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SubjectRe: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock
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Date
On 24.09.19 17:03, Qian Cai wrote:
> On Tue, 2019-09-24 at 16:36 +0200, David Hildenbrand wrote:
>> Since commit 3f906ba23689 ("mm/memory-hotplug: switch locking to a percpu
>> rwsem") we do a cpus_read_lock() in mem_hotplug_begin(). This was
>> introduced to fix a potential deadlock between get_online_mems() and
>> get_online_cpus() - the memory and cpu hotplug lock. The root issue was
>> that build_all_zonelists() -> stop_machine() required 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
>>
>> So memory hotplug never really required any cpu lock itself, only
>> stop_machine() and lru_add_drain_all() required it. Back then,
>> stop_machine_cpuslocked() and lru_add_drain_all_cpuslocked() were used
>> as the cpu hotplug lock was now obtained in the caller.
>>
>> Since commit 11cd8638c37f ("mm, page_alloc: remove stop_machine from build
>> all_zonelists"), the stop_machine_cpuslocked() call is gone.
>> build_all_zonelists() does no longer require the cpu lock and does no
>> longer make use of stop_machine().
>>
>> Since commit 9852a7212324 ("mm: drop hotplug lock from
>> lru_add_drain_all()"), lru_add_drain_all() "Doesn't need any cpu hotplug
>> locking because we do rely on per-cpu kworkers being shut down before our
>> page_alloc_cpu_dead callback is executed on the offlined cpu.". The
>> lru_add_drain_all_cpuslocked() variant was removed.
>>
>> So there is nothing left that requires the cpu hotplug lock. The memory
>> hotplug lock and the device hotplug lock are sufficient.
>>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>
>> RFC -> v1:
>> - Reword and add more details why the cpu hotplug lock was needed here
>> in the first place, and why we no longer require it.
>>
>> ---
>> mm/memory_hotplug.c | 2 --
>> 1 file changed, 2 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index c3e9aed6023f..5fa30f3010e1 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -88,14 +88,12 @@ __setup("memhp_default_state=", setup_memhp_default_state);
>>
>> void mem_hotplug_begin(void)
>> {
>> - cpus_read_lock();
>> percpu_down_write(&mem_hotplug_lock);
>> }
>>
>> void mem_hotplug_done(void)
>> {
>> percpu_up_write(&mem_hotplug_lock);
>> - cpus_read_unlock();
>> }
>>
>> u64 max_mem_size = U64_MAX;
>
> While at it, it might be a good time to rethink the whole locking over there, as
> it right now read files under /sys/kernel/slab/ could trigger a possible
> deadlock anyway.

We still have another (I think incorrect) splat when onlining/offlining
memory and then removing it e.g., via ACPI. It's also a (problematic ?)
"kn->count" lock in these cases.

E.g., when offlining memory (via sysfs) we first take the kn->count and
then the memory hotplug lock. When removing memory devices we first take
the memory hotplug lock and then the kn->count (to remove the sysfs
files). But at least there we have the device hotplug lock involved to
effectively prohibit any deadlocks. But it's somewhat similar to what
you pasted below (but there, it could as well be that we can just drop
the lock as Michal said).

I wonder if one general solution could be to replace the
device_hotplug_lock by a per-subsystem (memory, cpu) lock. Especially,
in drivers/base/core.c:online_store and when creating/deleting devices
we would take that subsystem lock directly, and not fairly down in the
call paths.

There are only a handful of places (e.g., node onlining/offlining) where
cpu and memory hotplug can race. But we can handle these cases differently.

Eventually we could get rid of the device_hotplug_lock() and use locks
per subsystem instead. That would at least be better compared to what we
have just now (we always need the device hotplug lock and the memory
hotplug lock in memory hotplug paths).

