From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail.skyhub.de ([2a01:4f8:120:8448::d00d]) by bombadil.infradead.org with esmtp (Exim 4.80.1 #2 (Red Hat Linux)) id 1a4QJB-0001Yt-TY for kexec@lists.infradead.org; Thu, 03 Dec 2015 09:36:15 +0000 Date: Thu, 3 Dec 2015 10:35:44 +0100 From: Borislav Petkov Subject: Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly Message-ID: <20151203093544.GC22271@pd.tnic> References: <20151120093641.4285.97253.stgit@softrs> <20151120093648.4285.17715.stgit@softrs> <20151125095457.GB29499@pd.tnic> <04EAB7311EE43145B2D3536183D1A84454A3B032@GSjpTKYDCembx31.service.hitachi.net> <20151202154023.GH3783@pd.tnic> <04EAB7311EE43145B2D3536183D1A84454A3CD95@GSjpTKYDCembx31.service.hitachi.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <04EAB7311EE43145B2D3536183D1A84454A3CD95@GSjpTKYDCembx31.service.hitachi.net> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "kexec" Errors-To: kexec-bounces+dwmw2=infradead.org@lists.infradead.org To: =?utf-8?B?5rKz5ZCI6Iux5a6PIC8gS0FXQUnvvIxISURFSElSTw==?= Cc: "x86@kernel.org" , Baoquan He , Jonathan Corbet , Peter Zijlstra , "linux-doc@vger.kernel.org" , "kexec@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Michal Hocko , Thomas Gleixner , "Eric W. Biederman" , "H. Peter Anvin" , =?utf-8?B?5bmz5p2+6ZuF5bezIC8gSElSQU1BVFXvvIxNQVNBTUk=?= , Andrew Morton , Ingo Molnar , Vivek Goyal T24gVGh1LCBEZWMgMDMsIDIwMTUgYXQgMDI6MDE6MzhBTSArMDAwMCwg5rKz5ZCI6Iux5a6PIC8g S0FXQUnvvIxISURFSElSTyB3cm90ZToKPiA+IE9uIFdlZCwgRGVjIDAyLCAyMDE1IGF0IDExOjU3 OjM4QU0gKzAwMDAsIOays+WQiOiLseWujyAvIEtBV0FJ77yMSElERUhJUk8gd3JvdGU6Cj4gPiA+ IFdlIGNhbiBkbyBzbywgYnV0IEkgdGhpbmsgcmVzZXR0aW5nIHBhbmljX2NwdSBhbHdheXMgd291 bGQgYmUKPiA+ID4gc2ltcGxlciBhbmQgc2FmZXIuCj4gCj4gSSdsbCBzdGF0ZSBpbiBkZXRhaWwu Cj4gCj4gV2hlbiB3ZSBjYWxsIGNyYXNoX2tleGVjKCkgd2l0aG91dCBlbnRlcmluZyBwYW5pYygp IGFuZCByZXR1cm4gZnJvbQo+IGl0LCBwYW5pYygpIHNob3VsZCBiZSBjYWxsZWQgZXZlbnR1YWxs eS4KCkh1aCwgdGhlIGNhbGwgY2hhaW4gaXMKCnBhbmljLT5jcmFzaF9rZXhlYwoKT3IgZG8geW91 IG1lYW4sIHdoZW4gY3Jhc2hfa2V4ZWMoKSBpcyBub3QgY2FsbGVkIGJ5IHBhbmljKCkgYnV0IGJ5 IHNvbWUKb2YgaXRzIG90aGVyIGNhbGxlcnM/Cgo+IEJ1dCB0aGUgY29kZSBwYXRocyBhcmUgYSBi aXQgY29tcGxpY2F0ZWQgYW5kIHRoZXJlIGFyZSBtYW55Cj4gaW1wbGVtZW50YXRpb25zIGZvciBl YWNoIGFyY2hpdGVjdHVyZS4gU28gb25lIGRheSwgdGhpcyBhc3N1bXB0aW9uIG1heQo+IGJlIGJy b2tlbjsgdGhlIENQVSBkb2Vzbid0IGNhbGwgcGFuaWMoKS4gT3IgdGhlIENQVSBtYXkgZmFpbCB0 byBjYWxsCj4gcGFuaWMoKSBiZWNhdXNlIHdlIGFyZSBhbHJlYWR5IGluIGluc2FuZSBzdGF0ZS4g SXQgd291bGQgYmUgbmVydm91cywKPiBidXQgYWxsb3dpbmcgYW5vdGhlciBDUFUgdG8gcHJvY2Vz cyBwYW5pYyByb3V0aW5lcyBieSByZXNldHRpbmcKPiBwYW5pY19jcHUgaXMgc2FmZXIgYXBwcm9h Y2guCgpNeSBzdWdnZXN0aW9uIHdhcyB0byBkbyB0aGlzIG9ubHkgb24gdGhlIHBhbmljIHBhdGgg LSBub3QgbmVjZXNzYXJpbHkgb24KdGhlIG90aGVycy4KCj4gU2luY2UgdGhpcyBjb2RlIGlzIGV4 ZWN1dGVkIG9ubHkgb25jZSBkdWUgdG8gcGFuaWNfY3B1LAo+IEkgdGhpbmsgaW50cm9kdWNpbmcg dGhpcyBsb2dpYyBpcyBub3QgbXVjaCB2YWx1YWJsZS4KPiBBbHNvLCBjdXJyZW50IGltcGxlbWVu dGF0aW9uIGlzIGFscmVhZHkgcXVpdGUgc2ltcGxlOgo+IAo+IHBhbmljKCkKPiB7Cj4gLi4uCj4g CV9fY3Jhc2hfa2V4ZWMoTlVMTCkgewo+IAkJaWYgKG11dGV4X3RyeWxvY2soJmtleGVjX211dGV4 KSkgewo+IAkJCWlmIChrZXhlY19jcmFzaF9pbWFnZSkgewo+IAkJCQkvKiBkb24ndCByZXR1cm4g Ki8KPiAJCQl9CgpJIGRvbid0IG1lYW4gdGhlIGtleGVjX2NyYXNoX2ltYWdlIGNhc2UgLSBJIG1l YW4gdGhlIG9wcG9zaXRlIG9uZToKIWtleGVjX2NyYXNoX2ltYWdlLiBBbmQgSSB0aGluayBJIGtu b3cgbm93IHdoYXQgeW91J3JlIHRyeWluZyB0byB0ZWxsCm1lOiB0aGUgZmlyc3QgQ1BVIHdoaWNo IGhpdHMgcGFuaWMsIHdpbGwgZmluaXNoIHBhbmljIGV2ZW50dWFsbHkgYW5kIHNvCml0IHdpbGwg dGFrZSBkb3duIHRoZSBtYWNoaW5lLgoKRXZlcnkgb3RoZXIgQ1BVIHdoaWNoIGhhcHBlbnMgdG8g