Re: [PATCH] kthreads: kill CLONE_KERNEL, change kernel_thread(kernel_init) to avoid CLONE_SIGHAND

[Sasha Levin <sasha.levin@oracle.com>]

On 04/13/2014 03:56 PM, Oleg Nesterov wrote:
> 1. Remove CLONE_KERNEL, it has no users and it is dangerous.
> 
>    The (old) comment says "List of flags we want to share for kernel
>    threads" but this is not true, we do not want to share ->sighand by
>    default. This flag can only be used if the caller is sure that both
>    parent/child will never play with signals (say, allow_signal/etc).
> 
> 2. Change rest_init() to clone kernel_init() without CLONE_SIGHAND.
> 
>    In this case CLONE_SIGHAND does not really hurt, and it looks like
>    optimization because copy_sighand() can avoid kmem_cache_alloc().
> 
>    But in fact this only adds the minor pessimization. kernel_init()
>    is going to exec the init process, and de_thread() will need to
>    unshare ->sighand and do kmem_cache_alloc(sighand_cachep) anyway,
>    but it needs to do more work and take tasklist_lock and siglock.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Hi Oleg,

This patch triggers a hang during boot in my KVM guest. There are no
messages or anything, it just hangs right before init is supposed to
start up.

I've narrowed it down a bit, and it's the removal of CLONE_SIGHAND
that's bothering it. Removing CLONE_FS and CLONE_FILES doesn't
cause the hang on boot.


Thanks,
Sasha