>
> [  442.258806][ T5224] WARNING: possible circular locking dependency detected
> [  442.265678][ T5224] 5.3.0-rc7-mm1+ #6 Tainted: G             L   
> [  442.271766][ T5224] ------------------------------------------------------
> [  442.278635][ T5224] cat/5224 is trying to acquire lock:
> [  442.283857][ T5224] ffff900012ac3120 (mem_hotplug_lock.rw_sem){++++}, at:
> show_slab_objects+0x94/0x3a8
> [  442.293189][ T5224] 
> [  442.293189][ T5224] but task is already holding lock:
> [  442.300404][ T5224] b8ff009693eee398 (kn->count#45){++++}, at:
> kernfs_seq_start+0x44/0xf0
> [  442.308587][ T5224] 
> [  442.308587][ T5224] which lock already depends on the new lock.
> [  442.308587][ T5224] 
> [  442.318841][ T5224] 
> [  442.318841][ T5224] the existing dependency chain (in reverse order) is:
> [  442.327705][ T5224] 
> [  442.327705][ T5224] -> #2 (kn->count#45){++++}:
> [  442.334413][ T5224]        lock_acquire+0x31c/0x360
> [  442.339286][ T5224]        __kernfs_remove+0x290/0x490
> [  442.344428][ T5224]        kernfs_remove+0x30/0x44
> [  442.349224][ T5224]        sysfs_remove_dir+0x70/0x88
> [  442.354276][ T5224]        kobject_del+0x50/0xb0
> [  442.358890][ T5224]        sysfs_slab_unlink+0x2c/0x38
> [  442.364025][ T5224]        shutdown_cache+0xa0/0xf0
> [  442.368898][ T5224]        kmemcg_cache_shutdown_fn+0x1c/0x34
> [  442.374640][ T5224]        kmemcg_workfn+0x44/0x64
> [  442.379428][ T5224]        process_one_work+0x4f4/0x950
> [  442.384649][ T5224]        worker_thread+0x390/0x4bc
> [  442.389610][ T5224]        kthread+0x1cc/0x1e8
> [  442.394052][ T5224]        ret_from_fork+0x10/0x18
> [  442.398835][ T5224] 
> [  442.398835][ T5224] -> #1 (slab_mutex){+.+.}:
> [  442.405365][ T5224]        lock_acquire+0x31c/0x360
> [  442.410240][ T5224]        __mutex_lock_common+0x16c/0xf78
> [  442.415722][ T5224]        mutex_lock_nested+0x40/0x50
> [  442.420855][ T5224]        memcg_create_kmem_cache+0x38/0x16c
> [  442.426598][ T5224]        memcg_kmem_cache_create_func+0x3c/0x70
> [  442.432687][ T5224]        process_one_work+0x4f4/0x950
> [  442.437908][ T5224]        worker_thread+0x390/0x4bc
> [  442.442868][ T5224]        kthread+0x1cc/0x1e8
> [  442.447307][ T5224]        ret_from_fork+0x10/0x18
> [  442.452090][ T5224] 
> [  442.452090][ T5224] -> #0 (mem_hotplug_lock.rw_sem){++++}:
> [  442.459748][ T5224]        validate_chain+0xd10/0x2bcc
> [  442.464883][ T5224]        __lock_acquire+0x7f4/0xb8c
> [  442.469930][ T5224]        lock_acquire+0x31c/0x360
> [  442.474803][ T5224]        get_online_mems+0x54/0x150
> [  442.479850][ T5224]        show_slab_objects+0x94/0x3a8
> [  442.485072][ T5224]        total_objects_show+0x28/0x34
> [  442.490292][ T5224]        slab_attr_show+0x38/0x54
> [  442.495166][ T5224]        sysfs_kf_seq_show+0x198/0x2d4
> [  442.500473][ T5224]        kernfs_seq_show+0xa4/0xcc
> [  442.505433][ T5224]        seq_read+0x30c/0x8a8
> [  442.509958][ T5224]        kernfs_fop_read+0xa8/0x314
