On Mon, May 09, 2022 at 09:16:37AM +0900, Byungchul Park wrote: > 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@xxxxxxx> 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*. > Yeah, in previous threads we discussed this [1] And the case was: scan_mutex -> object_lock -> kmemleak_lock -> object_lock And dept reported: object_lock -> kmemleak_lock, kmemleak_lock -> object_lock as deadlock. But IIUC - What DEPT reported happens only under scan_mutex and It is not simple just not to take them because the object can be removed from the list and freed while scanning via kmemleak_free() without kmemleak_lock and object_lock. Just I'm still not sure that someone will fix the warning in the future - even if the locking rule is not good - if it will not cause a real 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. > Sorry, It was not right just to say DEPT doesn't work with _nested() APIs. > Byungchul [1] https://lore.kernel.org/lkml/20220304002809.GA6112@X58A-UD3R/ -- Thanks, Hyeonggon
