Re: [PATCHv2 04/12] x86/virt/tdx: Add tdx_alloc/free_page() helpers

[Edgecombe, Rick P]

On Tue, 2025-09-16 at 10:22 +0100, Kiryl Shutsemau wrote:
> My git has comment for the check:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/tree/arch/x86/virt/vmx/tdx/tdx.c?h=tdx/dpamt&id=375706fe73a8499dbdddb22c13d19d7286280ad6#n2160
> 

Yes, I saw but wasn't enough for me.

> Consider the following scenario
> 
> 		CPU0					CPU1
> tdx_pamt_put()
>   atomic_dec_and_test() == true
>   					tdx_pamt_get()
> 					  atomic_inc_not_zero() == false
> 					  tdx_pamt_add()
> 					    <takes pamt_lock>
> 					    // CPU0 never removed PAMT memory
> 					    tdh_phymem_pamt_add() == HPA_RANGE_NOT_FREE
> 					    atomic_set(1);
> 					    <drops pamt_lock>
>   <takes pamt_lock>
>   // Lost the race to CPU1
>   atomic_read() > 0
>   <drop pamt_lock>
> 
> Does it make sense?

Ah, yes thanks. It falls out from the asymmetry of when the inc/dec happens
between get/put.