Re: [PATCH 00/22] add support for Clang LTO

Paul E. McKenney
2020-07-06T23:41:37Z

On Mon, Jul 06, 2020 at 09:40:12PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 06, 2020 at 11:39:33AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 06, 2020 at 08:29:26PM +0200, Peter Zijlstra wrote:
> > > On Mon, Jul 06, 2020 at 09:26:33AM -0700, Paul E. McKenney wrote:
>
> > > If they do not consider their Linux OS running correctly :-)
> >
> > Many of them really do not care at all. In fact, some would consider
> > Linux failing to run as an added bonus.
>
> This I think is why we have compiler people in the thread that care a
> lot more.

Here is hoping! ;-)

> > > > Nevertheless, yes, control dependencies also need attention.
> > >
> > > Today I added one more \o/
> >
> > Just make sure you continually check to make sure that compilers
> > don't break it, along with the others you have added. ;-)
>
> There's:
>
> kernel/locking/mcs_spinlock.h: smp_cond_load_acquire(l, VAL); \
> kernel/sched/core.c: smp_cond_load_acquire(&p->on_cpu, !VAL);
> kernel/smp.c: smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
>
> arch/x86/kernel/alternative.c: atomic_cond_read_acquire(&desc.refs, !VAL);
> kernel/locking/qrwlock.c: atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
> kernel/locking/qrwlock.c: atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
> kernel/locking/qrwlock.c: atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
> kernel/locking/qspinlock.c: atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
> kernel/locking/qspinlock.c: val = atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK));
>
> include/linux/refcount.h: smp_acquire__after_ctrl_dep();
> ipc/mqueue.c: smp_acquire__after_ctrl_dep();
> ipc/msg.c: smp_acquire__after_ctrl_dep();
> ipc/sem.c: smp_acquire__after_ctrl_dep();
> kernel/locking/rwsem.c: smp_acquire__after_ctrl_dep();
> kernel/sched/core.c: smp_acquire__after_ctrl_dep();
>
> kernel/events/ring_buffer.c:__perf_output_begin()
>
> And I'm fairly sure I'm forgetting some... One could argue there's too
> many of them to check already.
>
> Both GCC and CLANG had better think about it.

That would be good!

I won't list the number of address/data dependencies given that there
are well over a thousand of them.

Thanx, Paul