Linux 5.3 compat: retire rw_tryupgrade()

The Linux kernel's rwsem's have never provided an interface to
allow a reader to be upgraded to a writer.  Historically, this
functionality has been implemented by a SPL wrapper function.
However, this approach depends on internal knowledge of the
rw_semaphore and is therefore rather brittle.

Since the ZFS code must always be able to fallback to rw_exit()
and rw_enter() when an rw_tryupgrade() fails; this functionality
isn't critical.  Furthermore, the only potentially performance
sensitive consumer is dmu_zfetch() and no decrease in performance
was observed with this change applied.  See the PR comments for
additional testing details.

Therefore, it is being retired to make the build more robust and
to simplify the rwlock implementation.

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Tomohiro Kusumi <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #9029

1 files changed, 7 insertions, 53 deletions

diff --git a/include/spl/sys/rwlock.h b/include/spl/sys/rwlock.h
index 5e052b532..89e02fa8f 100644
--- a/include/spl/sys/rwlock.h
+++ b/include/spl/sys/rwlock.h
@@ -29,43 +29,6 @@
 #include <linux/rwsem.h>
 #include <linux/sched.h>
 
-/* Linux kernel compatibility */
-#if defined(CONFIG_PREEMPT_RT_FULL)
-#define	SPL_RWSEM_SINGLE_READER_VALUE	(1)
-#define	SPL_RWSEM_SINGLE_WRITER_VALUE	(0)
-#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
-#define	SPL_RWSEM_SINGLE_READER_VALUE	(1)
-#define	SPL_RWSEM_SINGLE_WRITER_VALUE	(-1)
-#elif defined(RWSEM_ACTIVE_MASK)
-#define	SPL_RWSEM_SINGLE_READER_VALUE	(RWSEM_ACTIVE_READ_BIAS)
-#define	SPL_RWSEM_SINGLE_WRITER_VALUE	(RWSEM_ACTIVE_WRITE_BIAS)
-#endif
-
-/* Linux 3.16 changed activity to count for rwsem-spinlock */
-#if defined(CONFIG_PREEMPT_RT_FULL)
-#define	RWSEM_COUNT(sem)	sem->read_depth
-#elif defined(HAVE_RWSEM_ACTIVITY)
-#define	RWSEM_COUNT(sem)	sem->activity
-/* Linux 4.8 changed count to an atomic_long_t for !rwsem-spinlock */
-#elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
-#define	RWSEM_COUNT(sem)	atomic_long_read(&(sem)->count)
-#else
-#define	RWSEM_COUNT(sem)	sem->count
-#endif
-
-#if defined(RWSEM_SPINLOCK_IS_RAW)
-#define	spl_rwsem_lock_irqsave(lk, fl)		raw_spin_lock_irqsave(lk, fl)
-#define	spl_rwsem_unlock_irqrestore(lk, fl)	\
-    raw_spin_unlock_irqrestore(lk, fl)
-#define	spl_rwsem_trylock_irqsave(lk, fl)	raw_spin_trylock_irqsave(lk, fl)
-#else
-#define	spl_rwsem_lock_irqsave(lk, fl)		spin_lock_irqsave(lk, fl)
-#define	spl_rwsem_unlock_irqrestore(lk, fl)	spin_unlock_irqrestore(lk, fl)
-#define	spl_rwsem_trylock_irqsave(lk, fl)	spin_trylock_irqsave(lk, fl)
-#endif /* RWSEM_SPINLOCK_IS_RAW */
-
-#define	spl_rwsem_is_locked(rwsem)		rwsem_is_locked(rwsem)
-
 typedef enum {
 	RW_DRIVER	= 2,
 	RW_DEFAULT	= 4,
@@ -133,7 +96,7 @@ spl_rw_lockdep_on_maybe(krwlock_t *rwp)			\
 static inline int
 RW_LOCK_HELD(krwlock_t *rwp)
 {
-	return (spl_rwsem_is_locked(SEM(rwp)));
+	return (rwsem_is_locked(SEM(rwp)));
 }
 
 static inline int
@@ -170,6 +133,12 @@ RW_READ_HELD(krwlock_t *rwp)
  */
 #define	rw_destroy(rwp)		((void) 0)
 
+/*
+ * Upgrading a rwsem from a reader to a writer is not supported by the
+ * Linux kernel.  The lock must be dropped and reacquired as a writer.
+ */
+#define	rw_tryupgrade(rwp)	RW_WRITE_HELD(rwp)
+
 #define	rw_tryenter(rwp, rw)						\
 ({									\
 	int _rc_ = 0;							\
@@ -228,24 +197,9 @@ RW_READ_HELD(krwlock_t *rwp)
 	spl_rw_lockdep_on_maybe(rwp);					\
 })
 
-#define	rw_tryupgrade(rwp)						\
-({									\
-	int _rc_ = 0;							\
-									\
-	if (RW_WRITE_HELD(rwp)) {					\
-		_rc_ = 1;						\
-	} else {							\
-		spl_rw_lockdep_off_maybe(rwp);				\
-		if ((_rc_ = rwsem_tryupgrade(SEM(rwp))))		\
-			spl_rw_set_owner(rwp);				\
-		spl_rw_lockdep_on_maybe(rwp);				\
-	}								\
-	_rc_;								\
-})
 /* END CSTYLED */
 
 int spl_rw_init(void);
 void spl_rw_fini(void);
-int rwsem_tryupgrade(struct rw_semaphore *rwsem);
 
 #endif /* _SPL_RWLOCK_H */