Re: [PATCH] SCSI: Fix some locking issues

[Elias Oltmanns <eo@nebensachen.de>]

James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> On Thu, 2008-07-03 at 09:12 +0200, Elias Oltmanns wrote:
>> Matthew Wilcox <matthew@wil.cx> wrote:
[...]
>> > 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?
>
> I think you're still confused about the difference between integral
> observation of values and serialisation.
>
> Assignment is always integral.  In your example above anything observing
> the variable always sees either the previous value or the one you set it
> to, nothing else.  So, if all you're doing is setting a flag and
> clearing it (as in setting it to an integral value, not setting it via a
> bit operation) and checking it, no locking is needed.  It's when you
> start doing operations on variables that require serialisation, or the
> variables themselves are absolute indicators of events that need
> serialisation that you start having to introduce locking.

Right. Thank you very much for the clarification.

Regards,

Elias