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Date: Fri, 11 Aug 2017 22:06:37 +0900 From: Byungchul Park <max.byungchul.park@...il.com> To: Byungchul Park <byungchul.park@....com> Cc: Boqun Feng <boqun.feng@...il.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, tglx@...utronix.de, Michel Lespinasse <walken@...gle.com>, kirill@...temov.name, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-mm@...ck.org, akpm@...ux-foundation.org, willy@...radead.org, npiggin@...il.com, kernel-team@....com Subject: Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring buffer overwrite On Fri, Aug 11, 2017 at 6:44 PM, Byungchul Park <byungchul.park@....com> 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
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