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Date: Mon, 9 May 2022 09:16:37 +0900 From: Byungchul Park <byungchul.park@....com> To: Hyeonggon Yoo <42.hyeyoo@...il.com> Cc: torvalds@...ux-foundation.org, damien.lemoal@...nsource.wdc.com, linux-ide@...r.kernel.org, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, mingo@...hat.com, linux-kernel@...r.kernel.org, peterz@...radead.org, will@...nel.org, tglx@...utronix.de, rostedt@...dmis.org, joel@...lfernandes.org, sashal@...nel.org, daniel.vetter@...ll.ch, chris@...is-wilson.co.uk, duyuyang@...il.com, johannes.berg@...el.com, tj@...nel.org, tytso@....edu, willy@...radead.org, david@...morbit.com, amir73il@...il.com, gregkh@...uxfoundation.org, kernel-team@....com, linux-mm@...ck.org, akpm@...ux-foundation.org, mhocko@...nel.org, minchan@...nel.org, hannes@...xchg.org, vdavydov.dev@...il.com, sj@...nel.org, jglisse@...hat.com, dennis@...nel.org, cl@...ux.com, penberg@...nel.org, rientjes@...gle.com, vbabka@...e.cz, ngupta@...are.org, linux-block@...r.kernel.org, paolo.valente@...aro.org, josef@...icpanda.com, linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk, jack@...e.cz, jack@...e.com, jlayton@...nel.org, dan.j.williams@...el.com, hch@...radead.org, djwong@...nel.org, dri-devel@...ts.freedesktop.org, airlied@...ux.ie, rodrigosiqueiramelo@...il.com, melissa.srw@...il.com, hamohammed.sa@...il.com Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker) On Sat, May 07, 2022 at 04:20:50PM +0900, Hyeonggon Yoo wrote: > On Fri, May 06, 2022 at 09:11:35AM +0900, Byungchul Park wrote: > > Linus wrote: > > > > > > On Wed, May 4, 2022 at 1:19 AM Byungchul Park <byungchul.park@....com> wrote: > > > > > > > > Hi Linus and folks, > > > > > > > > I've been developing a tool for detecting deadlock possibilities by > > > > tracking wait/event rather than lock(?) acquisition order to try to > > > > cover all synchonization machanisms. > > > > > > So what is the actual status of reports these days? > > > > > > Last time I looked at some reports, it gave a lot of false positives > > > due to mis-understanding prepare_to_sleep(). > > > > Yes, it was. I handled the case in the following way: > > > > 1. Stage the wait at prepare_to_sleep(), which might be used at commit. > > Which has yet to be an actual wait that Dept considers. > > 2. If the condition for sleep is true, the wait will be committed at > > __schedule(). The wait becomes an actual one that Dept considers. > > 3. If the condition is false and the task gets back to TASK_RUNNING, > > clean(=reset) the staged wait. > > > > That way, Dept only works with what actually hits to __schedule() for > > the waits through sleep. > > > > > For this all to make sense, it would need to not have false positives > > > (or at least a very small number of them together with a way to sanely > > > > Yes. I agree with you. I got rid of them that way I described above. > > > > IMHO DEPT should not report what lockdep allows (Not talking about No. > wait events). I mean lockdep allows some kind of nested locks but > DEPT reports them. You have already asked exactly same question in another thread of LKML. That time I answered to it but let me explain it again. --- CASE 1. lock L with depth n lock_nested L' with depth n + 1 ... unlock L' unlock L This case is allowed by Lockdep. This case is allowed by DEPT cuz it's not a deadlock. CASE 2. lock L with depth n lock A lock_nested L' with depth n + 1 ... unlock L' unlock A unlock L This case is allowed by Lockdep. This case is *NOT* allowed by DEPT cuz it's a *DEADLOCK*. --- The following scenario would explain why CASE 2 is problematic. THREAD X THREAD Y lock L with depth n lock L' with depth n lock A lock A lock_nested L' with depth n + 1 lock_nested L'' with depth n + 1 ... ... unlock L' unlock L'' unlock A unlock A unlock L unlock L' Yes. I need to check if the report you shared with me is a true one, but it's not because DEPT doesn't work with *_nested() APIs. Byungchul
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