Re: [GIT PULL rcu/next] RCU commits for 4.13

[Boqun Feng]

On Thu, Jun 29, 2017 at 09:02:41PM -0700, Paul E. McKenney wrote:
[...]
> > > o	net/netfilter/nf_conntrack_core.c nf_conntrack_lock()
> > > 	This instance of spin_unlock_wait() interacts with
> > > 	nf_conntrack_all_lock()'s instance of spin_unlock_wait().
> > > 	Although nf_conntrack_all_lock() has an smp_mb(), which I
> > > 	believe provides release semantics given current implementations,
> > > 	nf_conntrack_lock() just has smp_rmb().
> > > 
> > > 	I believe that the smp_rmb() needs to be smp_mb().  Am I missing
> > > 	something here that makes the current code safe on x86?
> > > 
> > 
> > actually i think the smp_rmb() or even along with the spin_unlock_wait()
> > in nf_conntrack_lock() is not needed, we could
> > implementnf_conntrack_lock() as:
> > 
> > 	
> > 	void nf_conntrack_lock(spinlock_t *lock) __acquires(lock)
> > 	{
> > 		spin_lock(lock);
> > 		while (unlikely(smp_load_acquire(nf_conntrack_locks_all))) {
> > 			spin_unlock(lock);
> > 			cpu_relaxed();
> > 			spin_lock(lock);
> > 		}
> > 	}
> > 
> > because in nf_conntrack_all_unlock(), we have:
> > 
> > 		smp_store_release(&nf_conntrack_locks_all, false);
> > 		spin_unlock(&nf_conntrack_locks_all_lock);
> > 
> > so if we exit the loop, which means we observe nf_conntrack_locks_all
> > being false, we actually hold the per bucket lock and observe everything
> > before the smp_store_release(), which is the same as everything in the
> > critical section of nf_conntrack_locks_all_lock. Otherwise, we observe
> > the nf_conntrack_locks_all being true, which means a global lock
> > critical section may be on its way, we simply drop the per bucket lock
> > and test whether the global lock is finished again some time later.
> > 
> > So I think spin_unlock_wait() in the nf_conntrack_lock() just requires
> > acquire semantics, at least.
> > 
> > Maybe I miss someting?
> 
> Or perhaps I was being too paranoid.
> 
> But does the same analysis work in the case where an nf_conntrack_lock
> races with an nf_contrack_all_lock()?
> 

You mean the smp_mb()+spin_unlock_wait() in nf_conntrack_all_lock(),
right? I think it's different, because nf_conntrack_all_lock() relies
this release-like operation to let all the next critical sections of per
bucket locks observe nf_conntrack_locks_all=true, otherwise
nf_conntrack_lock() will break out the loop and access some data while
the global lock crictial section is doing the same.

The variable @nf_conntrack_locks_all is used for synchronized between
two kinds of locks and is set by nf_conntrack_all_lock(), I think this
make things different.

> > > 	I believe that this code could use spin_lock+spin_unlock without
> > > 	significant performance penalties -- I do not believe that
> > > 	nf_conntrack_locks_all_lock gets significant contention.
> > > 
> > > raw_spin_unlock_wait() (Courtesy of Andrea Parri with added commentary):
> > > 
> > > o	kernel/exit.c do_exit()
> > > 	Seems to rely on both acquire and release semantics. The
> > > 	raw_spin_unlock_wait() primitive is preceded by a smp_mb().
> > > 	But this is task exit doing spin_unlock_wait() on the task's
> > > 	lock, so spin_lock+spin_unlock should work fine here.
> > > 
> > > o	kernel/sched/core.c do_task_dead()
> > > 	Seems to rely on the acquire semantics only. The
> > > 	raw_spin_unlock_wait() primitive is preceded by an inexplicable
> > > 	smp_mb().  Again, this is task exit doing spin_unlock_wait() on
> > > 	the task's lock, so spin_lock+spin_unlock should work fine here.
> > > 
> > > o	kernel/task_work.c task_work_run()
> > > 	Seems to rely on the acquire semantics only.  This is to handle
> > 
> > I think this one needs the stronger semantics, the smp_mb() is just
> > hidden in the cmpxchg() before the raw_spin_unlock_wait() ;-)
> > 
> > cmpxchg() sets a special value to indicate the task_work has been taken,
> > and raw_spin_unlock_wait() must wait until the next critical section of
> > ->pi_lock(in task_work_cancel()) could observe this, otherwise we may
> > cancel a task_work while executing it.
> 
> But either way, replacing the spin_unlock_wait() with a spin_lock()
> immediately followed by a spin_unlock() should work correctly, right?
> 

