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Date: Fri, 5 Aug 2011 16:23:38 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Q: cat /proc/mdstat blocking?
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On Fri, Aug 05, 2011 at 02:02:49PM +0200, Ulrich Windl wrote:
> Hi,
> 
> I thought that /proc/mdstat just displays the current state of the MD RAIDs, not querying them, thus never blocking. Unfortunately I could observer a "cat /proc/mdstat" hanging for several seconds when one of the RAID1 legs was unavailable and multipathd was queueing requests to the leg.
> 
> Practically this makes monitoring of the other RAIDs difficult when one bad device is enough to block /proc/mdstat.
> 
> Now my question: Does it work as designed (i.e. as intended), or is this not as intended?

I suspect that something is holding all_mddevs_lock too long; that, or one
of the subdrivers' spinlocks.  Try to find out what it blocks on and what
holds the lock in question...