Re: [PATCH v4] mm/memory_hotplug: Fix remove_memory() lockdep splat

[David Hildenbrand]

On 10.01.20 22:22, 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, and it wants to unwind the
> hot-add (via arch_remove_memory()) if the memblock device creation
> fails, 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>

I am not convinced this can actually happen. I explained somewhere else
already why a similar locksplat (reported by Pavel IIRC) on the ordinary
memory removal path is a false positive (because the device hotplug lock
actually protects us from such conditions). Can you elaborate why this
is stable material (and explain my tired eyes how the issue will
actually happen in real life)?

[...]
>  
>  When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
> -the device_hotplug_lock should be held to:
> +the device_hotplug_lock is held to:
>  
>  - synchronize against online/offline requests (e.g. via sysfs). This way, memory
>    block devices can only be accessed (.online/.state attributes) by user
> -  space once memory has been fully added. And when removing memory, we
> -  know nobody is in critical sections.
> +  space once memory has been fully added. And when removing memory, the
> +  memory block device is invalidated (mem->section count set to 0) under the
> +  lock to abort any in-flight online requests.

I don't think this is needed. See below.

>  - synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
>  
>  Especially, there is a possible lock inversion that is avoided using
> @@ -112,7 +113,13 @@ can result in a lock inversion.
>  
>  onlining/offlining of memory should be done via device_online()/
>  device_offline() - to make sure it is properly synchronized to actions
> -via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
> +via sysfs. Holding device_hotplug_lock is required to prevent online racing
> +removal. The device_hotplug_lock and memblock invalidation allows
> +remove_memory_block_devices() to run outside of mem_hotplug_lock to avoid lock
> +dependency conflicts with memblock-sysfs teardown. The add_memory() path
> +performs create_memory_block_devices() under mem_hotplug_lock so that if it
> +fails it can perform an arch_remove_memory() cleanup. There are no known lock
> +dependency problems with memblock-sysfs setup.
>  
>  When adding/removing/onlining/offlining memory or adding/removing
>  heterogeneous/device memory, we should always hold the mem_hotplug_lock in
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 42a672456432..5d5036370c92 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1146,6 +1146,11 @@ void unlock_device_hotplug(void)
>  	mutex_unlock(&device_hotplug_lock);
>  }
>  
> +void assert_held_device_hotplug(void)
> +{
> +	lockdep_assert_held(&device_hotplug_lock);
> +}
> +
>  int lock_device_hotplug_sysfs(void)
>  {
>  	if (mutex_trylock(&device_hotplug_lock))
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 799b43191dea..91c6fbd2383e 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -280,6 +280,10 @@ static int memory_subsys_online(struct device *dev)
>  	if (mem->state == MEM_ONLINE)
>  		return 0;
>  
> +	/* online lost the race with hot-unplug, abort */
> +	if (!mem->section_count)
> +		return -ENXIO;
> +

Huh, why is that needed? There is pages_correctly_probed(), which checks
that all sections are present already (but I also have a patch to rework
that in my queue, because it looks like it's not needed in the current
state).

(Especially, I don't see why this is necessary in the context of this
patch - nothing changed in that regard. Also, checks against "device
already removed" should logically belong into
device_online()/device_offline(). Other subsystems should have similar
issues, no?)

>  	/*
>  	 * If we are called from state_store(), online_type will be
>  	 * set >= 0 Otherwise we were called from the device online
> @@ -736,8 +740,6 @@ int create_memory_block_devices(unsigned long start, unsigned long size)
>   * Remove memory block devices for the given memory area. Start and size
>   * have to be aligned to memory block granularity. Memory block devices
>   * have to be offline.
> - *
> - * Called under device_hotplug_lock.
>   */

Why is that change needed? Especially with the radix tree rework, this
lock is required on this call path. Removing this looks wrong to me.


>  void remove_memory_block_devices(unsigned long start, unsigned long size)
>  {
> @@ -746,6 +748,8 @@ void remove_memory_block_devices(unsigned long start, unsigned long size)
>  	struct memory_block *mem;
>  	unsigned long block_id;
>  
> +	assert_held_device_hotplug();
> +
>  	if (WARN_ON_ONCE(!IS_ALIGNED(start, memory_block_size_bytes()) ||
>  			 !IS_ALIGNED(size, memory_block_size_bytes())))
>  		return;
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 96ff76731e93..a84654489c51 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1553,6 +1553,7 @@ static inline bool device_supports_offline(struct device *dev)
>  extern void lock_device_hotplug(void);
>  extern void unlock_device_hotplug(void);
>  extern int lock_device_hotplug_sysfs(void);
> +extern void assert_held_device_hotplug(void);
>  extern int device_offline(struct device *dev);
>  extern int device_online(struct device *dev);
>  extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 55ac23ef11c1..0158cd4cca48 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 +1775,15 @@ 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 before
> +	 * mem_hotplug_begin() (see Documentation/core-api/memory-hotplug.rst
> +	 * "Locking Internals").
> +	 */
>  	remove_memory_block_devices(start, size);
>  
> +	mem_hotplug_begin();
> +
>  	arch_remove_memory(nid, start, size, NULL);
>  	memblock_free(start, size);
>  	memblock_remove(start, size);
> 

That hunk looks good to me.

-- 
Thanks,

David / dhildenb