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Message-ID: <45603be5-65ae-4d45-bfbe-22c3c1e22280@oracle.com>
Date: Wed, 12 Jun 2024 12:33:19 +0100
From: John Garry <john.g.garry@...cle.com>
To: Tejun Heo <tj@...nel.org>, axboe@...nel.dk
Cc: linux-block@...r.kernel.org, cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel-team@...com, newella@...com,
hch@....de
Subject: Re: [PATCH 12/27] blk-iocost: grab ioc->lock for debt handling
On 01/09/2020 19:52, Tejun Heo wrote:
Background:
I have been trying to solve some block/ sparse issues, and it has led me
to digging up this old mail.
> Currently, debt handling requires only iocg->waitq.lock. In the future, we
> want to adjust and propagate inuse changes depending on debt status. Let's
> grab ioc->lock in debt handling paths in preparation.
>
> * Because ioc->lock nests outside iocg->waitq.lock, the decision to grab
> ioc->lock needs to be made before entering the critical sections.
>
> * Add and use iocg_[un]lock() which handles the conditional double locking.
>
> * Add @pay_debt to iocg_kick_waitq() so that debt payment happens only when
> the caller grabbed both locks.
>
> This patch is prepatory and the comments contain references to future
> changes.
>
> Signed-off-by: Tejun Heo<tj@...nel.org>
> ---
> block/blk-iocost.c | 92 ++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 73 insertions(+), 19 deletions(-)
>
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index f36988657594..23b173e34591 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -680,6 +680,26 @@ static void iocg_commit_bio(struct ioc_gq *iocg, struct bio *bio, u64 cost)
> atomic64_add(cost, &iocg->vtime);
> }
>
> +static void iocg_lock(struct ioc_gq *iocg, bool lock_ioc, unsigned long *flags)
> +{
> + if (lock_ioc) {
> + spin_lock_irqsave(&iocg->ioc->lock, *flags);
> + spin_lock(&iocg->waitq.lock);
> + } else {
> + spin_lock_irqsave(&iocg->waitq.lock, *flags);
> + }
> +}
This generates the following sparse warnings on mainline today:
CHECK block/blk-iocost.c
block/blk-iocost.c:685:9: warning: context imbalance in 'iocg_lock' -
wrong count at exit
block/blk-iocost.c:696:28: warning: context imbalance in 'iocg_unlock'
- unexpected unlock
If we try to break iocg_lock() into one version for lock_ioc set and
another for lock_ioc unset, we can solve the sparse issues for those
functions, but then we get another sparse issue from the callsites for
those functions:
block/blk-iocost.c:2679:17: warning: context imbalance in
'ioc_rqos_throttle' - different lock contexts for basic block
I tried to solve with a total ioc_rqos_throttle() re-org and much code
duplication by calling the different lock and unlock versions from
effectively 2x separate copies of ioc_rqos_throttle(), as sparse seems
confused with how we call these functions. It's a total no-go.
Any simpler idea to solve these? Or just something to live with?
Thanks,
John
> +
> +static void iocg_unlock(struct ioc_gq *iocg, bool unlock_ioc, unsigned long *flags)
> +{
> + if (unlock_ioc) {
> + spin_unlock(&iocg->waitq.lock);
> + spin_unlock_irqrestore(&iocg->ioc->lock, *flags);
> + } else {
> + spin_unlock_irqrestore(&iocg->waitq.lock, *flags);
> + }
> +}
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