Re: Warnings when suspending to disk

[Mikulas Patocka]

On Wed, 8 Jun 2022, Yazen Ghannam wrote:

> On Fri, Jun 03, 2022 at 01:34:26PM -0400, Mikulas Patocka wrote:
> 
> ...
> 
> > I tried this patch and it doesn't help.
> 
> Thanks Mikulas for testing.
> 
> I'm still not able to reproduce the exact issue. But I was able to reproduce
> the same symptom by hacking the kernel and doing CPU hotplug.

I also see the warnings when disabling cores.

> Can you please try the following patch? This seems to work in my hacked case.
> I also tried to write out a detailed description of the issue to the best of
> my knowledge.

This patch works - there are no longer any warnings on CPU disable or on 
suspend to disk.

Mikulas

> Thanks,
> Yazen
> 
> ========================
> 
> >From d1fa5cdc7f29bf810215f0a83f16bc7435e55240 Mon Sep 17 00:00:00 2001
> From: Yazen Ghannam <yazen.ghannam@xxxxxxx>
> Date: Mon, 6 Jun 2022 19:45:56 +0000
> Subject: [PATCH] x86/MCE/AMD: Decrement threshold_bank refcount when removing
>  threshold blocks
> 
> AMD systems from Family 10h to 16h share MCA bank 4 across multiple CPUs.
> Therefore, the threshold_bank structure for bank 4, and its threshold_block
> structures, will be initialized once at boot time. And the kobject for the
> shared bank will be added to each of the CPUs that share it. Furthermore,
> the threshold_blocks for the shared bank will be added again to the bank's
> kobject. These additions will increase the refcount for the bank's kobject.
> 
> For example, a shared bank with two blocks and shared across two CPUs will
> be set up like this:
> 
> CPU0 init
>   bank create and add; bank refcount = 1; threshold_create_bank()
>     block 0 init and add; bank refcount = 2; allocate_threshold_blocks()
>     block 1 init and add; bank refcount = 3; allocate_threshold_blocks()
> CPU1 init
>   bank add; bank refcount = 3; threshold_create_bank()
>     block 0 add; bank refcount = 4; __threshold_add_blocks()
>     block 1 add; bank refcount = 5; __threshold_add_blocks()
> 
> Currently in threshold_remove_bank(), if the bank is shared then
> __threshold_remove_blocks() is called. Here the shared bank's kobject and
> the bank's blocks' kobjects are deleted. This is done on the first call
> even while the structures are still shared. Subsequent calls from other
> CPUs that share the structures will attempt to delete the kobjects.
> 
> During kobject_del(), kobject->sd is removed. If the kobject is not part of
> a kset with default_groups, then subsequent kobject_del() calls seem safe
> even with kobject->sd == NULL.
> 
> Originally, the AMD MCA thresholding structures did not use default_groups.
> And so the above behavior was not apparent.
> 
> However, a recent change implemented default_groups for the thresholding
> structures. Therefore, kobject_del() will go down the sysfs_remove_groups()
> code path. In this case, the first kobject_del() may succeed and remove
> kobject->sd. But subsequent kobject_del() calls will give a WARNing in
> kernfs_remove_by_name_ns() since kobject->sd == NULL.
> 
> Use kobject_put() on the shared bank's kobject when "removing" blocks. This
> decrements the bank's refcount while keeping kobjects enabled until the
> bank is no longer shared. At that point, kobject_put() will be called on
> the blocks which drives their refcount to 0 and deletes them and also
> decrementing the bank's refcount. And finally kobject_put() will be called
> on the bank driving its refcount to 0 and deleting it.
> 
> With this patch and the example above:
> 
> CPU1 shutdown
>   bank is shared; bank refcount = 5; threshold_remove_bank()
>     block 0 put parent bank; bank refcount = 4; __threshold_remove_blocks()
>     block 1 put parent bank; bank refcount = 3; __threshold_remove_blocks()
> CPU0 shutdown
>   bank is no longer shared; bank refcount = 3; threshold_remove_bank()
>     block 0 put block; bank refcount = 2; deallocate_threshold_blocks()
>     block 1 put block; bank refcount = 1; deallocate_threshold_blocks()
>   put bank; bank refcount = 0; threshold_remove_bank()
> 
> Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>

Tested-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

> ---
>  arch/x86/kernel/cpu/mce/amd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mce/amd.c b/arch/x86/kernel/cpu/mce/amd.c
> index 2b7ee4a6c6ba..680b75d23a03 100644
> --- a/arch/x86/kernel/cpu/mce/amd.c
> +++ b/arch/x86/kernel/cpu/mce/amd.c
> @@ -1260,10 +1260,10 @@ static void __threshold_remove_blocks(struct threshold_bank *b)
>  	struct threshold_block *pos = NULL;
>  	struct threshold_block *tmp = NULL;
>  
> -	kobject_del(b->kobj);
> +	kobject_put(b->kobj);
>  
>  	list_for_each_entry_safe(pos, tmp, &b->blocks->miscj, miscj)
> -		kobject_del(&pos->kobj);
> +		kobject_put(b->kobj);
>  }
>  
>  static void threshold_remove_bank(struct threshold_bank *bank)
> -- 
> 2.25.1
>