Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem

[Vlastimil Babka]

On 07/04/2017 11:32 AM, Thomas Gleixner wrote:
> Andrey reported a potential deadlock with the memory hotplug lock and the
> cpu hotplug lock.
> 
> The reason is that memory hotplug takes the memory hotplug lock and then
> calls stop_machine() which calls get_online_cpus(). That's the reverse lock
> order to get_online_cpus(); get_online_mems(); in mm/slub_common.c
> 
> The problem has been there forever. The reason why this was never reported
> is that the cpu hotplug locking had this homebrewn recursive reader writer
> semaphore construct which due to the recursion evaded the full lock dep
> coverage. The memory hotplug code copied that construct verbatim and
> therefor has similar issues.
> 
> Three steps to fix this:
> 
> 1) Convert the memory hotplug locking to a per cpu rwsem so the potential
>    issues get reported proper by lockdep.
> 
> 2) Lock the online cpus in mem_hotplug_begin() before taking the memory
>    hotplug rwsem and use stop_machine_cpuslocked() in the page_alloc code
>    and use to avoid recursive locking.

     ^ s/and use // ?

> 
> 3) The cpu hotpluck locking in #2 causes a recursive locking of the cpu
>    hotplug lock via __offline_pages() -> lru_add_drain_all(). Solve this by
>    invoking lru_add_drain_all_cpuslocked() instead.
> 
> Reported-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/memory_hotplug.c |   89 ++++++++--------------------------------------------
>  mm/page_alloc.c     |    2 -
>  2 files changed, 16 insertions(+), 75 deletions(-)

Nice! Glad to see the crazy code go.