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

[Dave Hansen]

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:

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.

Does anyone disagree?