linux-kernel - [PATCH] locking/rwsem: Remove RWSEM_OWNER

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Message-ID: <20200204092228.GP14946@hirez.programming.kicks-ass.net>
Date:   Tue, 4 Feb 2020 10:22:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     mingo@...nel.org, will@...nel.org, oleg@...hat.com,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, juri.lelli@...hat.com, williams@...hat.com,
        bristot@...hat.com, longman@...hat.com, dave@...olabs.net,
        jack@...e.com
Subject: [PATCH] locking/rwsem: Remove RWSEM_OWNER_UNKNOWN

On Tue, Feb 04, 2020 at 09:50:49AM +0100, Peter Zijlstra wrote:

> Anyway, I'll go split, since you seem to care so deeply.

---

Subject: locking/rwsem: Remove RWSEM_OWNER_UNKNOWN
From: Peter Zijlstra <peterz@...radead.org>
Date: Tue Feb  4 09:34:37 CET 2020

Remove the now unused RWSEM_OWNER_UNKNOWN hack. This hack breaks
PREEMPT_RT and getting rid of it was the entire motivation for
re-writing the percpu rwsem.

The biggest problem is that it is fundamentally incompatible with any
form of Priority Inheritance, any exclusively held lock must have a
distinct owner.

Requested-by: Christoph Hellwig <hch@...radead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/rwsem.h  |    6 ------
 kernel/locking/rwsem.c |    2 --
 2 files changed, 8 deletions(-)

--- 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)
 {
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -659,8 +659,6 @@ static inline bool rwsem_can_spin_on_own
 	unsigned long flags;
 	bool ret = true;
 
-	BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
-
 	if (need_resched()) {
 		lockevent_inc(rwsem_opt_fail);
 		return false;