Re: [PATCH v2] x86/mm/pti: remove dead logic during user pagetable population

[Dave Hansen]

On 01/07/2018 04:05 AM, Thomas Gleixner wrote:
>> static DEFINE_SPINLOCK(shadow_table_allocation_lock);
>>
>> Now I have my suspicions why that's not needed anymore upstream but I'd
>> let tglx explain better.
> We got rid of all that runtime mapping stuff and the functions are only
> called from pti_init(). So the locking and therefor the tests above are not
> needed anymore. While at it we should mark all those function __init.

Yes, the double-test was part of an optimization where we attempted to
avoid using a global spinlock in the fork() path.  We would check for
unallocated mid-level page tables without the lock.  The lock was only
taken it when we needed to *make* an entry to avoid collisions.

Now that it is all single-threaded, there is no chance of a collision,
no need for a lock, and no need for the re-check.

^^ Just in case someone wants to include a bit more first-hand
information about wtf that code was doing there.