Messages in this thread | | | Subject | Re: [PATCH 0/7] isdn4linux: add drivers for Siemens Gigaset ISDN DECT PABX | From | Roland Dreier <> | Date | Thu, 02 Mar 2006 16:58:54 -0800 |
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> So you are saying that, for example > > spin_lock_irqsave(&cs->ev_lock, flags); > head = cs->ev_head; > tail = cs->ev_tail; > spin_unlock_irqrestore(&cs->ev_lock, flags); > > is (mutatis mutandis) actually cheaper than > > head = atomic_read(&cs->ev_head); > tail = atomic_read(&cs->ev_tail); > > ? That's interesting. I wouldn't have expected that after reading > Documentation/atomic_ops.txt and Documentation/spinlock.txt.
No, atomic_read() is cheap because it doesn't have to do a locked operation. However, operations like atomic_inc() that do need to do something special are quite expensive.
For example, on x86, each atomic_inc()/atomic_dec() is the same cost as a spin_lock(), since they all have to do some sort of "lock ; incX" or "lock ; decX". But then spin_unlock() is cheap, because it can do a simple unlocked mov.
So in other words,
spin_lock_irqsave(&lock, flags); ++head1; ++head2; spin_unlock_irqrestore(&lock, flags);
should be cheaper than
atomic_inc(&head1); atomic_inc(&head2);
On the other hand, if you use the spinlock variant, then you do incur an extra cost by requiring the lock for both reads and writes, instead of the cheap atomic_read().
But complex use of atomic_t is very hard to get right, so it's usually better to use a spinlock.
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