On Thu, Jun 29, 2017 at 11:11:26AM -0700, Paul E. McKenney wrote: > On Thu, Jun 29, 2017 at 11:59:27AM -0400, Alan Stern wrote: > > On Thu, 29 Jun 2017, Will Deacon wrote: > > > > > [turns out I've not been on cc for this thread, but Jade pointed me to it > > > and I see my name came up at some point!] > > > > > > On Wed, Jun 28, 2017 at 05:05:46PM -0700, Linus Torvalds wrote: > > > > On Wed, Jun 28, 2017 at 4:54 PM, Paul E. McKenney > > > > <paul...@linux.vnet.ibm.com> wrote: > > > > > > > > > > Linus, are you dead-set against defining spin_unlock_wait() to be > > > > > spin_lock + spin_unlock? For example, is the current x86 > > > > > implementation > > > > > of spin_unlock_wait() really a non-negotiable hard requirement? Or > > > > > would you be willing to live with the spin_lock + spin_unlock > > > > > semantics? > > > > > > > > So I think the "same as spin_lock + spin_unlock" semantics are kind of > > > > insane. > > > > > > > > One of the issues is that the same as "spin_lock + spin_unlock" is > > > > basically now architecture-dependent. Is it really the > > > > architecture-dependent ordering you want to define this as? > > > > > > > > So I just think it's a *bad* definition. If somebody wants something > > > > that is exactly equivalent to spin_lock+spin_unlock, then dammit, just > > > > do *THAT*. It's completely pointless to me to define > > > > spin_unlock_wait() in those terms. > > > > > > > > And if it's not equivalent to the *architecture* behavior of > > > > spin_lock+spin_unlock, then I think it should be descibed in terms > > > > that aren't about the architecture implementation (so you shouldn't > > > > describe it as "spin_lock+spin_unlock", you should describe it in > > > > terms of memory barrier semantics. > > > > > > > > And if we really have to use the spin_lock+spinunlock semantics for > > > > this, then what is the advantage of spin_unlock_wait at all, if it > > > > doesn't fundamentally avoid some locking overhead of just taking the > > > > spinlock in the first place? > > > > > > Just on this point -- the arm64 code provides the same ordering semantics > > > as you would get from a lock;unlock sequence, but we can optimise that > > > when compared to an actual lock;unlock sequence because we don't need to > > > wait in turn for our ticket. I suspect something similar could be done > > > if/when we move to qspinlocks. > > > > > > Whether or not this is actually worth optimising is another question, but > > > it is worth noting that unlock_wait can be implemented more cheaply than > > > lock;unlock, whilst providing the same ordering guarantees (if that's > > > really what we want -- see my reply to Paul). > > > > > > Simplicity tends to be my preference, so ripping this out would suit me > > > best ;) > > > > It would be best to know: > > > > (1). How spin_unlock_wait() is currently being used. > > > > (2). What it was originally intended for. > > > > Paul has done some research into (1). He can correct me if I get this > > wrong... Only a few (i.e., around one or two) of the usages don't seem > > to require the full spin_lock+spin_unlock semantics. I go along with > > Linus; the places which really do want it to behave like > > spin_lock+spin_unlock should simply use spin_lock+spin_unlock. There > > hasn't been any indication so far that the possible efficiency > > improvement Will mentions is at all important. > > > > According to Paul, most of the other places don't need anything more > > than the acquire guarantee (any changes made in earlier critical > > sections will be visible to the code following spin_unlock_wait). In > > which case, the semantics of spin_unlock_wait could be redefined in > > this simpler form. > > > > Or we could literally replace all the existing definitions with > > spin_lock+spin_unlock. Would that be so terrible? > > And here they are... > > spin_unlock_wait(): > > o drivers/ata/libata-eh.c ata_scsi_cmd_error_handler() > spin_unlock_wait(ap->lock) in else-clause where then-clause has > a full critical section for this same lock. This use case could > potentially require both acquire and release semantics. (I am > following up with the developers/maintainers, suggesting that > they convert to spin_lock+spin_unlock if they need release > semantics.) > > This is error-handling code, which should be rare, so > spin_lock+spin_unlock should work fine here. Probably shouldn't > have bugged the maintainer, but email already sent. :-/ > > o ipc/sem.c exit_sem() > This use case appears to need to wait only on prior critical > sections, as the only way we get here is if the entry has already > been removed from the list. An acquire-only spin_unlock_wait() > works here. However, this is sem-exit code, which is not a > fastpath, and the race should be rare, so spin_lock+spin_unlock > should work fine here. > > o kernel/sched/completion.c completion_done() > This use case appears to need to wait only on prior critical > sections, as the only way we get past the "if" is when the lock is > held by complete(), and you are only supposed to invoke complete() > once on a given completion. An acquire-only spin_unlock_wait() > works here, but the race should be rare, so spin_lock+spin_unlock > should also work fine here. > > 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?
> 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.
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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