Re: [RFC PATCH 0/3] restartable sequences: fast user-space percpu critical sections

[Paul Turner]

On Wed, Jun 24, 2015 at 5:07 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Jun 24, 2015 at 3:26 PM, Paul Turner <pjt@xxxxxxxxxx> wrote:
>> This is a fairly small series demonstrating a feature we've found to be quite
>> powerful in practice, "restartable sequences".
>>
>
> On an extremely short glance, I'm starting to think that the right
> approach, at least for x86, is to implement per-cpu gsbase.  Then you
> could do cmpxchg with a gs prefix to atomically take a percpu lock and
> atomically release a percpu lock and check whether someone else stole
> the lock from you.  (Note: cmpxchg, unlike lock cmpxchg, is very
> fast.)
>
> This is totally useless for other architectures, but I think it would
> be reasonable clean on x86.  Thoughts?

So this gives semantics that are obviously similar to this_cpu().
This provides allows reasonable per-cpu counters (which is alone
almost sufficient for a strong user-space RCU implementation giving
this some legs).

However, unless there's a nice implementation trick I'm missing, the
thing that stands out to me for locks (or other primitives) is that
this forces a two-phase commit.  There's no way (short of say,
cmpxchg16b) to perform a write conditional on the lock not having been
stolen from us (and subsequently release the lock).

e.g.
 1) We take the operation in some sort of speculative mode, that
another thread on the same cpu is stilled allowed to steal from us
 2) We prepare what we want to commit
 3) At this point we have to promote the lock taken in (1) to perform
our actual commit, or see that someone else has stolen (1)
 4) Release the promoted lock in (3)

However, this means that if we're preempted at (3) then no other
thread on that cpu can make progress until we've been rescheduled and
released the lock; a nice property of the model we have today is that
threads sharing a cpu can not impede each other beyond what the
scheduler allows.

A lesser concern, but worth mentioning, is that there are also
potential pitfalls in the interaction with signal handlers,
particularly if a 2-phase commit is used.

- Paul
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/