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@lge.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:
- Stage the wait at prepare_to_sleep(), which might be used at commit. Which has yet to be an actual wait that Dept considers.
- If the condition for sleep is true, the wait will be committed at __schedule(). The wait becomes an actual one that Dept considers.
- 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/