kstat to use private lock by default

While porting Illumos #3537 I found that ks_lock member of kstat_t
structure is different between Illumos and SPL. It is a pointer to
the kmutex_t in Illumos, but the mutex lock itself in SPL.
Apparently Illumos kstat API allows consumer to override the lock
if required. With SPL implementation it is not possible anymore.

Things were alright until the first attempt to actually override
the lock. Porting of Illumos #3537 introduced such code for the
first time.

In order to provide the Solaris/Illumos like functionality we:
  1. convert ks_lock to "kmutex_t *ks_lock"
  2. create a new field "kmutex_t ks_private_lock"
  3. On kstat_create() ks_lock = &ks_private_lock

Thus if consumer doesn't care we still have our internal lock in use.
If, however, consumer does care she has a chance to set ks_lock to
anything else before calling kstat_install().

The rest of the code will use ks_lock regardless of its origin.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#286

include/sys/kstat.h

@@ -114,7 +114,8 @@ struct kstat_s {
         struct proc_dir_entry *ks_proc;             /* proc linkage */
         kstat_update_t   *ks_update;                /* dynamic updates */
         void             *ks_private;               /* private data */
-        kmutex_t         ks_lock;                   /* kstat data lock */
+	kmutex_t         ks_private_lock;           /* kstat private data lock */
+	kmutex_t         *ks_lock;                  /* kstat data lock */
         struct list_head ks_list;                   /* kstat linkage */
 	kstat_module_t   *ks_owner;                 /* kstat module linkage */
 	kstat_raw_ops_t  ks_raw_ops;                /* ops table for raw type */

module/spl/spl-kstat.c

@@ -314,7 +314,7 @@ kstat_seq_start(struct seq_file *f, loff_t *pos)
         ASSERT(ksp->ks_magic == KS_MAGIC);
         SENTRY;
 
-        mutex_enter(&ksp->ks_lock);
+	mutex_enter(ksp->ks_lock);
 
         if (ksp->ks_type == KSTAT_TYPE_RAW) {
                 ksp->ks_raw_bufsize = PAGE_SIZE;
@@ -352,13 +352,13 @@ kstat_seq_next(struct seq_file *f, void *p, loff_t *pos)
 static void
 kstat_seq_stop(struct seq_file *f, void *v)
 {
-        kstat_t *ksp = (kstat_t *)f->private;
-        ASSERT(ksp->ks_magic == KS_MAGIC);
+	kstat_t *ksp = (kstat_t *)f->private;
+	ASSERT(ksp->ks_magic == KS_MAGIC);
 
 	if (ksp->ks_type == KSTAT_TYPE_RAW)
 		vmem_free(ksp->ks_raw_buf, ksp->ks_raw_bufsize);
 
-        mutex_exit(&ksp->ks_lock);
+	mutex_exit(ksp->ks_lock);
 }
 
 static struct seq_operations kstat_seq_ops = {
@@ -491,7 +491,8 @@ __kstat_create(const char *ks_module, int ks_instance, const char *ks_name,
 	mutex_exit(&kstat_module_lock);
 
         ksp->ks_magic = KS_MAGIC;
-	mutex_init(&ksp->ks_lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&ksp->ks_private_lock, NULL, MUTEX_DEFAULT, NULL);
+	ksp->ks_lock = &ksp->ks_private_lock;
 	INIT_LIST_HEAD(&ksp->ks_list);
 
 	ksp->ks_crtime = gethrtime();
@@ -576,7 +577,7 @@ __kstat_install(kstat_t *ksp)
 
 	list_add_tail(&ksp->ks_list, &module->ksm_kstat_list);
 
-	mutex_enter(&ksp->ks_lock);
+	mutex_enter(ksp->ks_lock);
 	ksp->ks_owner = module;
 	ksp->ks_proc = proc_create_data(ksp->ks_name, 0644,
 	    module->ksm_proc, &proc_kstat_operations, (void *)ksp);
@@ -585,7 +586,7 @@ __kstat_install(kstat_t *ksp)
 		if (list_empty(&module->ksm_kstat_list))
 			kstat_delete_module(module);
 	}
-	mutex_exit(&ksp->ks_lock);
+	mutex_exit(ksp->ks_lock);
 out:
 	mutex_exit(&kstat_module_lock);
 }
@@ -611,7 +612,8 @@ __kstat_delete(kstat_t *ksp)
 	if (!(ksp->ks_flags & KSTAT_FLAG_VIRTUAL))
 		kmem_free(ksp->ks_data, ksp->ks_data_size);
 
-	mutex_destroy(&ksp->ks_lock);
+	ksp->ks_lock = NULL;
+	mutex_destroy(&ksp->ks_private_lock);
 	kmem_free(ksp, sizeof(*ksp));
 
 	return;