On 08/11, Peter Zijlstra wrote:
On Tue, Aug 11, 2020 at 09:14:02AM +0200, Oleg Nesterov wrote:
On 08/11, Peter Zijlstra wrote:
On Tue, Aug 11, 2020 at 08:45:16AM +0200, Oleg Nesterov wrote:
->jobctl is always modified with ->siglock held, do we really need WRITE_ONCE() ?
In theory, yes. The compiler doesn't know about locks, it can tear writes whenever it feels like it.
Yes, but why does this matter? Could you spell please?
Ah, well, that I don't konw. Why do we need the READ_ONCE() ?
It does:
if (!(task->jobctl & JOBCTL_TASK_WORK) &&lock_task_sighand(task, &flags)) {and the lock_task_sighand() implies barrier(), so I thought the reason for the READ_ONCE() was load-tearing, and then we need WRITE_ONCE() to avoid store-tearing.
I don't think we really need READ_ONCE() for correctness, compiler can't reorder this LOAD with cmpxchg() above, and I think we don't care about load-tearing.
But I guess we need READ_ONCE() or data_race() to shut kcsan up.
Oleg.