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

[Thomas Gleixner <tglx@kernel.org>]

On Mon, Aug 10 2026 at 13:35, Usama Arif wrote:
> 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.

What's the point of that exercise?

If the user access fails and it can fail with a page fault because
it's preemptible context, then the operation is done and returns -EFAULT
to user space.

This special handling of private futexes does not buy any value at all,
actually it adds a pointless conditional and makes the code
incomprehensible no matter how much effort you put into a comment.

Thanks,

        tglx