Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932871Ab2JaLwh (ORCPT ); Wed, 31 Oct 2012 07:52:37 -0400 Received: from merlin.infradead.org ([205.233.59.134]:36013 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751199Ab2JaLwb (ORCPT ); Wed, 31 Oct 2012 07:52:31 -0400 Message-ID: <1351684339.8774.21.camel@shinybook.infradead.org> Subject: Re: [PATCH v2 2/3] pppoatm: fix race condition with destroying of vcc From: David Woodhouse To: Krzysztof Mazur Cc: davem@davemloft.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Date: Wed, 31 Oct 2012 11:52:19 +0000 In-Reply-To: <20121031113021.GA24093@shrek.podlesie.net> References: <1350926091-12642-1-git-send-email-krzysiek@podlesie.net> <1350926091-12642-2-git-send-email-krzysiek@podlesie.net> <1351589868.17077.34.camel@shinybook.infradead.org> <20121030190725.GA14044@shrek.podlesie.net> <20121030195224.GA2153@shrek.podlesie.net> <1351678578.8774.8.camel@shinybook.infradead.org> <20121031113021.GA24093@shrek.podlesie.net> Content-Type: multipart/signed; micalg="sha1"; protocol="application/x-pkcs7-signature"; boundary="=-eK6nA9wvGdafbfewHkSh" X-Mailer: Evolution 3.6.1 (3.6.1-1.fc18) Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 10263 Lines: 158 --=-eK6nA9wvGdafbfewHkSh Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2012-10-31 at 12:30 +0100, Krzysztof Mazur wrote: > Yes, original patch had also the same problem with sock_owned_by_user(), > so I just incorrectly assumed that we can do "goto nospace" after > pppoatm_may_send(), but ppooatm_may_send() must be the last test. >=20 > So I just moved all other tests earlier and and now pppoatm_may_send() > is also protected by ATM socket lock as you suggested earlier. I don't think that's sufficient. When we return zero from pppoatm_send(), the generic PPP code considers the channel to be blocked, and it won't send any more data to it, ever, until we call ppp_output_wakeup(). Which we do from a tasklet, triggered in pppoatm_pop() *iff* the BLOCKED flag is set. So we play silly buggers in pppoatm_may_send() to ensure that *if* we're going to return zero, we make damn sure the BLOCKED flag is set and that pppoatm_pop() is going to see that it's set. There are extensive comments in pppoatm_pop() and pppoatm_may_send() which try to explain this. It works because there's *always* going to be packet in flight if we say that the sk_wmem is full, so of course there's *always* going to be a later call to pppoatm_pop() to wake things up. However, if you're going to return zero from pppoatm_send() when sock_owned_by_user() is true, what guarantees that ppp_output_wakeup() will ever be called? --=20 dwmw2 --=-eK6nA9wvGdafbfewHkSh Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIIUbjCCBjQw ggQcoAMCAQICAR4wDQYJKoZIhvcNAQEFBQAwfTELMAkGA1UEBhMCSUwxFjAUBgNVBAoTDVN0YXJ0 Q29tIEx0ZC4xKzApBgNVBAsTIlNlY3VyZSBEaWdpdGFsIENlcnRpZmljYXRlIFNpZ25pbmcxKTAn BgNVBAMTIFN0YXJ0Q29tIENlcnRpZmljYXRpb24gQXV0aG9yaXR5MB4XDTA3MTAyNDIxMDE1NVoX DTE3MTAyNDIxMDE1NVowgYwxCzAJBgNVBAYTAklMMRYwFAYDVQQKEw1TdGFydENvbSBMdGQuMSsw KQYDVQQLEyJTZWN1cmUgRGlnaXRhbCBDZXJ0aWZpY2F0ZSBTaWduaW5nMTgwNgYDVQQDEy9TdGFy dENvbSBDbGFzcyAxIFByaW1hcnkgSW50ZXJtZWRpYXRlIENsaWVudCBDQTCCASIwDQYJKoZIhvcN AQEBBQADggEPADCCAQoCggEBAMcJg8zOLdgasSmkLhOrlr6KMoOMpohBllVHrdRvEg/q6r8jR+EK 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