linux-kernel - Re: [PATCH 1/2] kref: add kref_test_and

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Message-ID: <20110104205720.GA4180@kroah.com>
Date:	Tue, 4 Jan 2011 12:57:20 -0800
From:	Greg KH <greg@...ah.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <jaxboe@...ionio.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] kref: add kref_test_and_get

On Tue, Jan 04, 2011 at 04:52:26PM +0100, Jerome Marchand wrote:
> 
> Add kref_test_and_get() function, which atomically add a reference only if
> refcount is not zero. This prevent to add a reference to an object that is
> already being removed.

We just removed a function like this recently as it really isn't the
solution for what you need here at all.

Please use a lock that protects the desctruction of the kref, like the
rest of the kernel, to keep someone from grabing a reference while the
device could be going away.  That's the only way to safely do this.

See the archives for why this overall, isn't a good idea for a kref to
have this type of function.

thanks,

greg k-h
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