Re: [RFC PATCH v5 16/45] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Huang, Kai]

On Tue, 2026-02-10 at 14:50 -0800, Dave Hansen wrote:
> On 2/10/26 14:46, Huang, Kai wrote:
> > Sorry I am a bit confused.  But I think the "1=>0 and lock" are atomic
> > together?
> 
> Maybe I'm being pedantic. The 1=>0 happens under the lock, but the 1=>0
> and the lock acquisition itself are not atomic. You can see them
> happening at different times:

Oh I see.  Thanks.

> 
> int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
> {
>         /* Subtract 1 from counter unless that drops it to 0...
>         if (atomic_add_unless(atomic, -1, 1))
>                 return 0;
> 
>         /* Otherwise do it the slow way */
>         spin_lock(lock);
>         if (atomic_dec_and_test(atomic))
>                 return 1;
>         spin_unlock(lock);
>         return 0;
> }
> 
> tl;dr: Kirill was right, atomic_dec_and_test() doesn't work by itself here.
> 
> But I think atomic_dec_and_lock() will.

Agreed.