Re: [PATCH] Profile likely/unlikely macros

[Nick Piggin <nickpiggin@yahoo.com.au>]

Daniel Walker wrote:

>On Tue, 2006-04-25 at 20:11 +1000, Nick Piggin wrote:
>
>
>>I guess it is so it can be used in NMIs and interrupts without turning
>>interrupts off (so is somewhat lightweight).
>>
>>But please Daniel, just use spinlocks and trylock. This is buggy because
>>it doesn't get the required release consistency correct.
>>
>
>
>To use spinlock we would need to used the __raw_ types . As Hua
>explained all of the vanilla spinlock calls use the unlikely macro. The
>result is that we end up using atomic operations. So using them directly
>seems like the cleanest method .
>

Ah, I see. Then you should be OK with either your current scheme, or
Andrew's suggestion, so long as you have a memory barrier before the
unlock (eg. smp_mb__before_clear_bit()).

>
>I'm not exactly sure what you mean by "release consistency" ?
>

Without a barrier, the stores to the linked list may be visible to another
CPU after the store that unlocks the atomic_t. Ie. the critical section can
leak out of the lock.

--

Send instant messages to your online friends http://au.messenger.yahoo.com