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Subject: Re: [linux-usb-devel] Re: [PATCH 00/10] usb-serial: Switches from
	spin lock to atomic_t.
From: Arjan van de Ven <arjan@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oliver@neukum.org>, linux-usb-devel@lists.sourceforge.net,
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> 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... 

>   If 
> they weren't, atomic_t wouldn't belong in the kernel at all.

there's different usage patterns where either makes sense. 
In this case it looks just disgusting on very first sight; the atomic
are used to implement a lock, and that lock itself is then implemented
with a spinlock again. For me, again on first sight, the real solution
appears to be to use a linux primitive for the higher level lock in the
first place, instead of reimplementing <your own thing> with <another
own thing>.



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