On Wed, 17 Nov 2010 11:03:25 +0800 Huang Ying <ying.huang@xxxxxxxxx> wrote: > It seems that Steven thinks many architectures without NMI-safe cmpxchg > have no real NMI too. Could be. Really, we should nail this down and work out the matrix of what-works-with-what, and then arrange for the things which _won't_ work to be either non-Kconfigurable or non-compilable. The worst thing we could do would be to advertise some code as "nmi-safe!!", then to have someone go and use it on that basis, only for their users to later discover ghastly rare races. > In the patch description and comments, it is said that on architectures > without NMI-safe cmpxchg, gen_pool can not be used in NMI handler > safely. > > Or do you think it is better to use a spin_trylock based fallback if > NMI-safe cmpxchg is not available? Or require cmpxchg implementation > uses spin_trylock instead of spin_lock? As a first step, a typical thing to do would be to create CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG, define that in the appropriate architectures, make ftrace and perf and genpool and anything else dependent upon that at Kconfig-time. A spin_trylock_irqsave() implementation would do what? Rarely fail the memory allocation attempt if the trylock failed? I guess that's acceptable in the context of gen_pool, because memory allocators are expected to fail, and everyone carefully tests the allocation-failed error paths (lol). But rare failures may not be useful within the context of future clients of the "lockless" list implementation so I'd say that a safer approach would be to make the list implementation require CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG and be done with it. So that's all pretty simple so far. However... The list implementation is still useful to non-CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG clients, as long as they aren't using it from NMI context, yes? In which case I suppose we could add a library of lockless_list_foo() functions which are usable from non-NMI contexts and which are available to all configs. And on top of that implement a library of nmi_safe_lockless_list_foo() functions which are just wrappers around lockless_list_foo(), but which are only available if CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG=y. Which is getting to be a bit of a pain, so you might choose to disregard everything after "However..." ;) -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
