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SubjectRe: [PATCH] mm/memory_hotplug: Fix remove_memory() lockdep splat
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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.
*/

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. I properly documented the semantics of
add_memory_block_devices()/remove_memory_block_devices() already (that
they need the device hotplug lock).

--
Thanks,

David / dhildenb

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