[ofa-general] Incorrect atomic usage in ipath driver

[Benjamin Herrenschmidt]

Hi !

So I see this construct:

	/* There is already a thread processing this queue. */
	if (test_and_set_bit(0, &dd->ipath_rcv_pending))
		goto bail;

	.../...

done:
	clear_bit(0, &dd->ipath_rcv_pending);
	smp_mb__after_clear_bit();

So that's basically an attempt at doing a spinlock. The problem is your
barrier is wrong at the end. Better would be:


done:
	smp_mb__before_clear_bit();
	clear_bit(0, &dd->ipath_rcv_pending);


Though it's still less optimal that doing:

	if (!spin_trylock(...))
		goto bail;

	.../...

done:
	spin_unlock(...)

If you really want to stick to bitops, then you may want to look at
Nick's upcoming patches adding some bitops with appropriate lock
semantics.

Cheers,
Ben.