From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Fri, 17 Sep 2010 19:21:38 -0400
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Subject: Re: [9fans] interesting deadlock
From: Russ Cox <rsc@swtch.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
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> of mh->mount, which is nil. =C2=A0the code in question
> was a fileserver which was crashing, getting killed
> while concurrent io was being done to the fs.

i believe that.  the code is assuming that because
it found mh in the mount table, mh->mount !=3D nil.
that's only true until it releases the rlock, which it has.
if an unmount happens between the runlock and the
rlock, then mh->mount will be nil here.  it means
that nothing is mounted there anymore.  your fix
looks reasonable.

russ