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Subject: Re: [PATCH 2/4] configfs: Fix writing at a non-zero offset
To:     Bodo Stroesser <bostroesser@gmail.com>,
        Christoph Hellwig <hch@lst.de>
Cc:     Joel Becker <jlbec@evilplan.org>, linux-kernel@vger.kernel.org,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Yanko Kaneti <yaneti@declera.com>,
        Brendan Higgins <brendanhiggins@google.com>
References: <20210723212353.896343-1-bvanassche@acm.org>
 <20210723212353.896343-3-bvanassche@acm.org>
 <7bee65ce-f5f1-a525-c72d-221b5d23cf3e@gmail.com>
 <d12f24b6-7066-f9bb-1b88-6cc23c9c45c1@acm.org>
 <4055ca70-7669-d00d-7c08-86fe75a3d377@gmail.com>
 <618b2bdc-282b-0a1d-1fc5-020cf80d7a7e@acm.org>
 <c9cb1f3b-0b3b-c571-4a51-e647f3c1e90a@gmail.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <ab190c50-8c87-b215-1432-056c81bcd656@acm.org>
Date:   Mon, 26 Jul 2021 20:17:33 -0700
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On 7/26/21 5:54 PM, Bodo Stroesser wrote:
> The new behavior can also cause trouble with existing store handlers.
> Example:
> The tcmu attribute files cmd_time_out and qfull_time_out just take a
> string containing the decimal formatted number of seconds of the
> timeout. Each number up to now had to be transferred in a single write.
> Assume the old value is 30 and we want to change to 19. If userspace
> writes byte by byte, you end up calling
> store(item, "1\0", 1) and then
> store(item, "19\9", 2).
> If these quick changes do not cause trouble in tcmu's scsi cmd handling,
> then think what happens, if userspace is interrupted between the two
> writes. Allowing to split the writes cause a loss of "atomicity".

From Documentation/filesystems/configfs.rst, for normal attributes:
"Configfs expects write(2) to store the entire buffer at once." In other
words, the behavior for partial writes is undocumented. My changes
preserve the behavior if a buffer is written in its entirety. I do not
agree that my changes can cause trouble for existing store handlers.

Bart.