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

[Thomas Gleixner]

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