On 8/11/20 2:10 AM, Oleg Nesterov wrote:
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.
Thanks, reading through this thread makes me feel better. I agree that we'll need READ_ONCE() just to shut up analyzers.
I'd really like to get this done at the same time as the io_uring change. Are you open to doing the READ_ONCE() based JOBCTL_TASK_WORK addition for 5.9? Alternatively we can retain the 1/2 patch from this series and I'll open-code it in io_uring, but seems pointless as io_uring is the only user of TWA_SIGNAL in the kernel anyway.