percpu_ref: add PERCPU_REF_INIT_* flags

With the recent addition of percpu_ref_reinit(), percpu_ref now can be
used as a persistent switch which can be turned on and off repeatedly
where turning off maps to killing the ref and waiting for it to drain;
however, there currently isn't a way to initialize a percpu_ref in its
off (killed and drained) state, which can be inconvenient for certain
persistent switch use cases.

Similarly, percpu_ref_switch_to_atomic/percpu() allow dynamic
selection of operation mode; however, currently a newly initialized
percpu_ref is always in percpu mode making it impossible to avoid the
latency overhead of switching to atomic mode.

This patch adds @flags to percpu_ref_init() and implements the
following flags.

* PERCPU_REF_INIT_ATOMIC : start ref in atomic mode
* PERCPU_REF_INIT_DEAD : start ref killed and drained

These flags should be able to serve the above two use cases.

v2: target_core_tpg.c conversion was missing. Fixed.

Signed-off-by: Tejun Heo <[e-mail]>
Reviewed-by: Kent Overstreet <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Johannes Weiner <[e-mail]>

block/blk-mq.c

···
 		goto err_hctxs;
 
 	if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
-			    GFP_KERNEL))
+			    0, GFP_KERNEL))
 		goto err_map;
 
 	setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);

drivers/target/target_core_tpg.c

···
 {
 	int ret;
 
-	ret = percpu_ref_init(&lun->lun_ref, core_tpg_lun_ref_release,
+	ret = percpu_ref_init(&lun->lun_ref, core_tpg_lun_ref_release, 0,
 			      GFP_KERNEL);
 	if (ret < 0)
 		return ret;

fs/aio.c

···
 
 	INIT_LIST_HEAD(&ctx->active_reqs);
 
-	if (percpu_ref_init(&ctx->users, free_ioctx_users, GFP_KERNEL))
+	if (percpu_ref_init(&ctx->users, free_ioctx_users, 0, GFP_KERNEL))
 		goto err;
 
-	if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs, GFP_KERNEL))
+	if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs, 0, GFP_KERNEL))
 		goto err;
 
 	ctx->cpu = alloc_percpu(struct kioctx_cpu);

include/linux/percpu-refcount.h

···
 	__PERCPU_REF_FLAG_BITS	= 2,
 };
 
+/* @flags for percpu_ref_init() */
+enum {
+	/*
+	 * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
+	 * operation using percpu_ref_switch_to_percpu().
+	 */
+	PERCPU_REF_INIT_ATOMIC	= 1 << 0,
+
+	/*
+	 * Start dead w/ ref == 0 in atomic mode.  Must be revived with
+	 * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
+	 */
+	PERCPU_REF_INIT_DEAD	= 1 << 1,
+};
+
 struct percpu_ref {
 	atomic_long_t		count;
 	/*
···
 };
 
 int __must_check percpu_ref_init(struct percpu_ref *ref,
-				 percpu_ref_func_t *release, gfp_t gfp);
+				 percpu_ref_func_t *release, unsigned int flags,
+				 gfp_t gfp);
 void percpu_ref_exit(struct percpu_ref *ref);
 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_switch);

kernel/cgroup.c

···
 		goto out;
 	root_cgrp->id = ret;
 
-	ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, GFP_KERNEL);
+	ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, 0,
+			      GFP_KERNEL);
 	if (ret)
 		goto out;
 
···
 
 	init_and_link_css(css, ss, cgrp);
 
-	err = percpu_ref_init(&css->refcnt, css_release, GFP_KERNEL);
+	err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
 	if (err)
 		goto err_free_css;
 
···
 		goto out_unlock;
 	}
 
-	ret = percpu_ref_init(&cgrp->self.refcnt, css_release, GFP_KERNEL);
+	ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
 	if (ret)
 		goto out_free_cgrp;
 

lib/percpu-refcount.c

···
  * percpu_ref_init - initialize a percpu refcount
  * @ref: percpu_ref to initialize
  * @release: function which will be called when refcount hits 0
+ * @flags: PERCPU_REF_INIT_* flags
  * @gfp: allocation mask to use
  *
- * Initializes the refcount in single atomic counter mode with a refcount of 1;
- * analagous to atomic_long_set(ref, 1).
+ * Initializes @ref.  If @flags is zero, @ref starts in percpu mode with a
+ * refcount of 1; analagous to atomic_long_set(ref, 1).  See the
+ * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
  *
  * Note that @release must not sleep - it may potentially be called from RCU
  * callback context by percpu_ref_kill().
  */
 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
-		    gfp_t gfp)
+		    unsigned int flags, gfp_t gfp)
 {
 	size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
 			     __alignof__(unsigned long));
-
-	atomic_long_set(&ref->count, 1 + PERCPU_COUNT_BIAS);
+	unsigned long start_count = 0;
 
 	ref->percpu_count_ptr = (unsigned long)
 		__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
 	if (!ref->percpu_count_ptr)
 		return -ENOMEM;
+
+	if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
+		ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
+	else
+		start_count += PERCPU_COUNT_BIAS;
+
+	if (flags & PERCPU_REF_INIT_DEAD)
+		ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
+	else
+		start_count++;
+
+	atomic_long_set(&ref->count, start_count);
 
 	ref->release = release;
 	return 0;