linux-kernel - Re: A possible sys

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Message-Id: <20100701053442.D59F04C33C@magilla.sf.frob.com>
Date:	Wed, 30 Jun 2010 22:34:42 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: A possible sys_wait* bug

There are several other ways than wait that pathological user processes can
arrange to do tight loops taking tasklist_lock.  So if you call that a DoS
risk then it's not solved with anything specific to the wait syscalls.

Using different rwlock behavior (at least for that lock) would be one way
to address it.  Of course, changing the contention dynamics might have
other effects that you don't want (i.e. fork/exec/exit/reap drowning out
the reader uses, cascading to some bad pattern).  I don't have an opinion
on that, and wish you good luck figuring it out.

In the long run, things have been moving to finer-grained locking and/or
less direct locking.  We can imagine tasklist_lock might go this way one
day too, removing the general bottleneck (i.e. slicing it up into different
smaller, bottlenecks).  As things stand today, it's a bit hard to see
exactly how we might get there e.g. for wait/exit with the synchronization
requirements for reparenting and all that.  But it's clear that eventually
the big system-wide bottlenecks like tasklist_lock all get reduced.  I
don't know of anything like this to expect to see any time soon, however.

Thanks,
Roland
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