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

On Wed, Jun 24, 2015 at 5:07 PM, Andy Lutomirski [off-list ref] wrote:

On Wed, Jun 24, 2015 at 3:26 PM, Paul Turner [off-list ref] wrote:

quoted

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