Re: [PATCH] mm/memory_hotplug: Fix remove_memory() lockdep splat

[David Hildenbrand]

On 10.01.20 17:54, David Hildenbrand wrote:
> On 10.01.20 17:42, Dan Williams wrote:
>> On Fri, Jan 10, 2020 at 1:10 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>
>>> On 10.01.20 05:30, Dan Williams wrote:
>>>> The daxctl unit test for the dax_kmem driver currently triggers the
>>>> lockdep splat below. It results from the fact that
>>>> remove_memory_block_devices() is invoked under the mem_hotplug_lock()
>>>> causing lockdep entanglements with cpu_hotplug_lock().
>>>>
>>>> The mem_hotplug_lock() is not needed to synchronize the memory block
>>>> device sysfs interface vs the page online state, that is already handled
>>>> by lock_device_hotplug(). Specifically lock_device_hotplug()
>>>> is sufficient to allow try_remove_memory() to check the offline
>>>> state of the memblocks and be assured that subsequent online attempts
>>>> will be blocked. The device_online() path checks mem->section_count
>>>> before allowing any state manipulations and mem->section_count is
>>>> cleared in remove_memory_block_devices().
>>>>
>>>> The add_memory() path does create memblock devices under the lock, but
>>>> there is no lockdep report on that path, so it is left alone for now.
>>>>
>>>> This change is only possible thanks to the recent change that refactored
>>>> memory block device removal out of arch_remove_memory() (commit
>>>> 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before
>>>> arch_remove_memory()).
>>>>
>>>>     ======================================================
>>>>     WARNING: possible circular locking dependency detected
>>>>     5.5.0-rc3+ #230 Tainted: G           OE
>>>>     ------------------------------------------------------
>>>>     lt-daxctl/6459 is trying to acquire lock:
>>>>     ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80
>>>>
>>>>     but task is already holding lock:
>>>>     ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0
>>>>
>>>>     which lock already depends on the new lock.
>>>>
>>>>
>>>>     the existing dependency chain (in reverse order) is:
>>>>
>>>>     -> #2 (mem_hotplug_lock.rw_sem){++++}:
>>>>            __lock_acquire+0x39c/0x790
>>>>            lock_acquire+0xa2/0x1b0
>>>>            get_online_mems+0x3e/0xb0
>>>>            kmem_cache_create_usercopy+0x2e/0x260
>>>>            kmem_cache_create+0x12/0x20
>>>>            ptlock_cache_init+0x20/0x28
>>>>            start_kernel+0x243/0x547
>>>>            secondary_startup_64+0xb6/0xc0
>>>>
>>>>     -> #1 (cpu_hotplug_lock.rw_sem){++++}:
>>>>            __lock_acquire+0x39c/0x790
>>>>            lock_acquire+0xa2/0x1b0
>>>>            cpus_read_lock+0x3e/0xb0
>>>>            online_pages+0x37/0x300
>>>>            memory_subsys_online+0x17d/0x1c0
>>>>            device_online+0x60/0x80
>>>>            state_store+0x65/0xd0
>>>>            kernfs_fop_write+0xcf/0x1c0
>>>>            vfs_write+0xdb/0x1d0
>>>>            ksys_write+0x65/0xe0
>>>>            do_syscall_64+0x5c/0xa0
>>>>            entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>>
>>>>     -> #0 (kn->count#241){++++}:
>>>>            check_prev_add+0x98/0xa40
>>>>            validate_chain+0x576/0x860
>>>>            __lock_acquire+0x39c/0x790
>>>>            lock_acquire+0xa2/0x1b0
>>>>            __kernfs_remove+0x25f/0x2e0
>>>>            kernfs_remove_by_name_ns+0x41/0x80
>>>>            remove_files.isra.0+0x30/0x70
>>>>            sysfs_remove_group+0x3d/0x80
>>>>            sysfs_remove_groups+0x29/0x40
>>>>            device_remove_attrs+0x39/0x70
>>>>            device_del+0x16a/0x3f0
>>>>            device_unregister+0x16/0x60
>>>>            remove_memory_block_devices+0x82/0xb0
>>>>            try_remove_memory+0xb5/0x130
>>>>            remove_memory+0x26/0x40
>>>>            dev_dax_kmem_remove+0x44/0x6a [kmem]
>>>>            device_release_driver_internal+0xe4/0x1c0
>>>>            unbind_store+0xef/0x120
