Re: [PATCH v5] mm/memory_hotplug: Fix remove_memory() lockdep splat

[Michal Hocko <mhocko@kernel.org>]

On Fri 24-01-20 17:07:21, Dan Williams wrote:
> The daxctl unit test for the dax_kmem driver currently triggers the
> (false positive) 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() and sysfs (kernfs
> active state tracking). It is a false positive because the sysfs
> attribute path triggering the memory remove is not the same attribute
> path associated with memory-block device.
> 
> sysfs_break_active_protection() is not applicable since there is no real
> deadlock conflict, instead move memory-block device removal outside the
> lock. The mem_hotplug_lock() is not needed to synchronize the
> memory-block device removal 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 any in progress online attempts are
> flushed / blocked by kernfs_drain() / attribute removal.
> 
> The add_memory() path safely creates memblock devices under the
> mem_hotplug_lock(). There is no kernfs active state synchronization in
> the memblock device_register() path, so nothing to fix there.
> 
> 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()), and David's due diligence tracking down the
> guarantees afforded by kernfs_drain(). Not flagged for -stable since
> this only impacts ongoing development and lockdep validation, not a
> runtime issue.
> 
>     ======================================================
>     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 has been a long standing issue that predated the
> addition of kernfs lockdep annotations.
> 
> 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>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
> Changes since v4 [1]:
> - Drop the unnecessary consideration of mem->section_count.
>   kernfs_drain() + lock_device_hotplug() is sufficient protection
>   (David)
> 
> [1]: http://lore.kernel.org/r/157869128062.2451572.4093315441083744888.stgit@dwillia2-desk3.amr.corp.intel.com
> 
>  mm/memory_hotplug.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 55ac23ef11c1..65ddaf3a2a12 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 +1775,14 @@ 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 */
> +	/*
> +	 * Memory block device removal under the device_hotplug_lock is
> +	 * a barrier against racing online attempts.
> +	 */
>  	remove_memory_block_devices(start, size);
>  
> +	mem_hotplug_begin();
> +
>  	arch_remove_memory(nid, start, size, NULL);
>  	memblock_free(start, size);
>  	memblock_remove(start, size);

-- 
Michal Hocko
SUSE Labs