Re: [tip: locking/core] locking/rwsem: Remove the list_head from struct rw_semaphore

[Andrei Vagin]

On Mon, Mar 9, 2026 at 12:48 PM tip-bot2 for Matthew Wilcox (Oracle)
<tip-bot2@xxxxxxxxxxxxx> wrote:
>
> The following commit has been merged into the locking/core branch of tip:
>
> Commit-ID:     1ea4b473504b6dc6a0d21c298519aff2d52433c9
> Gitweb:        https://git.kernel.org/tip/1ea4b473504b6dc6a0d21c298519aff2d52433c9
> Author:        Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> AuthorDate:    Thu, 05 Mar 2026 19:55:41
> Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CommitterDate: Sun, 08 Mar 2026 11:06:51 +01:00
>
> locking/rwsem: Remove the list_head from struct rw_semaphore
>
> Instead of embedding a list_head in struct rw_semaphore, store a pointer
> to the first waiter.  The list of waiters remains a doubly linked list
> so we can efficiently add to the tail of the list, remove from the front
> (or middle) of the list.
>
> Some of the list manipulation becomes more complicated, but it's a
> reasonable tradeoff on the slow paths to shrink some core data structures
> like struct inode.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: https://patch.msgid.link/20260305195545.3707590-2-willy@xxxxxxxxxxxxx
> ---
>  include/linux/rwsem.h  |  8 ++--
>  kernel/locking/rwsem.c | 90 ++++++++++++++++++++++++++---------------
>  2 files changed, 62 insertions(+), 36 deletions(-)
>
...
> -static inline void
> -rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
> +static inline
> +bool __rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
>  {
> -       lockdep_assert_held(&sem->wait_lock);
> -       list_add_tail(&waiter->list, &sem->wait_list);
> -       /* caller will set RWSEM_FLAG_WAITERS */
> +       if (list_empty(&waiter->list)) {
> +               sem->first_waiter = NULL;
> +               return true;
> +       }
> +
> +       if (sem->first_waiter == waiter) {
> +               sem->first_waiter = list_first_entry(&waiter->list,
> +                                                    struct rwsem_waiter, list);
> +       }
> +       list_del(&waiter->list);
> +
> +       return false;
>  }
>
>  /*
> @@ -385,14 +392,23 @@ static inline bool
>  rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
>  {
>         lockdep_assert_held(&sem->wait_lock);
> -       list_del(&waiter->list);
> -       if (likely(!list_empty(&sem->wait_list)))
> +       if (__rwsem_del_waiter(sem, waiter))

__rwsem_del_waiter() returns true when the wait list becomes empty.
rwsem_del_waiter() is supposed to return true if the wait list is not empty...

>                 return true;
> -
>         atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count);
>         return false;
>  }

Thanks,
Andrei