[tip: locking/core] locking/rwsem: Remove RWSEM_OWNER_UNKNOWN

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     bcba67cd806800fa8e973ac49dbc7d2d8fb3e55e
Gitweb:        https://git.kernel.org/tip/bcba67cd806800fa8e973ac49dbc7d2d8fb3e55e
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 04 Feb 2020 09:34:37 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 11 Feb 2020 13:10:57 +01:00

locking/rwsem: Remove RWSEM_OWNER_UNKNOWN

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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Tested-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200204092228.GP14946@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 include/linux/rwsem.h  | 6 ------
 kernel/locking/rwsem.c | 2 --
 2 files changed, 8 deletions(-)

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 00d6054..8a418d9 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 81c0d75..e6f437b 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -659,8 +659,6 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
 	unsigned long flags;
 	bool ret = true;
 
-	BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
-
 	if (need_resched()) {
 		lockevent_inc(rwsem_opt_fail);
 		return false;