I noticed that the atomic_dec_and_lock_irqsave() in put_ucounts() loops
sometimes even during boot. Something like 2-3 iterations but still.
This series replaces the refcounting with rcuref_t and adds a RCU
lookup.
This allows a lockless lookup in alloc_ucounts() if the entry is
available and a cmpxchg()less put of the item.

Patch #1 adds HLIST_NULLS_HEAD_INIT for which I added the RCU folks.

Sebastian

Sebastian Andrzej Siewior (4):
  rcu: Provide a static initializer for hlist_nulls_head.
  ucount: Replace get_ucounts_or_wrap() with atomic_inc_not_zero().
  ucount: Use RCU for ucounts lookups.
  ucount: Use rcuref_t for reference counting.

 include/linux/list_nulls.h     |  1 +
 include/linux/user_namespace.h | 15 ++++--
 kernel/ucount.c                | 97 +++++++++++++++-------------------
 3 files changed, 55 insertions(+), 58 deletions(-)

-- 
2.47.2

On 2025-02-03 16:05:21 [+0100], To linux-kernel@vger.kernel.org wrote:
> I noticed that the atomic_dec_and_lock_irqsave() in put_ucounts() loops
> sometimes even during boot. Something like 2-3 iterations but still.
> This series replaces the refcounting with rcuref_t and adds a RCU
> lookup.
> This allows a lockless lookup in alloc_ucounts() if the entry is
> available and a cmpxchg()less put of the item.

Andrew, could you please take this series?

Sebastian

On Mon, Feb 03, 2025 at 04:05:21PM +0100, Sebastian Andrzej Siewior wrote:
> I noticed that the atomic_dec_and_lock_irqsave() in put_ucounts() loops
> sometimes even during boot. Something like 2-3 iterations but still.
> This series replaces the refcounting with rcuref_t and adds a RCU
> lookup.
> This allows a lockless lookup in alloc_ucounts() if the entry is
> available and a cmpxchg()less put of the item.
> 
> Patch #1 adds HLIST_NULLS_HEAD_INIT for which I added the RCU folks.

The modification and later elimination of get_ucounts() threw me for
a bit, but for the series:

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> Sebastian
> 
> Sebastian Andrzej Siewior (4):
>   rcu: Provide a static initializer for hlist_nulls_head.
>   ucount: Replace get_ucounts_or_wrap() with atomic_inc_not_zero().
>   ucount: Use RCU for ucounts lookups.
>   ucount: Use rcuref_t for reference counting.
> 
>  include/linux/list_nulls.h     |  1 +
>  include/linux/user_namespace.h | 15 ++++--
>  kernel/ucount.c                | 97 +++++++++++++++-------------------
>  3 files changed, 55 insertions(+), 58 deletions(-)
> 
> -- 
> 2.47.2
>