linux-kernel - Re: [PATCH RESEND net 3/7] net: hns3: Resolved the issue that the debugfs query result is inconsistent.

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Message-ID: <e4396ecc-7874-4caf-b25d-870a9d897eb1@huawei.com>
Date: Wed, 13 Nov 2024 13:59:32 +0800
From: Jijie Shao <shaojijie@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: <shaojijie@...wei.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<pabeni@...hat.com>, <andrew+netdev@...n.ch>, <horms@...nel.org>,
	<shenjian15@...wei.com>, <salil.mehta@...wei.com>, <liuyonglong@...wei.com>,
	<wangpeiyang1@...wei.com>, <chenhao418@...wei.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND net 3/7] net: hns3: Resolved the issue that the
 debugfs query result is inconsistent.

on 2024/11/12 9:25, Jakub Kicinski wrote:
> On Thu, 7 Nov 2024 21:30:19 +0800 Jijie Shao wrote:
>> Subject: [PATCH RESEND net 3/7] net: hns3: Resolved the issue that the debugfs query result is inconsistent.
>> Date: Thu, 7 Nov 2024 21:30:19 +0800
>> X-Mailer: git-send-email 2.30.0
>>
>> From: Hao Lan <lanhao@...wei.com>
>>
>> This patch modifies the implementation of debugfs:
>> When the user process stops unexpectedly, not all data of the file system
>> is read. In this case, the save_buf pointer is not released. When the user
>> process is called next time, save_buf is used to copy the cached data
>> to the user space. As a result, the queried data is inconsistent. To solve
>> this problem, determine whether the function is invoked for the first time
>> based on the value of *ppos. If *ppos is 0, obtain the actual data.
> What do you mean by "inconsistent" ?
> What if two processes read the file at once?

inconsistent：
Before this modification,
if the previous read operation is stopped before complete, the buffer is not released.
In the next read operation (perhaps after a long time), the driver does not read again.
Instead, the driver returns the bufffer content, which causes outdated data to be obtained.
As a result, the obtained data is inconsistent with the actual data.

In this patch, ppos is used to determine whether a new read operation is performed.
If yes, the driver updates the data in the buffer to ensure that the queried data is fresh.
But, if two processes read the same file at once, The read operation that ends first releases the buffer.
As a result, the other read operation re-alloc buffer memory. However, because the value of ppos is not 0,
the data is not updated again. As a result, the queried data is truncated.

This is a bug and I will fix it in the next version.

Thanks
Jijie Shao