linux-kernel - Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem

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Date:   Mon, 18 Nov 2019 16:52:23 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        will@...nel.org
Cc:     oleg@...hat.com, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, juri.lelli@...hat.com, williams@...hat.com,
        bristot@...hat.com, dave@...olabs.net, jack@...e.com
Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem

On 11/13/19 5:21 AM, Peter Zijlstra wrote:
> @@ -130,20 +131,12 @@ static inline void percpu_rwsem_release(
>  					bool read, unsigned long ip)
>  {
>  	lock_release(&sem->dep_map, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> -	if (!read)
> -		atomic_long_set(&sem->rw_sem.owner, RWSEM_OWNER_UNKNOWN);
> -#endif
>  }
>  

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 {
 #endif
 };
 
-/*
- * 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    (-2L)
-
 /* In all implementations count != 0 means locked */
 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 {
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
        unsigned long flags;
        bool ret = true;
 
-       BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
-
        if (need_resched()) {
                lockevent_inc(rwsem_opt_fail);
                return false;

Cheers,
Longman