linux-ext4 - Re: [BUG] bugs in jbd2_dev_to_name() (was Re: [PATCH 00/11] [GIT PULL] more updates for the tag format)

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Date:	Mon, 22 Jun 2009 09:36:28 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Theodore Tso <tytso@....edu>, Li Zefan <lizf@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [BUG] bugs in jbd2_dev_to_name() (was Re: [PATCH 00/11] [GIT
 PULL] more updates for the tag format)

Theodore Tso wrote:
> On Fri, Jun 19, 2009 at 04:14:23PM +0800, Li Zefan wrote:
>>> 	rcu_read_lock();
>>> 	if (devcache[i] && devcache[i]->device == device) {
>>> 		ret = devcache[i]->devname;
>>> 		rcu_read_unlock();
>>> 		return ret;
>> It doesn't seem safe to dereference @ret outside rcu read section.
> 
> Note the comments at the beginning of the function:
> 

Ah, I overlooked the comments.

But the patch that adds rcu locking around trace event prints
never gets merged:

http://lkml.org/lkml/2009/4/15/549

Steven?

>     The caller should use rcu_read_lock() in order to make sure the
>     device name stays valid until its done with it.  We use
>     rcu_read_lock() as well to make sure we're safe in case the caller
>     gets sloppy, and because rcu_read_lock() is cheap and can be safely
>     nested.
> 
> I suppose I should change the wording to indicate that it adds a bit
> more safety (as in, the crash won't happen inside this function, but
> as far as the caller is concerned, all bets are off!)
> 

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