> [  442.515007][ T5224]        __vfs_read+0x88/0x20c
> [  442.519620][ T5224]        vfs_read+0xd8/0x10c
> [  442.524060][ T5224]        ksys_read+0xb0/0x120
> [  442.528586][ T5224]        __arm64_sys_read+0x54/0x88
> [  442.533634][ T5224]        el0_svc_handler+0x170/0x240
> [  442.538768][ T5224]        el0_svc+0x8/0xc
> [  442.542858][ T5224] 
> [  442.542858][ T5224] other info that might help us debug this:
> [  442.542858][ T5224] 
> [  442.552936][ T5224] Chain exists of:
> [  442.552936][ T5224]   mem_hotplug_lock.rw_sem --> slab_mutex --> kn->count#45
> [  442.552936][ T5224] 
> [  442.565803][ T5224]  Possible unsafe locking scenario:
> [  442.565803][ T5224] 
> [  442.573105][ T5224]        CPU0                    CPU1
> [  442.578322][ T5224]        ----                    ----
> [  442.583539][ T5224]   lock(kn->count#45);
> [  442.587545][ T5224]                                lock(slab_mutex);
> [  442.593898][ T5224]                                lock(kn->count#45);
> [  442.600433][ T5224]   lock(mem_hotplug_lock.rw_sem);
> [  442.605393][ T5224] 
> [  442.605393][ T5224]  *** DEADLOCK ***
> [  442.605393][ T5224] 
> [  442.613390][ T5224] 3 locks held by cat/5224:
> [  442.617740][ T5224]  #0: 9eff00095b14b2a0 (&p->lock){+.+.}, at:
> seq_read+0x4c/0x8a8
> [  442.625399][ T5224]  #1: 0eff008997041480 (&of->mutex){+.+.}, at:
> kernfs_seq_start+0x34/0xf0
> [  442.633842][ T5224]  #2: b8ff009693eee398 (kn->count#45){++++}, at:
> kernfs_seq_start+0x44/0xf0
> [  442.642477][ T5224] 
> [  442.642477][ T5224] stack backtrace:
> [  442.648221][ T5224] CPU: 117 PID: 5224 Comm: cat Tainted:
> G             L    5.3.0-rc7-mm1+ #6
> [  442.656826][ T5224] Hardware name: HPE Apollo
> 70             /C01_APACHE_MB         , BIOS L50_5.13_1.11 06/18/2019
> [  442.667253][ T5224] Call trace:
> [  442.670391][ T5224]  dump_backtrace+0x0/0x248
> [  442.674743][ T5224]  show_stack+0x20/0x2c
> [  442.678750][ T5224]  dump_stack+0xd0/0x140
> [  442.682841][ T5224]  print_circular_bug+0x368/0x380
> [  442.687715][ T5224]  check_noncircular+0x248/0x250
> [  442.692501][ T5224]  validate_chain+0xd10/0x2bcc
> [  442.697115][ T5224]  __lock_acquire+0x7f4/0xb8c
> [  442.701641][ T5224]  lock_acquire+0x31c/0x360
> [  442.705993][ T5224]  get_online_mems+0x54/0x150
> [  442.710519][ T5224]  show_slab_objects+0x94/0x3a8
> [  442.715219][ T5224]  total_objects_show+0x28/0x34
> [  442.719918][ T5224]  slab_attr_show+0x38/0x54
> [  442.724271][ T5224]  sysfs_kf_seq_show+0x198/0x2d4
> [  442.729056][ T5224]  kernfs_seq_show+0xa4/0xcc
> [  442.733494][ T5224]  seq_read+0x30c/0x8a8
> [  442.737498][ T5224]  kernfs_fop_read+0xa8/0x314
> [  442.742025][ T5224]  __vfs_read+0x88/0x20c
> [  442.746118][ T5224]  vfs_read+0xd8/0x10c
> [  442.750036][ T5224]  ksys_read+0xb0/0x120
> [  442.754042][ T5224]  __arm64_sys_read+0x54/0x88
> [  442.758569][ T5224]  el0_svc_handler+0x170/0x240
> [  442.763180][ T5224]  el0_svc+0x8/0xc
>


--

Thanks,

David / dhildenb

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