Re: [linux-usb-devel] Re: [PATCH 00/10] usb-serial: Switches fromspin lock to atomic_t.

[Alan Stern]

On Wed, 7 Dec 2005, Arjan van de Ven wrote:

> > On the other hand, Oliver needs to be careful about claiming too much.  In 
> > general atomic_t operations _are_ superior to the spinlock approach.
> 
> No they're not. Both are just about equally expensive cpu wise,
> sometimes the atomic_t ones are a bit more expensive (like on parisc
> architecture). But on x86 in either case it's a locked cycle, which is
> just expensive no matter which side you flip the coin... 

You're overgeneralizing.

Sure, a locked cycle has a certain expense.  But it's a lot less than the 
expense of a contested spinlock.  On the other hand, many times UP systems 
can eliminate spinlocks entirely.  There are lots of variables and many 
possible tradeoffs.

Alan Stern

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