Messages in this thread | | | Subject | Re: [patch] Re: spin_unlock optimization(i386) | Date | Mon, 22 Nov 1999 09:54:36 +0000 (GMT) | From | Alan Cox <> |
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> An rmb() will prevent reads that occur before it from being > speculatively executed after it, and vice versa. rmb() expands to asm > volatile ("lock; addl $0,0(%esp)" : : : "memory"). Being a locked > operation it serves as a processor barrier for both reads and writes; > being in cache, it is fast on a PPro. The "memory" constraint makes it > a compiler memory barrier; register operations can still be reordered > around it in theory. That's ok.
Unfortunately the lock addl turns into as big an overhead as the rest of the spinlock clearing
> rmb() then "movl $0,%0".
We do seem to have some cases where we have implicit synchronization points before an unlock where a movl would work (ie would could use some kind of __spin_unlock(). Several drivers end outl(value, lp->base+offset); spin_unlock
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