Add rw_tryupgrade()

This implementation of rw_tryupgrade() behaves slightly differently
from its counterparts on other platforms.  It drops the RW_READER lock
and then acquires the RW_WRITER lock leaving a small window where no
lock is held.  On other platforms the lock is never released during
the upgrade process.  This is necessary under Linux because the kernel
does not provide an upgrade function.

There are currently no callers in the ZFS code where this change in
behavior is a problem.  In fact, in most cases the code is already
written such that if the upgrade fails the RW_READER lock is dropped
and the caller blocks waiting to acquire the lock as RW_WRITER.

Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tim Chase <[email protected]>
Signed-off-by: Matthew Thode <[email protected]>
Closes zfsonlinux/zfs#4388
Closes #534

2 files changed, 0 insertions, 66 deletions

diff --git a/module/spl/spl-rwlock.c b/module/spl/spl-rwlock.c
index 462a6f0de..98251c011 100644
--- a/module/spl/spl-rwlock.c
+++ b/module/spl/spl-rwlock.c
@@ -32,65 +32,5 @@
 
 #define DEBUG_SUBSYSTEM S_RWLOCK
 
-#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
-
-/*
- * From lib/rwsem-spinlock.c but modified such that the caller is
- * responsible for acquiring and dropping the sem->wait_lock.
- */
-struct rwsem_waiter {
-        struct list_head list;
-        struct task_struct *task;
-        unsigned int flags;
-#define RWSEM_WAITING_FOR_READ  0x00000001
-#define RWSEM_WAITING_FOR_WRITE 0x00000002
-};
-
-/* wake a single writer */
-static struct rw_semaphore *
-__rwsem_wake_one_writer_locked(struct rw_semaphore *sem)
-{
-        struct rwsem_waiter *waiter;
-        struct task_struct *tsk;
-
-        sem->activity = -1;
-
-        waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
-        list_del(&waiter->list);
-
-        tsk = waiter->task;
-        smp_mb();
-        waiter->task = NULL;
-        wake_up_process(tsk);
-        put_task_struct(tsk);
-        return sem;
-}
-
-/* release a read lock on the semaphore */
-void
-__up_read_locked(struct rw_semaphore *sem)
-{
-        if (--sem->activity == 0 && !list_empty(&sem->wait_list))
-                (void)__rwsem_wake_one_writer_locked(sem);
-}
-EXPORT_SYMBOL(__up_read_locked);
-
-/* trylock for writing -- returns 1 if successful, 0 if contention */
-int
-__down_write_trylock_locked(struct rw_semaphore *sem)
-{
-        int ret = 0;
-
-        if (sem->activity == 0 && list_empty(&sem->wait_list)) {
-                sem->activity = -1;
-                ret = 1;
-        }
-
-        return ret;
-}
-EXPORT_SYMBOL(__down_write_trylock_locked);
-
-#endif
-
 int spl_rw_init(void) { return 0; }
 void spl_rw_fini(void) { }
diff --git a/module/splat/splat-rwlock.c b/module/splat/splat-rwlock.c
index 284f77370..abd6a0e60 100644
--- a/module/splat/splat-rwlock.c
+++ b/module/splat/splat-rwlock.c
@@ -588,7 +588,6 @@ splat_rwlock_test6(struct file *file, void *arg)
 		goto out;
 	}
 
-#if defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
 	/* With one reader upgrade should never fail. */
 	rc = rw_tryupgrade(&rwp->rw_rwlock);
 	if (!rc) {
@@ -610,11 +609,6 @@ splat_rwlock_test6(struct file *file, void *arg)
 	rc = 0;
 	splat_vprint(file, SPLAT_RWLOCK_TEST6_NAME, "%s",
 		     "rwlock properly upgraded\n");
-#else
-	rc = 0;
-	splat_vprint(file, SPLAT_RWLOCK_TEST6_NAME, "%s",
-		     "rw_tryupgrade() is disabled for this arch\n");
-#endif
 out:
 	rw_exit(&rwp->rw_rwlock);
 	rw_destroy(&rwp->rw_rwlock);