On Mon, 23 Nov 2020 at 17:08, Peter Zijlstra <[email protected]> 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.
