Davide Libenzi a écrit :

quoted text
> On Fri, 26 Jun 2009, Eric Dumazet wrote:

>

>> Davide Libenzi a écrit :

>>> On Thu, 25 Jun 2009, Oleg Nesterov wrote:

>>>

>>>> Can't really comment this patch, except this all looks reasonable to me.

>>>> Add more CCs.

>>> While this can work, IMO it'd be cleaner to have the smp_mb() moved from

>>> fs/select.c to the ->poll() function.

>>> Having a barrier that matches another one in another susbsystem, because

>>> of the special locking logic of such subsystem, is not too shiny IMHO.

>>>

>> Yes but barrier is necessary only if add_wait_queue() was actually called, and __pollwait()

>> does this call.

>>

>> Adding a plain smp_mb() in tcp_poll() for example would slowdown select()/poll() with NULL

>> timeout.

>

> Do you think of it as good design adding an MB on a subsystem, because of

> the special locking logic of another one?

> The (eventual) slowdown, IMO can be argued sideways, by saying that

> non-socket users will pay the price for their polls.

> 

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().
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