[PATCH 3/5] blkcg: perpcu_ref init/exit should be done from blkg_alloc/free()

[Tejun Heo]

blkg alloc is performed as a separate step from the rest of blkg
creation so that GFP_KERNEL allocations can be used when creating
blkgs from configuration file writes because otherwise user actions
may fail due to failures of opportunistic GFP_NOWAIT allocations.

While making blkgs use percpu_ref, 7fcf2b033b84 ("blkcg: change blkg
reference counting to use percpu_ref") incorrectly added unconditional
opportunistic percpu_ref_init() to blkg_create() breaking this
guarantee.

This patch moves percpu_ref_init() to blkg_alloc() so makes it use
@gfp_mask that blkg_alloc() is called with.  Also, percpu_ref_exit()
is moved to blkg_free() for consistency.

Signed-off-by: Tejun Heo <t...@kernel.org>
Fixes: 7fcf2b033b84 ("blkcg: change blkg reference counting to use percpu_ref")
Cc: Dennis Zhou <den...@kernel.org>
---
 block/blk-cgroup.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index e4715b35d42c..04d286934c5e 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -79,6 +79,7 @@ static void blkg_free(struct blkcg_gq *blkg)
 
        blkg_rwstat_exit(&blkg->stat_ios);
        blkg_rwstat_exit(&blkg->stat_bytes);
+       percpu_ref_exit(&blkg->refcnt);
        kfree(blkg);
 }
 
@@ -86,8 +87,6 @@ static void __blkg_release(struct rcu_head *rcu)
 {
        struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);
 
-       percpu_ref_exit(&blkg->refcnt);
-
        /* release the blkcg and parent blkg refs this blkg has been holding */
        css_put(&blkg->blkcg->css);
        if (blkg->parent)
@@ -132,6 +131,9 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, 
struct request_queue *q,
        if (!blkg)
                return NULL;
 
+       if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask))
+               goto err_free;
+
        if (blkg_rwstat_init(&blkg->stat_bytes, gfp_mask) ||
            blkg_rwstat_init(&blkg->stat_ios, gfp_mask))
                goto err_free;
@@ -244,11 +246,6 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
                blkg_get(blkg->parent);
        }
 
-       ret = percpu_ref_init(&blkg->refcnt, blkg_release, 0,
-                             GFP_NOWAIT | __GFP_NOWARN);
-       if (ret)
-               goto err_cancel_ref;
-
        /* invoke per-policy init */
        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];
@@ -281,8 +278,6 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
        blkg_put(blkg);
        return ERR_PTR(ret);
 
-err_cancel_ref:
-       percpu_ref_exit(&blkg->refcnt);
 err_put_congested:
        wb_congested_put(wb_congested);
 err_put_css:
-- 
2.17.1