On Mon, Jun 04, 2018 at 10:42:43AM +0100, Will Deacon wrote: > Hi Russell, > > On Fri, Jun 01, 2018 at 12:00:16PM +0100, Russell King wrote: > > Executing loops such as: > > > > while (1) > > cpu_relax(); > > > > with interrupts disabled results in a livelock of the entire system, > > as other CPUs are prevented making progress. This is most noticable > > as a failure of crashdump kexec, which stops just after issuing: > > > > Loading crashdump kernel... > > > > to the system console. Two other locations of these loops within the > > ARM code have been identified and fixed up. > > Can you confirm that this only happens if CONFIG_ARM_ERRATA_754327=y? CONFIG_ARM_ERRATA_754327=y + patch => works CONFIG_ARM_ERRATA_754327=y => fails CONFIG_ARM_ERRATA_754327=n => works > The only erratum I can find for A9 that matches this behaviour exists > when the body of the tight loop contains a DMB and some of the possible > workarounds are: > > - Add ten NOPs after the DMB > - Use DSB instead of DMB in the tight loop > - Set bit 16 in the diagnostic control register (p15, c1, 5, 0, c0, 1) Yes, I think you pointed me at that. It may be appropriate to mitigate the cases where we have a tight loop where the loop has a termination condition, but in these cases, all the loops are infinite - finding some way to avoid spinning in this case is probably a good idea in any case. What I'm more interested in this patch is to fix kexec crashdump when CONFIG_ARM_ERRATA_754327=y on OMAP4 (and similar) platforms. > WFE is probably fine (the write-up isn't clear), but if this only occurs > due to CONFIG_ARM_ERRATA_754327=y it would be nice to mitigate it in the > alternative cpu_relax() definition itself, which isn't generally possible > with WFE. With the WFE, it is no longer "a tight loop", although WFE is just a hint to the processor, it could ultimately ignore it. That said, in all these cases, either: - we're either talking about a secondary CPU, so SMP must be supported (which presumably guarantees implementation of SEV/WFE) or: - we're the only CPU so this problem doesn't apply to the infinite loop case. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
