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Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
To:     Peter Zijlstra <peterz@infradead.org>, mingo@kernel.org,
        will@kernel.org
Cc:     oleg@redhat.com, tglx@linutronix.de, linux-kernel@vger.kernel.org,
        bigeasy@linutronix.de, juri.lelli@redhat.com, williams@redhat.com,
        bristot@redhat.com, dave@stgolabs.net, jack@suse.com
References: <20191113102115.116470462@infradead.org>
 <20191113102855.925208237@infradead.org>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <7b412d15-b796-3b8d-0e98-22362377093f@redhat.com>
Date:   Mon, 18 Nov 2019 16:52:23 -0500
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On 11/13/19 5:21 AM, Peter Zijlstra wrote:
> @@ -130,20 +131,12 @@ static inline void percpu_rwsem_release(
>  =09=09=09=09=09bool read, unsigned long ip)
>  {
>  =09lock_release(&sem->dep_map, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> -=09if (!read)
> -=09=09atomic_long_set(&sem->rw_sem.owner, RWSEM_OWNER_UNKNOWN);
> -#endif
>  }
> =20

This is the only place that set RWSEM_OWNER_UNKNOWN. So you may as well
remove all references to it:

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 00d6054687dd..8a418d9eeb7a 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -53,12 +53,6 @@ struct rw_semaphore {
=C2=A0#endif
=C2=A0};
=C2=A0
-/*
- * Setting all bits of the owner field except bit 0 will indicate
- * that the rwsem is writer-owned with an unknown owner.
- */
-#define RWSEM_OWNER_UNKNOWN=C2=A0=C2=A0=C2=A0 (-2L)
-
=C2=A0/* In all implementations count !=3D 0 means locked */
=C2=A0static inline int rwsem_is_locked(struct rw_semaphore *sem)
=C2=A0{
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index eef04551eae7..622842754c73 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -660,8 +660,6 @@ static inline bool rwsem_can_spin_on_owner(struct
rw_semapho
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 unsigned long flags;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bool ret =3D true;
=C2=A0
-=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & =
RWSEM_NONSPINNABLE));
-
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (need_resched()) {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0 lockevent_inc(rwsem_opt_fail);
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0 return false;

Cheers,
Longman