Re: [PATCH] SCSI: Fix some locking issues

[Elias Oltmanns <eo@nebensachen.de>]

Matthew Wilcox <matthew@wil.cx> wrote:
> On Wed, Jul 02, 2008 at 05:59:31PM +0200, Elias Oltmanns wrote:
>> > The reason no locks are necessary is that there's no race to mediate.
>
>> > The checks are only is it set or not ...
>> 
>> I'm not sure whether that is of any consequence. Don't get me wrong, I
>> really don't know and you may well be right. But how exactly does
>> decrementing from 2 to 1 work? Do we know for sure that there will
>> always be at least one bit set so reading that address will reliably
>> return a non zero value?
>
> The assumption we make (and it is believed to be true on all SMP systems)
> is that a write to a naturally aligned memory location that is sized <=
> sizeof(long) is atomic.  That is, a reader will get either the previous
> value or the subsequent value, not a mixture.  The RCU code relies
> heavily on this assumption.

Does that mean that where ever I have

spin_lock_irqsave(some_lock, flags);
var = some_val;
spin_unlock_irqrestore(some_lock, flags);

I could just as well discard the locking provided that
sizeof(var) <= sizeof(long)
because the assignment of some_val to var will be atomic anyway?

Regards,

Elias