Linux 5.3 compat: rw_semaphore owner

Commit https://github.com/torvalds/linux/commit/94a9717b updated the
rwsem's owner field to contain additional flags describing the rwsem's
state.  Rather then update the wrappers to mask out these bits, the
code no longer relies on the owner stored by the kernel.  This does
increase the size of a krwlock_t but it makes the implementation
less sensitive to future kernel changes.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9029

include/spl/sys/rwlock.h

@@ -78,15 +78,9 @@ typedef enum {
 	RW_READER	= 2
 } krw_t;
 
-/*
- * If CONFIG_RWSEM_SPIN_ON_OWNER is defined, rw_semaphore will have an owner
- * field, so we don't need our own.
- */
 typedef struct {
 	struct rw_semaphore rw_rwlock;
-#ifndef CONFIG_RWSEM_SPIN_ON_OWNER
 	kthread_t *rw_owner;
-#endif
 #ifdef CONFIG_LOCKDEP
 	krw_type_t	rw_type;
 #endif /* CONFIG_LOCKDEP */
@@ -97,31 +91,19 @@ typedef struct {
 static inline void
 spl_rw_set_owner(krwlock_t *rwp)
 {
-/*
- * If CONFIG_RWSEM_SPIN_ON_OWNER is defined, down_write, up_write,
- * downgrade_write and __init_rwsem will set/clear owner for us.
- */
-#ifndef CONFIG_RWSEM_SPIN_ON_OWNER
 	rwp->rw_owner = current;
-#endif
 }
 
 static inline void
 spl_rw_clear_owner(krwlock_t *rwp)
 {
-#ifndef CONFIG_RWSEM_SPIN_ON_OWNER
 	rwp->rw_owner = NULL;
-#endif
 }
 
 static inline kthread_t *
 rw_owner(krwlock_t *rwp)
 {
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
-	return (SEM(rwp)->owner);
-#else
 	return (rwp->rw_owner);
-#endif
 }
 
 #ifdef CONFIG_LOCKDEP
@@ -148,6 +130,11 @@ spl_rw_lockdep_on_maybe(krwlock_t *rwp)			\
 #define	spl_rw_lockdep_on_maybe(rwp)
 #endif /* CONFIG_LOCKDEP */
 
+static inline int
+RW_LOCK_HELD(krwlock_t *rwp)
+{
+	return (spl_rwsem_is_locked(SEM(rwp)));
+}
 
 static inline int
 RW_WRITE_HELD(krwlock_t *rwp)
@@ -155,55 +142,10 @@ RW_WRITE_HELD(krwlock_t *rwp)
 	return (rw_owner(rwp) == current);
 }
 
-static inline int
-RW_LOCK_HELD(krwlock_t *rwp)
-{
-	return (spl_rwsem_is_locked(SEM(rwp)));
-}
-
 static inline int
 RW_READ_HELD(krwlock_t *rwp)
 {
-	if (!RW_LOCK_HELD(rwp))
+	return (RW_LOCK_HELD(rwp) && rw_owner(rwp) == NULL);
-		return (0);
-
-	/*
-	 * rw_semaphore cheat sheet:
-	 *
-	 * < 3.16:
-	 * There's no rw_semaphore.owner, so use rwp.owner instead.
-	 * If rwp.owner == NULL then it's a reader
-	 *
-	 * 3.16 - 4.7:
-	 * rw_semaphore.owner added (https://lwn.net/Articles/596656/)
-	 * and CONFIG_RWSEM_SPIN_ON_OWNER introduced.
-	 * If rw_semaphore.owner == NULL then it's a reader
-	 *
-	 * 4.8 - 4.16.16:
-	 * RWSEM_READER_OWNED added as an internal #define.
-	 * (https://lore.kernel.org/patchwork/patch/678590/)
-	 * If rw_semaphore.owner == 1 then it's a reader
-	 *
-	 * 4.16.17 - 4.19:
-	 * RWSEM_OWNER_UNKNOWN introduced as ((struct task_struct *)-1L)
-	 * (https://do-db2.lkml.org/lkml/2018/5/15/985)
-	 * If rw_semaphore.owner == 1 then it's a reader.
-	 *
-	 * 4.20+:
-	 * RWSEM_OWNER_UNKNOWN changed to ((struct task_struct *)-2L)
-	 * (https://lkml.org/lkml/2018/9/6/986)
-	 * If rw_semaphore.owner & 1 then it's a reader, and also the reader's
-	 * task_struct may be embedded in rw_semaphore->owner.
-	 */
-#if	defined(CONFIG_RWSEM_SPIN_ON_OWNER) && defined(RWSEM_OWNER_UNKNOWN)
-	if (RWSEM_OWNER_UNKNOWN == (struct task_struct *)-2L) {
-		/* 4.20+ kernels with CONFIG_RWSEM_SPIN_ON_OWNER */
-		return ((unsigned long) SEM(rwp)->owner & 1);
-	}
-#endif
-
-	/* < 4.20 kernel or !CONFIG_RWSEM_SPIN_ON_OWNER */
-	return (rw_owner(rwp) == NULL || (unsigned long) rw_owner(rwp) == 1);
 }
 
 /*

module/spl/spl-rwlock.c

@@ -119,9 +119,6 @@ rwsem_tryupgrade(struct rw_semaphore *rwsem)
 	if (__rwsem_tryupgrade(rwsem)) {
 		rwsem_release(&rwsem->dep_map, 1, _RET_IP_);
 		rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
-		rwsem->owner = current;
-#endif
 		return (1);
 	}
 	return (0);