linux-kernel - Re: semaphore: lockless fastpath using atomic_{inc,dec}

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Date:	Wed, 9 Jul 2008 13:05:04 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Bruno Santos <bsantos@...it.pt>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: semaphore: lockless fastpath using atomic_{inc,dec}_return

> >   
>  From your words if we do:
> 
> spin_lock()
> val = --foo;
> spin_unlock();
> 
> Has the same cost than:
> 
> val = atomic_dec_return(&foo);

yes


> How come I have the same number of atomic ops?
> 
> Let's consider the fast case scenario (semaphore is unlocked for the 
> 'down' and has no waiters for 'up') in x86:
> - with the spinlock only approach we have 2 atomic ops, xadd for
> lock, inc for unlock. The unlock doesn't come for free in x86 after
> all.

the unlock does not use an atomic operation.


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