linux-kernel - Re: rwlock_t unfairness and tasklist

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Message-ID: <20130109174922.GA31211@redhat.com>
Date:	Wed, 9 Jan 2013 18:49:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Salman Qazi <sqazi@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: rwlock_t unfairness and tasklist_lock

On 01/08, Michel Lespinasse wrote:
>
> Like others before me, I have discovered how easy it is to DOS a
> system by abusing the rwlock_t unfairness and causing the
> tasklist_lock read side to be continuously held

Yes. Plus it has perfomance problems.

It should die. We still need the global lock to protect, say,
init_task.tasks list, but otherwise we need the per-process locking.

> - Would there be any fundamental objection to implementing a fair
> rwlock_t and dealing with the reentrancy issues in tasklist_lock ? My
> proposal there would be along the lines of:

I don't really understand your proposal in details, but until we kill
tasklist_lock, perhaps it makes sense to implement something simple, say,
write-biased rwlock and add "int task_struct->tasklist_read_lock_counter"
to avoid the read-write-read deadlock.

Oleg.

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