Davide Libenzi a écrit :

On Fri, 26 Jun 2009, Eric Dumazet wrote:

quoted

Davide Libenzi a écrit :

quoted

On Thu, 25 Jun 2009, Oleg Nesterov wrote:

quoted

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().