linux-kernel - Re: [PATCH V2] x86/sgx: Fix free page accounting

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Date:   Wed, 10 Nov 2021 20:01:37 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        "Luck, Tony" <tony.luck@...el.com>
Cc:     Reinette Chatre <reinette.chatre@...el.com>,
        dave.hansen@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
        mingo@...hat.com, linux-sgx@...r.kernel.org, x86@...nel.org,
        seanjc@...gle.com, hpa@...or.com, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH V2] x86/sgx: Fix free page accounting

On 11/10/21 7:50 PM, Jarkko Sakkinen wrote:
>>               CPU_A                                 CPU_B
>>               -----                                 -----
>>          spin_lock(&nodeA->lock);              spin_lock(&nodeB->lock);
>>          ...                                   ...
>>          sgx_nr_free_pages--;  /* NOT SAFE */  sgx_nr_free_pages--;
>>
>>          spin_unlock(&nodeA->lock);            spin_unlock(&nodeB->lock);
>>
>> Maybe you missed the "NOT SAFE" hidden in the middle of
>> the picture?
>>
>> -Tony
> For me from that the ordering is not clear. E.g. compare to
> https://www.kernel.org/doc/Documentation/memory-barriers.txt

Jarkko,

Reinette's explanation looks great to me.  Something "protected" by two
different locks is not protected at all.  I don't think we need to fret
over this too much.

We don't need memory barriers or anything fancy at all to explain this.