From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Hurley Subject: Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close() Date: Wed, 09 Jul 2014 09:57:50 -0400 Message-ID: <53BD4A5E.90608@hurleysoftware.com> References: <1402924639-5164-1-git-send-email-peter@hurleysoftware.com> <1402924639-5164-15-git-send-email-peter@hurleysoftware.com> <4575870.N9RCpZ4UMg@wuerfel> <53A01F02.7000202@hurleysoftware.com> <063D6719AE5E284EB5DD2968C1650D6D1725DAF6@AcuExch.aculab.com> <53A0273F.70400@hurleysoftware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mailout32.mail01.mtsvc.net ([216.70.64.70]:36446 "EHLO n23.mail01.mtsvc.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754886AbaGIN5z (ORCPT ); Wed, 9 Jul 2014 09:57:55 -0400 In-Reply-To: <53A0273F.70400@hurleysoftware.com> Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: David Laight , Arnd Bergmann , One Thousand Gnomes Cc: "linuxppc-dev@lists.ozlabs.org" , Greg Kroah-Hartman , Karsten Keil , "linux-kernel@vger.kernel.org" , "linux-serial@vger.kernel.org" On 06/17/2014 07:32 AM, Peter Hurley wrote: > On 06/17/2014 07:03 AM, David Laight wrote: >> From: Peter Hurley >> ... >>>> I don't understand the second half of the changelog, it doesn't seem >>>> to fit here: there deadlock that we are trying to avoid here happens >>>> when the *same* tty needs the lock to complete the function that >>>> sends the pending data. I don't think we do still do that any more, >>>> but it doesn't seem related to the tty lock being system-wide or not. >>> >>> The tty lock is not used in the i/o path; it's purpose is to >>> mutually exclude state changes in open(), close() and hangup(). >>> >>> The commit that added this [1] comments that _other_ ttys may wait >>> for this tty to complete, and comments in the code note that this >>> function should be removed when the system-wide tty mutex was removed >>> (which happened with the commit noted in the changelog). >> >> What happens if another process tries to do a non-blocking open >> while you are sleeping in close waiting for output to drain? >> >> Hopefully this returns before that data has drained. > > Good point. > > tty_open() should be trylocking both mutexes anyway in O_NONBLOCK. Further, the tty lock should not be nested within the tty_mutex lock in a reopen, regardless of O_NONBLOCK. AFAICT, the tty_mutex in the reopen scenario is only protecting the tty count bump of the linked tty (if the tty is a pty). I think with some refactoring and returning with a tty reference held from both tty_open_current_tty() and tty_driver_lookup_tty(), the tty lock in tty_open() can be attempted without nesting in the tty_mutex. Regardless, I'll be splitting this series and I'll be sure to cc you all when I resubmit these changes (after testing). Regards, Peter Hurley From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from n23.mail01.mtsvc.net (mailout32.mail01.mtsvc.net [216.70.64.70]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 33AD51A00C1 for ; Wed, 9 Jul 2014 23:57:57 +1000 (EST) Message-ID: <53BD4A5E.90608@hurleysoftware.com> Date: Wed, 09 Jul 2014 09:57:50 -0400 From: Peter Hurley MIME-Version: 1.0 To: David Laight , Arnd Bergmann , One Thousand Gnomes Subject: Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close() References: <1402924639-5164-1-git-send-email-peter@hurleysoftware.com> <1402924639-5164-15-git-send-email-peter@hurleysoftware.com> <4575870.N9RCpZ4UMg@wuerfel> <53A01F02.7000202@hurleysoftware.com> <063D6719AE5E284EB5DD2968C1650D6D1725DAF6@AcuExch.aculab.com> <53A0273F.70400@hurleysoftware.com> In-Reply-To: <53A0273F.70400@hurleysoftware.com> Content-Type: text/plain; charset=UTF-8; format=flowed Cc: Greg Kroah-Hartman , Karsten Keil , "linuxppc-dev@lists.ozlabs.org" , "linux-kernel@vger.kernel.org" , "linux-serial@vger.kernel.org" List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 06/17/2014 07:32 AM, Peter Hurley wrote: > On 06/17/2014 07:03 AM, David Laight wrote: >> From: Peter Hurley >> ... >>>> I don't understand the second half of the changelog, it doesn't seem >>>> to fit here: there deadlock that we are trying to avoid here happens >>>> when the *same* tty needs the lock to complete the function that >>>> sends the pending data. I don't think we do still do that any more, >>>> but it doesn't seem related to the tty lock being system-wide or not. >>> >>> The tty lock is not used in the i/o path; it's purpose is to >>> mutually exclude state changes in open(), close() and hangup(). >>> >>> The commit that added this [1] comments that _other_ ttys may wait >>> for this tty to complete, and comments in the code note that this >>> function should be removed when the system-wide tty mutex was removed >>> (which happened with the commit noted in the changelog). >> >> What happens if another process tries to do a non-blocking open >> while you are sleeping in close waiting for output to drain? >> >> Hopefully this returns before that data has drained. > > Good point. > > tty_open() should be trylocking both mutexes anyway in O_NONBLOCK. Further, the tty lock should not be nested within the tty_mutex lock in a reopen, regardless of O_NONBLOCK. AFAICT, the tty_mutex in the reopen scenario is only protecting the tty count bump of the linked tty (if the tty is a pty). I think with some refactoring and returning with a tty reference held from both tty_open_current_tty() and tty_driver_lookup_tty(), the tty lock in tty_open() can be attempted without nesting in the tty_mutex. Regardless, I'll be splitting this series and I'll be sure to cc you all when I resubmit these changes (after testing). Regards, Peter Hurley