Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753212AbdHKNGm (ORCPT ); Fri, 11 Aug 2017 09:06:42 -0400 Received: from mail-lf0-f49.google.com ([209.85.215.49]:35934 "EHLO mail-lf0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753162AbdHKNGk (ORCPT ); Fri, 11 Aug 2017 09:06:40 -0400 MIME-Version: 1.0 In-Reply-To: <20170811094448.GJ20323@X58A-UD3R> References: <1502089981-21272-1-git-send-email-byungchul.park@lge.com> <1502089981-21272-7-git-send-email-byungchul.park@lge.com> <20170810115922.kegrfeg6xz7mgpj4@tardis> <016b01d311d1$d02acfa0$70806ee0$@lge.com> <20170810125133.2poixhni4d5aqkpy@tardis> <20170810131737.skdyy4qcxlikbyeh@tardis> <20170811034328.GH20323@X58A-UD3R> <20170811080329.3ehu7pp7lcm62ji6@tardis> <20170811085201.GI20323@X58A-UD3R> <20170811094448.GJ20323@X58A-UD3R> From: Byungchul Park Date: Fri, 11 Aug 2017 22:06:37 +0900 Message-ID: Subject: Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring buffer overwrite To: Byungchul Park Cc: Boqun Feng , Peter Zijlstra , Ingo Molnar , tglx@linutronix.de, Michel Lespinasse , kirill@shutemov.name, "linux-kernel@vger.kernel.org" , linux-mm@kvack.org, akpm@linux-foundation.org, willy@infradead.org, npiggin@gmail.com, kernel-team@lge.com Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3531 Lines: 97 On Fri, Aug 11, 2017 at 6:44 PM, Byungchul Park wrote: > On Fri, Aug 11, 2017 at 05:52:02PM +0900, Byungchul Park wrote: >> On Fri, Aug 11, 2017 at 04:03:29PM +0800, Boqun Feng wrote: >> > Thanks for taking a look at it ;-) >> >> I rather appriciate it. >> >> > > > @@ -5005,7 +5003,7 @@ static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock) >> > > > static void commit_xhlocks(struct cross_lock *xlock) >> > > > { >> > > > unsigned int cur = current->xhlock_idx; >> > > > - unsigned int prev_hist_id = xhlock(cur).hist_id; >> > > > + unsigned int prev_hist_id = cur + 1; >> > > >> > > I should have named it another. Could you suggest a better one? >> > > >> > >> > I think "prev" is fine, because I thought the "previous" means the >> > xhlock item we visit _previously_. >> > >> > > > unsigned int i; >> > > > >> > > > if (!graph_lock()) >> > > > @@ -5030,7 +5028,7 @@ static void commit_xhlocks(struct cross_lock *xlock) >> > > > * hist_id than the following one, which is impossible >> > > > * otherwise. >> > > >> > > Or we need to modify the comment so that the word 'prev' does not make >> > > readers confused. It was my mistake. >> > > >> > >> > I think the comment needs some help, but before you do it, could you >> > have another look at what Peter proposed previously? Note you have a >> > same_context_xhlock() check in the commit_xhlocks(), so the your >> > previous overwrite case actually could be detected, I think. >> >> What is the previous overwrite case? >> >> ppppppppppwwwwwwwwwwwwiiiiiiiii >> iiiiiiiiiiiiiii................ >> >> Do you mean this one? I missed the check of same_context_xhlock(). Yes, >> peterz's suggestion also seems to work. >> >> > However, one thing may not be detected is this case: >> > >> > ppppppppppppppppppppppppppppppppppwwwwwwww >> > wrapped > wwwwwww >> >> To be honest, I think your suggestion is more natual, with which this >> case would be also covered. >> >> > >> > where p: process and w: worker. >> > >> > , because p and w are in the same task_irq_context(). I discussed this >> > with Peter yesterday, and he has a good idea: unconditionally do a reset >> > on the ring buffer whenever we do a crossrelease_hist_end(XHLOCK_PROC). > > Ah, ok. You meant 'whenever _process_ context exit'. > > I need more time to be sure, but anyway for now it seems to work with > giving up some chances for remaining xhlocks. > > But, I am not sure if it's still true even in future and the code can be > maintained easily. I think your approach is natural and neat enough for > that purpose. What problem exists with yours? Let me list up the possible approaches: 0. Byungchul's approach 1. Boqun's approach 2. Peterz's approach 3. Reset on process exit I like Boqun's approach most but, _whatever_. It's ok if it solves the problem. The last one is not bad when it is used for syscall exit, but we have to give up valid dependencies unnecessarily in other cases. And I think Peterz's approach should be modified a bit to make it work neatly, like: crossrelease_hist_end(...) { ... invalidate_xhlock(&xhlock(cur->xhlock_idx_max)); for (c = 0; c < XHLOCK_CXT_NR; c++) if ((cur->xhlock_idx_max - cur->xhlock_idx_hist[c]) >= MAX_XHLOCKS_NR) invalidate_xhlock(&xhlock(cur->xhlock_idx_hist[c])); ... } And then Peterz's approach can also work, I think. --- Thanks, Byungchul