linux-kernel - Re: [GIT PULL] notification tree

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Message-ID: <201008171045.26155.tvrtko.ursulin@sophos.com>
Date:	Tue, 17 Aug 2010 10:45:25 +0100
From:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
To:	Eric Paris <eparis@...hat.com>
CC:	Andreas Gruenbacher <agruen@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	Matt Helsley <matthltc@...ibm.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Michael Kerrisk <michael.kerrisk@...il.com>
Subject: Re: [GIT PULL] notification tree - try 37!

On Tuesday 17 Aug 2010 04:39:47 Eric Paris wrote:
> > Q: What prevents the system from going out of memory when a listener
> > decides to stop reading events or simply can't keep up?  There doesn't
> > seem to be a limit on the queue depth.  Listeners currently need
> > CAP_SYS_ADMIN, but somehow limiting the queue depth and throttling when
> > things start to go bad still sounds like a reasonable thing to do,
> > right?)
>
> It's an interesting question and obviously one that I've thought about.
> You remember when we talked previously I said the hardest part left was
> allowing non-root users to use the interface.  It gets especially
> difficult when thinking about perm-events.  I was specifically told not
> to timeout or drop those.  But when dealing with non-root users using
> perm events?   As for pure notification we can do something like inotify
> does quite easily.

Why no timeouts? It sounds like a feasible way to work around listeners which
have stopped working. (Timeout and -ETIME for example to be clear, not
allowing access).

Alternative might be to expose queue size per group (and some additional group
info) so a daemon could keep an eye on listeners which are not making progress
and act accordingly. Sometimes appropriate action would be to restart, or to
kill, or even spawn a new one. Last bit is especially useful with some FUSE
filesystems to avoid deadlocks. Otherwise listener can get a perm event for
the top level, and then another perm event is generated when FUSE opens the
underlying object and there is noone to handle it.

But this can also work together with timeouts.

Tvrtko

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