Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 24 Dec 2010 10:26:05 -0800 | Subject | Re: [RFC][PATCH 05/17] x86: Optimize arch_spin_unlock_wait() |
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On Fri, Dec 24, 2010 at 4:23 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > Only wait for the current holder to release the lock. > > spin_unlock_wait() can only be about the current holder, since > completion of this function is inherently racy with new contenders. > Therefore, there is no reason to wait until the lock is completely > unlocked.
Is there really any reason for this patch? I'd rather keep the simpler and more straightforward code unless you have actual numbers.
> +static inline void __ticket_spin_unlock_wait(arch_spinlock_t *lock) > +{ > + int tmp = ACCESS_ONCE(lock->slock); > + > + if (!(((tmp >> TICKET_SHIFT) ^ tmp) & TICKET_MASK)) > + return; /* not locked */ > + > + /* wait until the current lock holder goes away */ > + while ((lock->slock & TICKET_MASK) == (tmp & TICKET_MASK)) > + cpu_relax(); > }
Also, the above is just ugly. You've lost the ACCESS_ONCE() on the lock access, and it's using another model of masking than the regular one. Both of which may be intentional (maybe you are _trying_ to get the compiler to just load the low bytes and avoid the 'and'), but the whole open-coding of the logic - twice, and with different looking masking - just makes my skin itch.
Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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