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Re: kern/50375: layerfs (nullfs) locking problem leading to livelock
The following reply was made to PR kern/50375; it has been noted by GNATS.
From: "J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>
Cc: Konrad Schroder <perseant%hhhh.org@localhost>
Subject: Re: kern/50375: layerfs (nullfs) locking problem leading to livelock
Date: Fri, 30 Oct 2015 09:55:12 +0100
> A quick read of hannken's analysis makes me think that the basic problem
> here is that the two imposed locking orders (parent directory before
> subdirectory, and upper before lower) are in direct conflict when a
> directory is null-mounted onto a subdirectory of itself.
Sure, loops would deadlock very fast.
For the deadlock described here it is sufficient to mount a path on
two different null mounts like:
dev on /data type ffs
/nulla on /data type null
/nullb on /data type null
The deadlock arises from one thread cleaning a node from /nulla while
another thread tries to vget a node on /nullb and both nodes point to
the same node on /data.
J. Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig (Germany)
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