Re: [PATCH] futex: Avoid hash-bucket locking for mismatched waits

[Usama Arif]

On 08/08/2026 10:19, Thomas Gleixner wrote:
> On Fri, Jul 31 2026 at 12:26, Usama Arif wrote:
>>  
>> +	/*
>> +	 * A mismatch here refuses the wait without locating the hash bucket;
>> +	 * a match is rechecked under the lock below before queueing.
>> +	 *
>> +	 * get_futex_key() runs get_user_pages_fast() only for shared futexes,
>> +	 * so their page is resident and the non-faulting read suffices, with
>> +	 * the locked path recovering if it does not.  A private futex may
> 
> That's a fairy tale:
> 
>        get_futex_key()
>           ...
>           // shared
>           get_user_pages_fast();        // gets a page reference
>           ...
>           folio = page_folio(page);
>           ...
> 
>        out:
>           folio_put(folio);	       // drops the page reference
>           
> Thanks,
> 
>         tglx

I will fix the comment if the discussion in the other thread hopefully
works out and there is a v2. folio_put() does not immediately evict the
page; however as you point out it removes the guarantee that it stays
available.

get_futex_key() has just resolved the shared page, so a non-faulting
read is "very likely" to succeed. If it doesn't, we get -EFAULT and will
then reach the existing locked check and continues to do what is done
without the patch.