linux-ext4 - Re: [PATCH 2/8] exofs: remove lock/unlock super

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Message-ID: <502CF259.9040300@panasas.com>
Date:	Thu, 16 Aug 2012 16:15:05 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	<dedekind1@...il.com>
CC:	Marco Stornelli <marco.stornelli@...il.com>, <bhalevy@...ian.com>,
	<jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	<adilger.kernel@...ger.ca>, <tytso@....edu>,
	<hirofumi@...l.parknet.co.jp>, <mikulas@...ax.karlin.mff.cuni.cz>,
	Al Viro <viro@...IV.linux.org.uk>, <hch@...radead.org>,
	<dushistov@...l.ru>, <osd-dev@...n-osd.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/8] exofs: remove lock/unlock super

On 08/16/2012 04:10 PM, Artem Bityutskiy wrote:

> On Thu, 2012-08-16 at 15:32 +0300, Boaz Harrosh wrote:
>> On 08/16/2012 03:20 PM, Artem Bityutskiy wrote:
>>
>>> On Thu, 2012-08-16 at 12:00 +0200, Marco Stornelli wrote:
>>>> From: Marco Stornelli <marco.stornelli@...il.com>
>>>>
>>>> Remove lock and unlock super operation.
>>>>
>>>> Signed-off-by: Marco Stornelli <marco.stornelli@...il.com>
>>>
>>> Acked-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
>>>
>>
>>
>> Are you sure? It used to be that exofs_sync_fs() could be called
>> concurrently.
>>
>> What about two "bash -c sync" calls or a sync and an unmount
>> in parallel. anything protecting that?
>>
>> If so then sure, but please let me test first.
> 
> Umm, actually we will probably end up writing the same twice without the
> lock. 
> 


No we are not allowed to run exofs_sync_fs() concurrently because it uses
a per-alllocated scratch buffer to do it's stuff so you can end up with data
corruption on disk.

And we cannot use a spin-lock because we might sleep in ore_write()

There are some optimizations I can do here, but lets for now just do
the sb->s_lock thing, and I might decide to completely revamp the
all thing later.

Thanks
Boaz
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