linux-kernel - Re: [PATCH v3] SUNRPC: set desired file system root before connecting local transports

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Message-ID: <50A4A93D.5010001@parallels.com>
Date:	Thu, 15 Nov 2012 12:35:09 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	"Trond.Myklebust@...app.com" <Trond.Myklebust@...app.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3] SUNRPC: set desired file system root before connecting
 local transports

15.11.2012 01:01, J. Bruce Fields пишет:
> On Mon, Nov 12, 2012 at 12:37:54PM +0400, Stanislav Kinsbursky wrote:
>> 07.11.2012 22:33, J. Bruce Fields пишет:
>>> On Tue, Nov 06, 2012 at 08:36:05AM -0500, J. Bruce Fields wrote:
>>>> On Tue, Nov 06, 2012 at 08:10:18AM -0500, Christoph Hellwig wrote:
>>>>> On Tue, Nov 06, 2012 at 08:07:06AM -0500, J. Bruce Fields wrote:
>>>>>> So you're worried that a bug in the nfs code could modify the root and
>>>>>> then not restore it?
>>>>>
>>>>> At least the link you pointed to earlier never sets it back.
>>>>
>>>> This? http://thread.gmane.org/gmane.linux.kernel/1259986/focus=47687
>>>>
>>>> 	+	get_fs_root(current->fs, &root);
>>>> 	+	set_fs_root(current->fs, &transport->root);
>>>> 	+
>>>> 	 	status = xs_local_finish_connecting(xprt, sock);
>>>> 	+
>>>> 	+	set_fs_root(current->fs, &root);
>>>> 	+	path_put(&root);
>>>>
>>>>> Instead
>>>>> of messing with it I'd rather have the sunrpc code use vfs_path_lookup
>>>>> and not care about current->fs->root at all.
>>>>
>>>> The annoyance is that the lookup happens somewhere lower down in the
>>>> networking code (net/unix/af_unix.c:unix_find_other, I think).  So we'd
>>>> need some new (internal) API.  We'd likely be the only user of that new
>>>> API.
>>>
>>> So, if the only drawback is really just the risk of introducing a bug
>>> that leaves the fs_root changed--the above seems simple enough for that
>>> not to be a great risk, right?
>>>
>>
>> If we unshare rpciod fs struct (which is exported already), then we
>
> I'm not sure what you mean by that.  Do workqueues actually have their
> own dedicated set of associated tasks?  I thought all workqueues shared
> a common pool of tasks these days.
>

Any kernel thread is cloned in kthreadd context with CLONE_FS flag.
I.e. all of them shares same fs struct, and changing fs->root in one of kthreads 
will affect all others.
That's why either fs struct have to be unshared to swap fs->root or fs->cwd, or 
the whole fs struct have to swapped.


-- 
Best regards,
Stanislav Kinsbursky
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