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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=Yq5XynenixoA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=jiCTI4zE5U7BLdzWsZGv:22 a=3I1J8UUJPc9JN9BFgKH3:22 a=VwQbUJbxAAAA:8 a=pGLkceISAAAA:8 a=yPCof4ZbAAAA:8 a=qbnnC1PJtrjm2xo4jsIA:9 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMzE3MDA1OSBTYWx0ZWRfXyt4RY+Z1oXgh ZuJwK5xlSjWT/R9y3sdTZfeVMPVhjA1PG9IObCnbxi9WH8UUoGF2OPdvvlVVXicteCSZeK5fZwj YebaobYsgGl/V72LbE6XgEJ1BdLEpsV2nYBaf61Yj8jqde7KaOifdGXyN1yNJfkUcTfybAZC2La GeIrcoDdD7OwTKzpyBkek5pa2pnn9ai3VGlp+88yP9A1RSzgYGu6pLE/ppvPsooaaXX0p+i+wLL lpqtW54E5F9fAvWF+Q0oBuVPspXjQtmRoqGaZrlpemHzBJSPPUXQDQ13A9jmOGebxtG80C8ZRuN dJv3RgvL9p0hdeHZfsazv72VMKSqwl0DGoLjcXA9H4hfngJoSek5ApjAoQjGDsUa6Pl0NIGPubP Ba7SDb4hykldy2f6Lte+ZnQNe2nj00dYMCwuoZTn/ogpK7K6sAJLzE3NnU1isB7e03rN1F4yd19 YTvg9ie2ruGBJogC7Jg== X-Proofpoint-GUID: p1gNkWsm2BCB0dj_BBzufOjw1_QUIpgC X-Proofpoint-ORIG-GUID: p1gNkWsm2BCB0dj_BBzufOjw1_QUIpgC David Laight writes: > On Mon, 16 Mar 2026 15:08:07 -0700 > Ankur Arora wrote: > > ... >> However, as David Laight pointed out in this thread >> (https://lore.kernel.org/lkml/20260214113122.70627a8b@pumpkin/) >> that this would be fine so long as the polling is on memory, but would >> need some work to handle MMIO. > > I'm not sure the current code works with MMIO on arm64. > > I was looking at the osq_lock() code, it uses smp_cond_load() with 'expr' > being 'VAL || need_resched()' expecting to get woken by the IPI associated > with the preemption being requested. Yeah, osq_lock() has: if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() || vcpu_is_preempted(node_cpu(node->prev)))) This works on x86 even with a non-stub vcpu_is_preempted() because we poll on the node->locked while evaluating the conditional. Won't work on arm64 since there we are just waiting on node->locked. (And so we depend on the IPI there.) > But the arm64 code relies on 'wfe' being woken when the memory write > 'breaks' the 'ldx' for the monitored location. Agreed. > That will only work for cached addresses. Yeah I was forgetting that. LDXR won't really work with non cacheable memory. So a generic MMIO version might not be possible at all. > For osq_lock(), while an IPI will wake it up, there is also a small timing > window where the IPI can happen before the ldx and so not actually wake up it. > This is true whenever 'expr' is non-trivial. Yeah because we are checking the state of more than one address in the conditional but the waiting monitor only does one memory location at a time. If only CPUs had vectored waiting mechanisms... I think the smp_cond_load_*() in poll_idle() and in rqspinlock monitor a single address (and in the conditional) so those two should be okay. > On arm64 I think you could use explicit sev and wfe - but that will wake all > 'sleeping' cpu; and you may not want the 'thundering herd'. Wouldn't we still have the same narrow window where the CPU disregards the IPI? -- ankur