Subject: Re: [RFC PATCH powerpc] Protect remove_memory() with device hotplug lock

From: Nathan Fontenot <[email-masked]>
Date: Wed, 09 Apr 2014 11:14:08 -0500

On 04/09/2014 03:54 AM, Li Zhong wrote:
> While testing memory hot-remove, I found following dead lock:
> 
> Process #1141 is drmgr, trying to remove some memory, i.e. memory499. 
> It holds the memory_hotplug_mutex, and blocks when trying to remove file
> "online" under dir memory499, in kernfs_drain(), at 
>         wait_event(root->deactivate_waitq,
>                    atomic_read(&kn->active) == KN_DEACTIVATED_BIAS);
> 
> Process #1120 is trying to online memory499 by 
>    echo 1 > memory499/online
> 
> In .kernfs_fop_write, it uses kernfs_get_active() to increase
> &kn->active, thus blocking process #1141. While itself is blocked later
> when trying to acquire memory_hotplug_mutex, which is held by process
> #1141.
> 
> The backtrace of both processes are shown below:
> 
> # cat /proc/1120/stack 
> [<c000000001b18600>] 0xc000000001b18600
> [<c000000000015044>] .__switch_to+0x144/0x200
> [<c000000000263ca4>] .online_pages+0x74/0x7b0
> [<c00000000055b40c>] .memory_subsys_online+0x9c/0x150
> [<c00000000053cbe8>] .device_online+0xb8/0x120
> [<c00000000053cd04>] .online_store+0xb4/0xc0
> [<c000000000538ce4>] .dev_attr_store+0x64/0xa0
> [<c00000000030f4ec>] .sysfs_kf_write+0x7c/0xb0
> [<c00000000030e574>] .kernfs_fop_write+0x154/0x1e0
> [<c000000000268450>] .vfs_write+0xe0/0x260
> [<c000000000269144>] .SyS_write+0x64/0x110
> [<c000000000009ffc>] syscall_exit+0x0/0x7c
> 
> 
> # cat /proc/1141/stack 
> [<c000000001b18600>] 0xc000000001b18600
> [<c000000000015044>] .__switch_to+0x144/0x200
> [<c00000000030be14>] .__kernfs_remove+0x204/0x300
> [<c00000000030d428>] .kernfs_remove_by_name_ns+0x68/0xf0
> [<c00000000030fb38>] .sysfs_remove_file_ns+0x38/0x60
> [<c000000000539354>] .device_remove_attrs+0x54/0xc0
> [<c000000000539fd8>] .device_del+0x158/0x250
> [<c00000000053a104>] .device_unregister+0x34/0xa0
> [<c00000000055bc14>] .unregister_memory_section+0x164/0x170
> [<c00000000024ee18>] .__remove_pages+0x108/0x4c0
> [<c00000000004b590>] .arch_remove_memory+0x60/0xc0
> [<c00000000026446c>] .remove_memory+0x8c/0xe0
> [<c00000000007f9f4>] .pseries_remove_memblock+0xd4/0x160
> [<c00000000007fcfc>] .pseries_memory_notifier+0x27c/0x290
> [<c0000000008ae6cc>] .notifier_call_chain+0x8c/0x100
> [<c0000000000d858c>] .__blocking_notifier_call_chain+0x6c/0xe0
> [<c00000000071ddec>] .of_property_notify+0x7c/0xc0
> [<c00000000071ed3c>] .of_update_property+0x3c/0x1b0
> [<c0000000000756cc>] .ofdt_write+0x3dc/0x740
> [<c0000000002f60fc>] .proc_reg_write+0xac/0x110
> [<c000000000268450>] .vfs_write+0xe0/0x260
> [<c000000000269144>] .SyS_write+0x64/0x110
> [<c000000000009ffc>] syscall_exit+0x0/0x7c
> 
> This patch uses lock_device_hotplug() to protect remove_memory() called
> in pseries_remove_memblock(), which is also stated before function
> remove_memory():
> 
>  * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
>  * and online/offline operations before this call, as required by
>  * try_offline_node().
>  */     
> void __ref remove_memory(int nid, u64 start, u64 size)
> 
> With this lock held, the other process(#1120 above) trying to online the
> memory block will retry the system call when calling
> lock_device_hotplug_sysfs(), and finally find No such device error. 
> 
> Signed-off-by: Li Zhong <[email-masked]>
> ---
>  arch/powerpc/platforms/pseries/hotplug-memory.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
> index 573b488..e96357c 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> @@ -109,10 +109,12 @@ static int pseries_remove_memblock(unsigned long base, unsigned int memblock_siz
>  	sections_per_block = block_sz / MIN_MEMORY_BLOCK_SIZE;
>  	nid = memory_add_physaddr_to_nid(base);
> 
> +	lock_device_hotplug();
>  	for (i = 0; i < sections_per_block; i++) {
>  		remove_memory(nid, base, MIN_MEMORY_BLOCK_SIZE);
>  		base += MIN_MEMORY_BLOCK_SIZE;
>  	}
> +	unlock_device_hotplug();
> 

Should we be releasing the lock here? I think we want to hold the lock
until we exit pseries_remove_memblock(). This would ensure that are able
to finish all the work needed to remove memory while holding the lock.

-Nathan

>  	/* Update memory regions for memory remove */
>  	memblock_remove(base, memblock_size);
> 
>