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

Byungchul Park
2017-08-11T03:44:45Z

On Thu, Aug 10, 2017 at 09:17:37PM +0800, Boqun Feng wrote:
> > > > > @@ -4826,6 +4851,7 @@ static inline int depend_after(struct held_lock
> > > > *hlock)
> > > > > * Check if the xhlock is valid, which would be false if,
> > > > > *
> > > > > * 1. Has not used after initializaion yet.
> > > > > + * 2. Got invalidated.
> > > > > *
> > > > > * Remind hist_lock is implemented as a ring buffer.
> > > > > */
> > > > > @@ -4857,6 +4883,7 @@ static void add_xhlock(struct held_lock *hlock)
> > > > >
> > > > > /* Initialize hist_lock's members */
> > > > > xhlock->hlock = *hlock;
> > > > > + xhlock->hist_id = current->hist_id++;
> >
> > Besides, is this code correct? Does this just make xhlock->hist_id
> > one-less-than the curr->hist_id, which cause the invalidation every time
> > you do ring buffer unwinding?
> >
> > Regards,
> > Boqun
> >
>
> So basically, I'm suggesting do this on top of your patch, there is also
> a fix in commit_xhlocks(), which I think you should swap the parameters
> in before(...), no matter using task_struct::hist_id or using
> task_struct::xhlock_idx as the timestamp.
>
> Hope this could make my point more clear, and if I do miss something,
> please point it out, thanks ;-)

Sorry for mis-understanding. I like your patch. I think it works.

Additionally.. See below..

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 074872f016f8..886ba79bfc38 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -854,9 +854,6 @@ struct task_struct {
> unsigned int xhlock_idx;
> /* For restoring at history boundaries */
> unsigned int xhlock_idx_hist[XHLOCK_NR];
> - unsigned int hist_id;
> - /* For overwrite check at each context exit */
> - unsigned int hist_id_save[XHLOCK_NR];
> #endif
>
> #ifdef CONFIG_UBSAN
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 699fbeab1920..04c6c8d68e18 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -4752,10 +4752,8 @@ void crossrelease_hist_start(enum xhlock_context_t c)
> {
> struct task_struct *cur = current;
>
> - if (cur->xhlocks) {
> + if (cur->xhlocks)
> cur->xhlock_idx_hist[c] = cur->xhlock_idx;
> - cur->hist_id_save[c] = cur->hist_id;
> - }
> }
>
> void crossrelease_hist_end(enum xhlock_context_t c)
> @@ -4769,7 +4767,7 @@ void crossrelease_hist_end(enum xhlock_context_t c)
> cur->xhlock_idx = idx;
>
> /* Check if the ring was overwritten. */
> - if (h->hist_id != cur->hist_id_save[c])
> + if (h->hist_id != idx)
> invalidate_xhlock(h);
> }
> }
> @@ -4849,7 +4847,7 @@ static void add_xhlock(struct held_lock *hlock)
>
> /* Initialize hist_lock's members */
> xhlock->hlock = *hlock;
> - xhlock->hist_id = current->hist_id++;
> + xhlock->hist_id = idx;
>
> xhlock->trace.nr_entries = 0;
> xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
> @@ -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?

> 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.

Thanks,
Byungchul