linux-kernel - Re: [PATCH] dax: make sure inodes are flushed before destroy cache

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Date:   Mon, 14 Feb 2022 09:59:05 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Tong Zhang <ztong0001@...il.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, nvdimm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dax: make sure inodes are flushed before destroy cache

On Fri, Feb 11, 2022 at 11:11:11PM -0800, Tong Zhang wrote:
> A bug can be triggered by following command
> 
> $ modprobe nd_pmem && modprobe -r nd_pmem
> 
> [   10.060014] BUG dax_cache (Not tainted): Objects remaining in dax_cache on __kmem_cache_shutdown()
> [   10.060938] Slab 0x0000000085b729ac objects=9 used=1 fp=0x000000004f5ae469 flags=0x200000000010200(slab|head|node)
> [   10.062433] Call Trace:
> [   10.062673]  dump_stack_lvl+0x34/0x44
> [   10.062865]  slab_err+0x90/0xd0
> [   10.063619]  __kmem_cache_shutdown+0x13b/0x2f0
> [   10.063848]  kmem_cache_destroy+0x4a/0x110
> [   10.064058]  __x64_sys_delete_module+0x265/0x300
> 
> This is caused by dax_fs_exit() not flushing inodes before destroy cache.
> To fix this issue, call rcu_barrier() before destroy cache.

I don't doubt that this fixes the bug.  However, I can't help but think this is
hiding a bug, or perhaps a missing step, in the kmem_cache layer?  As far as I
can see dax does not call call_rcu() and only uses srcu not rcu?  I was tempted
to suggest srcu_barrier() but dax does not call call_srcu() either.

So I'm not clear about what is really going on and why this fixes it.  I know
that dax is not using srcu is a standard way so perhaps this helps in a way I
don't quite grok?  If so perhaps a comment here would be in order?

Ira

> 
> Signed-off-by: Tong Zhang <ztong0001@...il.com>
> ---
>  drivers/dax/super.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index e3029389d809..6bd565fe2e63 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -476,6 +476,7 @@ static int dax_fs_init(void)
>  static void dax_fs_exit(void)
>  {
>  	kern_unmount(dax_mnt);
> +	rcu_barrier();
>  	kmem_cache_destroy(dax_cache);
>  }
>  
> -- 
> 2.25.1
> 
>