Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring buffer overwrite

Byungchul Park
2017-08-11T08:53:20Z

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).
> Basically it means we empty the lock history whenever we finished a
> worker function in a worker thread or we are about to return to
> userspace after we finish the syscall. This could further save some
> memory and so I think this may be better than my approach.

Do you mean reset _whenever_ hard irq exit, soft irq exit or work exit?
Why should we give up chances to check dependencies of remaining xhlocks
whenever each exit? Am I understanding correctly?

I am just curious. Does your approach have some problems?

Thanks,
Byungchul