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SubjectRe: [PATCH] mm/memory-hotplug: Switch locking to a percpu rwsem
[CC Vladimir and Heiko who were touching this area lately]

On Thu 29-06-17 18:11:15, 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

I always considered the stop_machine usage there totally gross. But
never had time to look into it properly. Memory hotplug locking is a
story of its own.

> 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.
>
> Two 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
> to avoid recursive locking.

So I like this simplification a lot! Even if we can get rid of the
stop_machine eventually this patch would be an improvement. A short
comment on why the per-cpu semaphore over the regular one is better
would be nice.

I cannot give my ack yet, I have to mull over the patch some
more because this has been an area of subtle bugs (especially
the lock dependency with the hotplug device locking - look at
lock_device_hotplug_sysfs if you dare) but it looks good from the first
look. Give me few days, please.

> Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: linux-mm@kvack.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> ---
>
> Note 1:
> Applies against -next or
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git smp/hotplug
>
> which contains the hotplug locking rework including stop_machine_cpuslocked()
>
> Note 2:
>
> Most of the call sites of get_online_mems() are also calling get_online_cpus().
>
> So we could switch the whole machinery to use the CPU hotplug locking for
> protecting both memory and CPU hotplug. That actually works and removes
> another 40 lines of code.
>
> ---
> mm/memory_hotplug.c | 85 +++++++---------------------------------------------
> mm/page_alloc.c | 2 -
> 2 files changed, 14 insertions(+), 73 deletions(-)
>
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -52,32 +52,17 @@ static void generic_online_page(struct p
> static online_page_callback_t online_page_callback = generic_online_page;
> static DEFINE_MUTEX(online_page_callback_lock);
>
> -/* The same as the cpu_hotplug lock, but for memory hotplug. */
> -static struct {
> - struct task_struct *active_writer;
> - struct mutex lock; /* Synchronizes accesses to refcount, */
> - /*
> - * Also blocks the new readers during
> - * an ongoing mem hotplug operation.
> - */
> - int refcount;
> +DEFINE_STATIC_PERCPU_RWSEM(mem_hotplug_lock);
>
> -#ifdef CONFIG_DEBUG_LOCK_ALLOC
> - struct lockdep_map dep_map;
> -#endif
> -} mem_hotplug = {
> - .active_writer = NULL,
> - .lock = __MUTEX_INITIALIZER(mem_hotplug.lock),
> - .refcount = 0,
> -#ifdef CONFIG_DEBUG_LOCK_ALLOC
> - .dep_map = {.name = "mem_hotplug.lock" },
> -#endif
> -};
> +void get_online_mems(void)
> +{
> + percpu_down_read(&mem_hotplug_lock);
> +}
>
> -/* Lockdep annotations for get/put_online_mems() and mem_hotplug_begin/end() */
> -#define memhp_lock_acquire_read() lock_map_acquire_read(&mem_hotplug.dep_map)
> -#define memhp_lock_acquire() lock_map_acquire(&mem_hotplug.dep_map)
> -#define memhp_lock_release() lock_map_release(&mem_hotplug.dep_map)
> +void put_online_mems(void)
> +{
> + percpu_up_read(&mem_hotplug_lock);
> +}
>
> #ifndef CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
> bool memhp_auto_online;
> @@ -97,60 +82,16 @@ static int __init setup_memhp_default_st
> }
> __setup("memhp_default_state=", setup_memhp_default_state);
>
> -void get_online_mems(void)
> -{
> - might_sleep();
> - if (mem_hotplug.active_writer == current)
> - return;
> - memhp_lock_acquire_read();
> - mutex_lock(&mem_hotplug.lock);
> - mem_hotplug.refcount++;
> - mutex_unlock(&mem_hotplug.lock);
> -
> -}
> -
> -void put_online_mems(void)
> -{
> - if (mem_hotplug.active_writer == current)
> - return;
> - mutex_lock(&mem_hotplug.lock);
> -
> - if (WARN_ON(!mem_hotplug.refcount))
> - mem_hotplug.refcount++; /* try to fix things up */
> -
> - if (!--mem_hotplug.refcount && unlikely(mem_hotplug.active_writer))
> - wake_up_process(mem_hotplug.active_writer);
> - mutex_unlock(&mem_hotplug.lock);
> - memhp_lock_release();
> -
> -}
> -
> -/* Serializes write accesses to mem_hotplug.active_writer. */
> -static DEFINE_MUTEX(memory_add_remove_lock);
> -
> void mem_hotplug_begin(void)
> {
> - mutex_lock(&memory_add_remove_lock);
> -
> - mem_hotplug.active_writer = current;
> -
> - memhp_lock_acquire();
> - for (;;) {
> - mutex_lock(&mem_hotplug.lock);
> - if (likely(!mem_hotplug.refcount))
> - break;
> - __set_current_state(TASK_UNINTERRUPTIBLE);
> - mutex_unlock(&mem_hotplug.lock);
> - schedule();
> - }
> + cpus_read_lock();
> + percpu_down_write(&mem_hotplug_lock);
> }
>
> void mem_hotplug_done(void)
> {
> - mem_hotplug.active_writer = NULL;
> - mutex_unlock(&mem_hotplug.lock);
> - memhp_lock_release();
> - mutex_unlock(&memory_add_remove_lock);
> + percpu_up_write(&mem_hotplug_lock);
> + cpus_read_unlock();
> }
>
> /* add this memory to iomem resource */
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5216,7 +5216,7 @@ void __ref build_all_zonelists(pg_data_t
> #endif
> /* we have to stop all cpus to guarantee there is no user
> of zonelist */
> - stop_machine(__build_all_zonelists, pgdat, NULL);
> + stop_machine_cpuslocked(__build_all_zonelists, pgdat, NULL);
> /* cpuset refresh routine should be here */
> }
> vm_total_pages = nr_free_pagecache_pages();

--
Michal Hocko
SUSE Labs

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