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 <tj@kernel.org>
Reviewed-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>

1 files changed, 17 insertions, 1 deletions

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index cd7e20f0fe47..b0293f268cd2 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -63,6 +63,21 @@ enum {
         __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;
         /*
@@ -76,7 +91,8 @@ struct percpu_ref {
 };
 
 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);