linux-kernel - Re: [PATCH 04/10] staging: erofs: fix `erofs_workgroup_{try_to

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Message-ID: <ee829ce6-e59e-3ed6-b9a4-16c29c9eb59f@huawei.com>
Date:   Thu, 22 Nov 2018 18:29:34 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     <devel@...verdev.osuosl.org>, <linux-erofs@...ts.ozlabs.org>,
        Chao Yu <yuchao0@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>, <weidu.du@...wei.com>,
        Miao Xie <miaoxie@...wei.com>
Subject: Re: [PATCH 04/10] staging: erofs: fix
 `erofs_workgroup_{try_to_freeze, unfreeze}'

Hi Greg,

On 2018/11/22 18:21, Greg Kroah-Hartman wrote:
> On Tue, Nov 20, 2018 at 10:34:19PM +0800, Gao Xiang wrote:
>> There are two minor issues in the current freeze interface:
>>
>>    1) Freeze interfaces have not related with CONFIG_DEBUG_SPINLOCK,
>>       therefore fix the incorrect conditions;
>>
>>    2) For SMP platforms, it should also disable preemption before
>>       doing atomic_cmpxchg in case that some high priority tasks
>>       preempt between atomic_cmpxchg and disable_preempt, then spin
>>       on the locked refcount later.
> 
> spinning on a refcount implies that you are trying to do your own type
> of locking.  Why not use the in-kernel locking api instead?  It will
> always do better than trying to do your own logic as the developers
> there know locking across all types of cpus better than filesystem
> developers :)

It is because refcount also plays a role as a spinlock on a specific value
(== EROFS_LOCKED_MAGIC), no need to introduce such a value since the spin
window is small.

Thanks,
Gao Xiang

> 
> thanks,
> 
> greg k-h
>