linux-kernel - Re: [RFC PATCH v5 16/45] x86/virt/tdx: Add tdx_alloc/free_control

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Message-ID: <b9b4b1aa3efad11e20b5935e08287d78825fefdd.camel@intel.com>
Date: Tue, 10 Feb 2026 23:02:10 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "Hansen, Dave" <dave.hansen@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "x86@...nel.org" <x86@...nel.org>, "kas@...nel.org"
	<kas@...nel.org>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"bp@...en8.de" <bp@...en8.de>, "tglx@...nel.org" <tglx@...nel.org>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>
CC: "ackerleytng@...gle.com" <ackerleytng@...gle.com>, "sagis@...gle.com"
	<sagis@...gle.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "Annapurve, Vishal" <vannapurve@...gle.com>
Subject: Re: [RFC PATCH v5 16/45] x86/virt/tdx: Add
 tdx_alloc/free_control_page() helpers

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.