>>>>            kernfs_fop_write+0xcf/0x1c0
>>>>            vfs_write+0xdb/0x1d0
>>>>            ksys_write+0x65/0xe0
>>>>            do_syscall_64+0x5c/0xa0
>>>>            entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>>
>>>>     other info that might help us debug this:
>>>>
>>>>     Chain exists of:
>>>>       kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem
>>>>
>>>>      Possible unsafe locking scenario:
>>>>
>>>>            CPU0                    CPU1
>>>>            ----                    ----
>>>>       lock(mem_hotplug_lock.rw_sem);
>>>>                                    lock(cpu_hotplug_lock.rw_sem);
>>>>                                    lock(mem_hotplug_lock.rw_sem);
>>>>       lock(kn->count#241);
>>>>
>>>>      *** DEADLOCK ***
>>>>
>>>> No fixes tag as this seems to have been a long standing issue that
>>>> likely predated the addition of kernfs lockdep annotations.
>>>>
>>>> Cc: <stable@xxxxxxxxxxxxxxx>
>>>> Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
>>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>>>> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>>>> ---
>>>>  mm/memory_hotplug.c |   12 +++++++++---
>>>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>>> index 55ac23ef11c1..a4e7dadded08 100644
>>>> --- a/mm/memory_hotplug.c
>>>> +++ b/mm/memory_hotplug.c
>>>> @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
>>>>
>>>>       BUG_ON(check_hotplug_memory_range(start, size));
>>>>
>>>> -     mem_hotplug_begin();
>>>> -
>>>>       /*
>>>>        * All memory blocks must be offlined before removing memory.  Check
>>>>        * whether all memory blocks in question are offline and return error
>>>> @@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
>>>>       /* remove memmap entry */
>>>>       firmware_map_remove(start, start + size, "System RAM");
>>>>
>>>> -     /* remove memory block devices before removing memory */
>>>> +     /*
>>>> +      * Remove memory block devices before removing memory, and do
>>>> +      * not hold the mem_hotplug_lock() over kobject removal
>>>> +      * operations. lock_device_hotplug() keeps the
>>>> +      * check_memblock_offlined_cb result valid until the entire
>>>> +      * removal process is complete.
>>>> +      */
>>>
>>> Maybe shorten that to
>>>
>>> /*
>>>  * Remove memory block devices before removing memory. Protected
>>>  * by the device_hotplug_lock only.
>>>  */
>>
>> Why make someone dig for the reasons this lock is sufficient?
> 
> I think 5 LOC of comment are too much for something that is documented
> e.g., in Documentation/core-api/memory-hotplug.rst ("Locking
> Internals"). But whatever you prefer.
> 
>>
>>>
>>> AFAIK, the device hotplug lock is sufficient here. The memory hotplug
>>> lock / cpu hotplug lock is only needed when calling into arch code
>>> (especially for PPC). We hold both locks when onlining/offlining memory.
>>>
>>>>       remove_memory_block_devices(start, size);
>>>>
>>>> +     mem_hotplug_begin();
>>>> +
>>>>       arch_remove_memory(nid, start, size, NULL);
>>>>       memblock_free(start, size);
>>>>       memblock_remove(start, size);
>>>>
>>>
>>> I'd suggest to do the same in the adding part right away (if easily
>>> possible) to make it clearer.
>>
>> Let's let this fix percolate upstream for a bit to make sure there was
>> no protection the mem_hotplug_begin() was inadvertently providing.
> 
> Yeah, why not.
> 
>>
>>> I properly documented the semantics of
>>> add_memory_block_devices()/remove_memory_block_devices() already (that
>>> they need the device hotplug lock).
>>
>> I see that, but I prefer lockdep_assert_held() in the code rather than
>> comments. I'll send a patch to fix that up.
> 
> That won't work as early boot code from ACPI won't hold it while it adds
> memory. And we decided (especially Michal :) ) to keep it like that.
> 

Was only thinking about the adding part, it could work for the removal
part, though :)

-- 
Thanks,

David / dhildenb