ZW50ZXIgcGFuaWMgaW4gYmV0d2VlbiB0aGUgZmlyc3QgQ1BVCmFuZCB0aGUgbWFjaGluZSBiZWlu ZyB0YWtlbiBkb3duLCBkb2Vzbid0IG1hdHRlciBiZWNhdXNlLCB3ZWxsLCB3aG8KY2FyZXMsIHdl J3JlIHBhbmlja2luZyBhbHJlYWR5LgoKQW0gSSBjbG9zZT8KCi0tIApSZWdhcmRzL0dydXNzLAog ICAgQm9yaXMuCgpFQ08gdGlwICMxMDE6IFRyaW0geW91ciBtYWlscyB3aGVuIHlvdSByZXBseS4K Cl9fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fCmtleGVjIG1h aWxpbmcgbGlzdAprZXhlY0BsaXN0cy5pbmZyYWRlYWQub3JnCmh0dHA6Ly9saXN0cy5pbmZyYWRl YWQub3JnL21haWxtYW4vbGlzdGluZm8va2V4ZWMK From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759491AbbLCJfz (ORCPT ); Thu, 3 Dec 2015 04:35:55 -0500 Received: from mail.skyhub.de ([78.46.96.112]:56556 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757763AbbLCJfu (ORCPT ); Thu, 3 Dec 2015 04:35:50 -0500 Date: Thu, 3 Dec 2015 10:35:44 +0100 From: Borislav Petkov To: =?utf-8?B?5rKz5ZCI6Iux5a6PIC8gS0FXQUnvvIxISURFSElSTw==?= Cc: Jonathan Corbet , Peter Zijlstra , Ingo Molnar , "Eric W. Biederman" , "H. Peter Anvin" , Andrew Morton , Thomas Gleixner , Vivek Goyal , Baoquan He , "linux-doc@vger.kernel.org" , "x86@kernel.org" , "kexec@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Michal Hocko , =?utf-8?B?5bmz5p2+6ZuF5bezIC8gSElSQU1BVFXvvIxNQVNBTUk=?= Subject: Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly Message-ID: <20151203093544.GC22271@pd.tnic> References: <20151120093641.4285.97253.stgit@softrs> <20151120093648.4285.17715.stgit@softrs> <20151125095457.GB29499@pd.tnic> <04EAB7311EE43145B2D3536183D1A84454A3B032@GSjpTKYDCembx31.service.hitachi.net> <20151202154023.GH3783@pd.tnic> <04EAB7311EE43145B2D3536183D1A84454A3CD95@GSjpTKYDCembx31.service.hitachi.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <04EAB7311EE43145B2D3536183D1A84454A3CD95@GSjpTKYDCembx31.service.hitachi.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 03, 2015 at 02:01:38AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote: > > On Wed, Dec 02, 2015 at 11:57:38AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote: > > > We can do so, but I think resetting panic_cpu always would be > > > simpler and safer. > > I'll state in detail. > > When we call crash_kexec() without entering panic() and return from > it, panic() should be called eventually. Huh, the call chain is panic->crash_kexec Or do you mean, when crash_kexec() is not called by panic() but by some of its other callers? > But the code paths are a bit complicated and there are many > implementations for each architecture. So one day, this assumption may > be broken; the CPU doesn't call panic(). Or the CPU may fail to call > panic() because we are already in insane state. It would be nervous, > but allowing another CPU to process panic routines by resetting > panic_cpu is safer approach. My suggestion was to do this only on the panic path - not necessarily on the others. > Since this code is executed only once due to panic_cpu, > I think introducing this logic is not much valuable. > Also, current implementation is already quite simple: > > panic() > { > ... > __crash_kexec(NULL) { > if (mutex_trylock(&kexec_mutex)) { > if (kexec_crash_image) { > /* don't return */ > } I don't mean the kexec_crash_image case - I mean the opposite one: !kexec_crash_image. And I think I know now what you're trying to tell me: the first CPU which hits panic, will finish panic eventually and so it will take down the machine. Every other CPU which happens to enter panic in between the first CPU and the machine being taken down, doesn't matter because, well, who cares, we're panicking already. Am I close? -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply.