linux-kernel - Re: [PATCH] net: fix race in the receive/select

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Date:	Thu, 25 Jun 2009 20:14:22 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Oleg Nesterov <oleg@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	Tejun Heo <htejun@...il.com>
Subject: Re: [PATCH] net: fix race in the receive/select

On Fri, 26 Jun 2009, Eric Dumazet wrote:

> I wont argue with you David, just try to correct bugs.
> 
> fs/ext4/ioctl.c line 182
> 
> 	set_current_state(TASK_INTERRUPTIBLE);
> 	add_wait_queue(&EXT4_SB(sb)->ro_wait_queue, &wait);
> 	if (timer_pending(&EXT4_SB(sb)->turn_ro_timer)) {
> 		schedule();
> 
> Another example of missing barrier after add_wait_queue()
> 
> Because add_wait_queue() misses a barrier, we have to add one after each call.
> 
> Maybe it would be safer to add barrier in add_wait_queue() itself, not in _pollwait().

Not all the code that uses add_wait_queue() does need to have the MB,
like code that does the most common pattern:

xxx_poll(...) {
	poll_wait(...);
	lock();
	flags = calc_flags(->status);
	unlock();
	return flags;
}

xxx_update(...) {
	lock();
	->status = ...;
	unlock();
	if (waitqueue_active())
		wake_up();
}

It's the code that does the lockless flags calculation in ->poll that 
might need it.
I dunno what the amount of changes are, but cross-matching MB across 
subsystems does not look nice.
IMHO that's a detail of the subsystem locking, and should be confined 
inside the subsystem itself.
No?



- Davide


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