Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755469Ab0BIRqV (ORCPT ); Tue, 9 Feb 2010 12:46:21 -0500 Received: from mail-fx0-f220.google.com ([209.85.220.220]:33832 "EHLO mail-fx0-f220.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755345Ab0BIRqU (ORCPT ); Tue, 9 Feb 2010 12:46:20 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=TO64hbE8gMgJd1GhHODvFD2AIh+ynwFkmwkvdrq3B4pYhUWA37f7DTOiuUp3hD7Yfm 0Q8kKCY4WqSS18WpkjHecNeeLX6Ygh6VboyvzUDS8dURmU2okSIX+iNOSYQIFoB+0pha 3VMm9RWYS8LCTWdlyjM5//U/M/7BgzMdyDJX0= Subject: Re: [PATCH 01/17] MTD: create lockless versions of {get,put}_mtd_device This will be used to resolve deadlock in block translation layer. From: Maxim Levitsky To: Peter Zijlstra Cc: David Woodhouse , Artem Bityutskiy , linux-mtd , linux-kernel , Alex Dubov , joern , Thomas Gleixner , "stanley.miao" , Vitaly Wool In-Reply-To: <1265736206.9195.4.camel@maxim-laptop> References: <1265734665-22656-1-git-send-email-maximlevitsky@gmail.com> <1265734665-22656-2-git-send-email-maximlevitsky@gmail.com> <1265735693.11509.253.camel@laptop> <1265736206.9195.4.camel@maxim-laptop> Content-Type: text/plain; charset="UTF-8" Date: Tue, 09 Feb 2010 19:46:14 +0200 Message-ID: <1265737574.9195.16.camel@maxim-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2213 Lines: 75 On Tue, 2010-02-09 at 19:23 +0200, Maxim Levitsky wrote: > On Tue, 2010-02-09 at 18:14 +0100, Peter Zijlstra wrote: > > On Tue, 2010-02-09 at 18:57 +0200, Maxim Levitsky wrote: > > > These functions can be used as long as we don't need access to global mtd table, but have > > > a pointer to the mtd device. > > > > > > Signed-off-by: Maxim Levitsky > > > --- > > > drivers/mtd/mtdcore.c | 60 ++++++++++++++++++++++++++++++---------------- > > > include/linux/mtd/mtd.h | 3 +- > > > 2 files changed, 41 insertions(+), 22 deletions(-) > > > > > +int __get_mtd_device(struct mtd_info *mtd) > > > +{ > > > + int err; > > > + > > > + if (!try_module_get(mtd->owner)) > > > + return -ENODEV; > > > + > > > + if (mtd->get_device) { > > > + > > > + err = mtd->get_device(mtd); > > > + > > > + if (err) { > > > + module_put(mtd->owner); > > > + return err; > > > + } > > > + } > > > + mtd->usecount++; > > > + return 0; > > > } > > > > > +void __put_mtd_device(struct mtd_info *mtd) > > > +{ > > > + --mtd->usecount; > > > + BUG_ON(mtd->usecount < 0); > > > + > > > if (mtd->put_device) > > > mtd->put_device(mtd); > > > > > > module_put(mtd->owner); > > > } > > > > That's racy, use kref. > > > Couldn't agree with you more. > > However, these functions aren't intended for general use, and probably > will be used by mtd translation layer only. I do have a lock that > protects concurrent use of these functions. > > Thus, I better add a comment about this? However on second thought, there is still a race if two FTLs access same mtd device. While this might seem impossible, and I say it is quite dangerous. I can imagine using both some FTL and mtdblock for testing. Just using kref (nothing against it) won't help here. The mtd->get_device/put_device aren't expecting to be called concurrently ether... I can add per mtd lock, but it is a bit ugly... What do you think? Best regards, Maxim Levitsky -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/