[syzbot] [afs?] BUG: sleeping function called from invalid context in __do_fault

[syzbot]

> #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

This crash does not have a reproducer. I cannot test it.

>
> commit db7a516159869b19f237c73bd75599bbe0bfcc4d
> Author: David Howells <dhowells at redhat.com>
> Date:   Fri Mar 28 16:46:58 2025 +0000
>
>     afs: Fix afs_dynroot_readdir() to not use the RCU read lock
>     
>     afs_dynroot_readdir() uses the RCU read lock to walk the cell list whilst
>     emitting cell automount entries - but dir_emit() may write to a userspace
>     buffer, thereby causing a fault to occur and waits to happen.
>     
>     Fix afs_dynroot_readdir() to get a shared lock on net->cells_lock instead.
>     
>     Fixes: 1d0b929fc070 ("afs: Change dynroot to create contents on demand")
>     Reported-by: syzbot+3b6c5c6a1d0119b687a1 at syzkaller.appspotmail.com
>     Signed-off-by: David Howells <dhowells at redhat.com>
>     cc: Marc Dionne <marc.dionne at auristor.com>
>     cc: linux-afs at lists.infradead.org
>     cc: linux-fsdevel at vger.kernel.org
>
> diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
> index 691e0ae607a1..8c6130789fde 100644
> --- a/fs/afs/dynroot.c
> +++ b/fs/afs/dynroot.c
> @@ -348,9 +348,9 @@ static int afs_dynroot_readdir(struct file *file, struct dir_context *ctx)
>  	}
>  
>  	if ((unsigned long long)ctx->pos <= AFS_MAX_DYNROOT_CELL_INO) {
> -		rcu_read_lock();
> +		down_read(&net->cells_lock);
>  		ret = afs_dynroot_readdir_cells(net, ctx);
> -		rcu_read_unlock();
> +		up_read(&net->cells_lock);
>  	}
>  	return ret;
>  }
>