On 06/17/2016 11:45 AM, Will Deacon wrote:
On Fri, Jun 17, 2016 at 11:26:41AM -0400, Waiman Long wrote:On 06/16/2016 08:48 PM, Boqun Feng wrote:On Thu, Jun 16, 2016 at 05:35:54PM -0400, Waiman Long wrote:If you look into the actual code: next = xchg_release(&node->next, NULL); if (next) { WRITE_ONCE(next->locked, 1); return; } There is a control dependency that WRITE_ONCE() won't happen untilBut a control dependency only orders LOAD->STORE pairs, right? And here the control dependency orders the LOAD part of xchg_release() and the WRITE_ONCE(). Along with the fact that RELEASE only orders the STORE part of xchg with the memory operations preceding the STORE part, so for the following code: WRTIE_ONCE(x,1); next = xchg_release(&node->next, NULL); if (next) WRITE_ONCE(next->locked, 1); such a reordering is allowed to happen on ARM64v8 next = ldxr [&node->next] // LOAD part of xchg_release() if (next) WRITE_ONCE(next->locked, 1); WRITE_ONCE(x,1); stlxr NULL [&node->next] // STORE part of xchg_releae() Am I missing your point here?My understanding of the release barrier is that both prior LOADs and STOREs can't move after the barrier. If WRITE_ONCE(x, 1) can move to below as shown above, it is not a real release barrier and we may need to change the barrier code.You seem to be missing the point. {READ,WRITE}_ONCE accesses appearing in program order after a release are not externally ordered with respect to the release unless they access the same location. This is illustrated by Boqun's example, which shows two WRITE_ONCE accesses being reordered before a store-release forming the write component of an xchg_release. In both cases, WRITE_ONCE(x, 1) remains ordered before the store-release. Will
I am sorry that I misread the mail. I am not used to treating xchg as two separate instructions. Yes, it is a problem. In that case, we have to either keep the xchg() function as it is or use smp_store_release(&next->locked, 1). So which one is a better alternative for ARM or PPC?
Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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