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SubjectRe: [PATCH] mm/memory_hotplug: Fix remove_memory() lockdep splat
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On 10.01.20 17:42, Dan Williams wrote:
> On Fri, Jan 10, 2020 at 1:10 AM David Hildenbrand <david@redhat.com> 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@vger.kernel.org>
>>> Cc: Vishal Verma <vishal.l.verma@intel.com>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>>> ---
>>> 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.

--
Thanks,

David / dhildenb

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