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SubjectRe: [PATCH v2] kcsan: Avoid scheduler recursion by using non-instrumented preempt_{disable,enable}()
On Mon, 23 Nov 2020 at 17:08, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Nov 23, 2020 at 04:57:46PM +0100, Marco Elver wrote:
> > Let me know what you prefer.
> >
>
> > @@ -288,27 +288,19 @@ static u32 kcsan_prandom_u32_max(u32 ep_ro)
> > u32 res;
> >
> > /*
> > + * Avoid recursion with scheduler by disabling KCSAN because
> > + * preempt_enable_notrace() will still call into scheduler code.
> > */
> > + kcsan_disable_current();
> > preempt_disable_notrace();
> > state = raw_cpu_ptr(&kcsan_rand_state);
> > res = prandom_u32_state(state);
> > + preempt_enable_notrace();
> > + kcsan_enable_current_nowarn();
> >
> > return (u32)(((u64) res * ep_ro) >> 32);
> > }
>
> This is much preferred over the other. The thing with _no_resched is that
> you can miss a preemption for an unbounded amount of time, which is bad.

Ah, I think this is rubbish, too. Because it might fix one problem,
but now I'm left with the problem that kcsan_prandom_u32_max() is
called for udelay() later and at that point we lose skip_count
randomness entirely.

I think relying on lib/random32.c already caused too many headaches,
so I'm tempted to just get rid of that dependency entirely. And
instead do the simplest possible thing, which might just be calling
get_cycles() (all we need is to introduce some non-determinism).

> The _only_ valid use of _no_resched is when there's a call to schedule()
> right after it.

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