Re: lockdep null-ptr-deref

Boqun Feng
2020-10-02T12:36:10Z

On Wed, Sep 30, 2020 at 09:02:28PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 30, 2020 at 08:18:18PM +0800, Boqun Feng wrote:
>
> > For one thing, I do think that LOCK_READ_USED trace is helpful for
> > better reporting, because if there is a read lock in the dependency path
> > which causes the deadlock, it's better to have the LOCK_READ_USED trace
> > to know at least the initial READ usage. For example, if we have
> >
> > void f1(...)
> > {
> > write_lock(&A);
> > spin_lock(&C);
> > // A -> C
> > ...
> > }
> >
> > void g(...)
> > {
> > read_lock(&A);
> > ...
> > }
> > void f2(...)
> > {
> > spin_lock(&B);
> > g(...);
> > // B -> A
> > }
> >
> > void f3(...) {
> > spin_lock(&C);
> > spin_lock(&B);
> > // C -> B, trigger lockdep splat
> > }
> >
> > when lockdep reports the deadlock (at the time f3() is called), it will
> > be useful if we have a trace like:
> >
> > INITIAL READ usage at:
> > g+0x.../0x...
> > f2+0x.../0x...
> >
> > Thoughts?
>
> Wouldn't that also be in LOCK_ENABLED_*_READ ?
>

But what if f2() is called with interrupt disabled? Or f2() disables
interrupt inside the function, like:

void f2(...)
{
local_irq_disable();
spin_lock(&B);
g(...);
...
local_irq_enable();
}

In this case, there wouldn't be any LOCK_ENABLED_*_READ usage for
rwlock_t A. As a result, we won't see it in the lockdep splat.

Regards,
Boqun

> That is, with PROVE_LOCKING on, the initial usage is bound to set more
> states, except for !check||trylock usage, and those aren't really all
> that interesting.