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, 18 insertions, 5 deletions

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 5a6d43baccc5..ed280fb1e5b5 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -45,27 +45,40 @@ static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
  * 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;
 }