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 <ztong0...@gmail.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
>
>
