Re: [PATCH] starfire reads irq before pci_enable_device.

Manfred Spraul
2001-02-10T14:48:54Z

Hi Jes,

I read through your acenic driver and noticed that you replaced
spinlocks with bitops.

Is that a good idea? I always avoid bitops and replace them with
spinlocks:

* On uniprocessor they are obviously slower.
* on SMP i386 spin_lock() / spin_unlock() is faster than
test_and_set_bit()/clear_bit(): the spinlock operations have a
direction, and thus no memory barrier is required in spin_unlock,
Intel's default memory ordering is sufficient. clear_bit() doesn't know
that it will be used to end a protected area, thus it needs a full
memory barrier.

* on ia64 spinlocks are probably faster, and it seems that clear_bit()
instead of spin_unlock() might even cause races:
spin_unlock() needs a 'release' memory barrier, but clear_bit() contains
an 'acquire' memory barrier.

I only see 2 advantages for bitops:
* you can avoid disabling local interrupts in hard_tx_xmit() or other
bottom half handlers, but often you only need the disabled interrupts
for a few instructions.
* you won't spin - but spinning should be rare, or you can use
spin_trylock().

--
Manfred