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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