08/22/2013 08:52 PM, Andy Lutomirski wrote:
> My program is occasionally seeing slow page faults.  latencytop says
> they're slow because they're waiting for read access to mmap_sem, but
> latencytop isn't showing any other thread in the process blocking.
> Is there any straightforward way to find out who called down_write on
> mmap_sem when down_read is slow?

Build with CONFIG_LOCKDEP (Kernel Debugging -> Lock Debugging ->
"Lock Debugging: prove locking correctness").

When you experience the slowdown, sysrq + d (displays all currently
held locks). sysrq can also be programmatically triggered with
"echo d > /proc/sysrq-trigger" or equivalent.

Peter Hurley

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