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SubjectRe: [PATCH -next 2/5] blk-iocost: don't release 'ioc->lock' while updating params
On Tue, Oct 11, 2022 at 04:35:44PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
>
> ioc_qos_write() and ioc_cost_model_write() are the same:
>
> 1) hold lock to read 'ioc->params' to local variable;
> 2) update params to local variable without lock;
> 3) hold lock to write local variable to 'ioc->params';
>
> In theroy, if user updates params concurrenty, the params might be lost:
>
> t1: update params a t2: update params b
> spin_lock_irq(&ioc->lock);
> memcpy(qos, ioc->params.qos, sizeof(qos))
> spin_unlock_irq(&ioc->lock);
>
> qos[a] = xxx;
>
> spin_lock_irq(&ioc->lock);
> memcpy(qos, ioc->params.qos, sizeof(qos))
> spin_unlock_irq(&ioc->lock);
>
> qos[b] = xxx;
>
> spin_lock_irq(&ioc->lock);
> memcpy(ioc->params.qos, qos, sizeof(qos));
> ioc_refresh_params(ioc, true);
> spin_unlock_irq(&ioc->lock);
>
> spin_lock_irq(&ioc->lock);
> // updates of a will be lost
> memcpy(ioc->params.qos, qos, sizeof(qos));
> ioc_refresh_params(ioc, true);
> spin_unlock_irq(&ioc->lock);
>
> Althrough this is not common case, the problem can by fixed easily by
> holding the lock through the read, update, write process.
>
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

--
tejun

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