Subject: Re: [PATCH] net: fix race in the receive/select

From: Jarek Poplawski <[email-masked]>
Date: Fri, 26 Jun 2009 05:42:30 +0000

On 26-06-2009 05:14, Davide Libenzi wrote:
> 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?

How about poll_wait_mb() and waitqueue_active_mb() (with mb and
additional check for NULL of wait_queue_head)?

Jarek P.