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    SubjectRe: [PATCH RFC] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
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    Hi, Jan!

    在 2022/12/27 11:15, Yu Kuai 写道:
    > From: Yu Kuai <yukuai3@huawei.com>
    >
    > The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
    > during code review, we found that bfq_pd_free() update 'bfqg->ref'
    > without holding the lock, which is problematic:
    >
    > 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
    > 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
    > which can concurrenty with 1).
    >
    > Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
    > be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.
    >
    > Signed-off-by: Yu Kuai <yukuai3@huawei.com>
    > ---
    > block/bfq-cgroup.c | 8 +++-----
    > block/bfq-iosched.h | 2 +-
    > 2 files changed, 4 insertions(+), 6 deletions(-)
    >
    > diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
    > index 1b2829e99dad..aa9c4f02e3a3 100644
    > --- a/block/bfq-cgroup.c
    > +++ b/block/bfq-cgroup.c
    > @@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
    >
    > static void bfqg_get(struct bfq_group *bfqg)
    > {
    > - bfqg->ref++;
    > + refcount_inc(&bfqg->ref);
    > }
    >
    > static void bfqg_put(struct bfq_group *bfqg)
    > {
    > - bfqg->ref--;
    > -
    > - if (bfqg->ref == 0)
    > + if (refcount_dec_and_test(bfqg->ref))
    Sorry that here should be '&bfqg->ref'.

    Anyway, I'll wait for you, and send a new version if you think this
    patch make sense.

    Thanks,
    Kuai

    > kfree(bfqg);
    > }
    >
    > @@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
    > }
    >
    > /* see comments in bfq_bic_update_cgroup for why refcounting */
    > - bfqg_get(bfqg);
    > + refcount_set(&bfqg->ref, 1);
    > return &bfqg->pd;
    > }
    >
    > diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
    > index 41aa151ccc22..466e4865ace6 100644
    > --- a/block/bfq-iosched.h
    > +++ b/block/bfq-iosched.h
    > @@ -928,7 +928,7 @@ struct bfq_group {
    > char blkg_path[128];
    >
    > /* reference counter (see comments in bfq_bic_update_cgroup) */
    > - int ref;
    > + refcount_t ref;
    > /* Is bfq_group still online? */
    > bool online;
    >
    >

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