On Wed, Mar 01, 2017 at 07:52:18AM -0800, Dan Williams wrote:

On Wed, Mar 1, 2017 at 4:51 AM, Heiko Carstens
[off-list ref] wrote:

quoted

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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