Yep ;-) I was thinking about the case that we kept spin_unlock_wait()
with a simpler acquire semantics, and if so, we would actually have to
do the replace. But I saw your patchset of removing it, so it doesn't
matter.

Regards,
Boqun

> 							Thanx, Paul
> 
> > Regards,
> > Boqun
> > > 	a race with task_work_cancel(), which appears to be quite rare.
> > > 	So the spin_lock+spin_unlock should work fine here.
> > > 
> > > spin_lock()/spin_unlock():
> > > 
> > > o	ipc/sem.c complexmode_enter()
> > > 	This used to be spin_unlock_wait(), but was changed to a
> > > 	spin_lock()/spin_unlock() pair by 27d7be1801a4 ("ipc/sem.c:
> > > 	avoid using spin_unlock_wait()").
> > > 
> > > Looks to me like we really can drop spin_unlock_wait() in favor of
> > > momentarily acquiring the lock.  There are so few use cases that I don't
> > > see a problem open-coding this.  I will put together yet another patch
> > > series for my spin_unlock_wait() collection of patch serieses.  ;-)
> > > 
> > > > As regards (2), I did a little digging.  spin_unlock_wait was
> > > > introduced in the 2.1.36 kernel, in mid-April 1997.  I wasn't able to
> > > > find a specific patch for it in the LKML archives.  At the time it
> > > > was used in only one place in the entire kernel (in kernel/exit.c):
> > > > 
> > > > void release(struct task_struct * p)
> > > > {
> > > > 	int i;
> > > > 
> > > > 	if (!p)
> > > > 		return;
> > > > 	if (p == current) {
> > > > 		printk("task releasing itself\n");
> > > > 		return;
> > > > 	}
> > > > 	for (i=1 ; i<NR_TASKS ; i++)
> > > > 		if (task[i] == p) {
> > > > #ifdef __SMP__
> > > > 			/* FIXME! Cheesy, but kills the window... -DaveM */
> > > > 			while(p->processor != NO_PROC_ID)
> > > > 				barrier();
> > > > 			spin_unlock_wait(&scheduler_lock);
> > > > #endif
> > > > 			nr_tasks--;
> > > > 			task[i] = NULL;
> > > > 			REMOVE_LINKS(p);
> > > > 			release_thread(p);
> > > > 			if (STACK_MAGIC != *(unsigned long *)p->kernel_stack_page)
> > > > 				printk(KERN_ALERT "release: %s kernel stack corruption. Aiee\n", p->comm);
> > > > 			free_kernel_stack(p->kernel_stack_page);
> > > > 			current->cmin_flt += p->min_flt + p->cmin_flt;
> > > > 			current->cmaj_flt += p->maj_flt + p->cmaj_flt;
> > > > 			current->cnswap += p->nswap + p->cnswap;
> > > > 			free_task_struct(p);
> > > > 			return;
> > > > 		}
> > > > 	panic("trying to release non-existent task");
> > > > }
> > > > 
> > > > I'm not entirely clear on the point of this call.  It looks like it 
> > > > wanted to wait until p was guaranteed not to be running on any 
> > > > processor ever again.  (I don't see why it couldn't have just acquired 
> > > > the scheduler_lock -- was release() a particularly hot path?)
> > > > 
> > > > Although it doesn't matter now, this would mean that the original
> > > > semantics of spin_unlock_wait were different from what we are
> > > > discussing.  It apparently was meant to provide the release guarantee:
> > > > any future critical sections would see the values that were visible
> > > > before the call.  Ironic.
> > > 
> > > Cute!!!  ;-)
> > > 
> > > 							Thanx, Paul
> > > 
> 
>

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