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, 0 insertions, 101 deletions

diff --git a/module/spl/spl-rwlock.c b/module/spl/spl-rwlock.c
index 886e16924..10f7c38db 100644
--- a/module/spl/spl-rwlock.c
+++ b/module/spl/spl-rwlock.c
@@ -24,106 +24,5 @@
  *  Solaris Porting Layer (SPL) Reader/Writer Lock Implementation.
  */
 
-#include <sys/rwlock.h>
-#include <linux/module.h>
-
-#if defined(CONFIG_PREEMPT_RT_FULL)
-
-#include <linux/rtmutex.h>
-#define	RT_MUTEX_OWNER_MASKALL	1UL
-
-static int
-__rwsem_tryupgrade(struct rw_semaphore *rwsem)
-{
-#if defined(READER_BIAS) && defined(WRITER_BIAS)
-	/*
-	 * After the 4.9.20-rt16 kernel the realtime patch series lifted the
-	 * single reader restriction.  While this could be accommodated by
-	 * adding additional compatibility code assume the rwsem can never
-	 * be upgraded.  All caller must already cleanly handle this case.
-	 */
-	return (0);
-#else
-	ASSERT((struct task_struct *)
-	    ((unsigned long)rwsem->lock.owner & ~RT_MUTEX_OWNER_MASKALL) ==
-	    current);
-
-	/*
-	 * Prior to 4.9.20-rt16 kernel the realtime patch series, rwsem is
-	 * implemented as a single mutex held by readers and writers alike.
-	 * However, this implementation would prevent a thread from taking
-	 * a read lock twice, as the mutex would already be locked on
-	 * the second attempt. Therefore the implementation allows a
-	 * single thread to take a rwsem as read lock multiple times
-	 * tracking that nesting as read_depth counter.
-	 */
-	if (rwsem->read_depth <= 1) {
-		/*
-		 * In case, the current thread has not taken the lock
-		 * more than once as read lock, we can allow an
-		 * upgrade to a write lock. rwsem_rt.h implements
-		 * write locks as read_depth == 0.
-		 */
-		rwsem->read_depth = 0;
-		return (1);
-	}
-	return (0);
-#endif
-}
-#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
-static int
-__rwsem_tryupgrade(struct rw_semaphore *rwsem)
-{
-	int ret = 0;
-	unsigned long flags;
-	spl_rwsem_lock_irqsave(&rwsem->wait_lock, flags);
-	if (RWSEM_COUNT(rwsem) == SPL_RWSEM_SINGLE_READER_VALUE &&
-	    list_empty(&rwsem->wait_list)) {
-		ret = 1;
-		RWSEM_COUNT(rwsem) = SPL_RWSEM_SINGLE_WRITER_VALUE;
-	}
-	spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
-	return (ret);
-}
-#elif defined(RWSEM_ACTIVE_MASK)
-#if defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
-static int
-__rwsem_tryupgrade(struct rw_semaphore *rwsem)
-{
-	long val;
-	val = atomic_long_cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
-	    SPL_RWSEM_SINGLE_WRITER_VALUE);
-	return (val == SPL_RWSEM_SINGLE_READER_VALUE);
-}
-#else
-static int
-__rwsem_tryupgrade(struct rw_semaphore *rwsem)
-{
-	typeof(rwsem->count) val;
-	val = cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
-	    SPL_RWSEM_SINGLE_WRITER_VALUE);
-	return (val == SPL_RWSEM_SINGLE_READER_VALUE);
-}
-#endif
-#else
-static int
-__rwsem_tryupgrade(struct rw_semaphore *rwsem)
-{
-	return (0);
-}
-#endif
-
-int
-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_);
-		return (1);
-	}
-	return (0);
-}
-EXPORT_SYMBOL(rwsem_tryupgrade);
-
 int spl_rw_init(void) { return 0; }
 void spl_rw_fini(void) { }