Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

[Oleg Nesterov]

On 10/22, Marco Elver wrote:
>
> On Tue, 22 Oct 2019 at 17:49, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >
> > Just for example. Suppose that task->state = TASK_UNINTERRUPTIBLE, this task
> > does __set_current_state(TASK_RUNNING), another CPU does wake_up_process(task)
> > which does the same UNINTERRUPTIBLE -> RUNNING transition.
> >
> > Looks like, this is the "data race" according to kcsan?
>
> Yes, they are "data races". They are probably not "race conditions" though.
>
> This is a fair distinction to make, and we never claimed to find "race
> conditions" only

I see, thanks, just wanted to be sure...

> KCSAN's goal is to find *data races* according to the LKMM.  Some data
> races are race conditions (usually the more interesting bugs) -- but
> not *all* data races are race conditions. Those are what are usually
> referred to as "benign", but they can still become bugs on the wrong
> arch/compiler combination. Hence, the need to annotate these accesses
> with READ_ONCE, WRITE_ONCE or use atomic_t:

Well, if I see READ_ONCE() in the code I want to understand why it was
used. Is it really needed for correctness or we want to shut up kcsan?
Say, why should wait_event(wq, *ptr) use READ_ONCE()? Nevermind, please
forget.

Btw, why __kcsan_check_watchpoint() does user_access_save() before
try_consume_watchpoint() ?

Oleg.