Messages in this thread | | | Date | Wed, 1 Mar 2017 18:04:29 +0100 | From | Heiko Carstens <> | Subject | Re: [PATCH] mm, add_memory_resource: hold device_hotplug lock over mem_hotplug_{begin, done} |
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On Wed, Mar 01, 2017 at 07:52:18AM -0800, Dan Williams wrote: > On Wed, Mar 1, 2017 at 4:51 AM, Heiko Carstens > <heiko.carstens@de.ibm.com> wrote: > > Since it is anything but obvious why Dan wrote in changelog of b5d24fda9c3d > > ("mm, devm_memremap_pages: hold device_hotplug lock over > > mem_hotplug_{begin, done}") that write accesses to > > mem_hotplug.active_writer are coordinated via lock_device_hotplug() I'd > > rather propose a new private memory_add_remove_lock which has similar > > semantics like the cpu_add_remove_lock for cpu hotplug (see patch below). > > > > However instead of sprinkling locking/unlocking of that new lock around all > > calls of mem_hotplug_begin() and mem_hotplug_end() simply include locking > > and unlocking into these two functions. > > > > This still allows get_online_mems() and put_online_mems() to work, while at > > the same time preventing mem_hotplug.active_writer corruption. > > > > Any opinions? > > Sorry, yes, I didn't make it clear that I derived that locking > requirement from store_mem_state() and its usage of > lock_device_hotplug_sysfs(). > > That routine is trying very hard not trip the soft-lockup detector. It > seems like that wants to be an interruptible wait.
If you look at commit 5e33bc4165f3 ("driver core / ACPI: Avoid device hot remove locking issues") then lock_device_hotplug_sysfs() was introduced to avoid a different subtle deadlock, but it also sleeps uninterruptible, but not for more than 5ms ;)
However I'm not sure if the device hotplug lock should also be used to fix an unrelated bug that was introduced with the get_online_mems() / put_online_mems() interface. Should it?
If so, we need to sprinkle around a couple of lock_device_hotplug() calls near mem_hotplug_begin() calls, like Sebastian already started, and give it additional semantics (protecting mem_hotplug.active_writer), and hope it doesn't lead to deadlocks anywhere.
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