linux-kernel - Re: [PATCH] refcount_t: add ACQUIRE ordering on success for dec(sub)_and

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Date:   Wed, 30 Jan 2019 04:33:33 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] refcount_t: add ACQUIRE ordering on success for
 dec(sub)_and_test variants

> So, you are saying that ACQUIRE does not guarantee that "po-later stores
> on the same CPU and all propagated stores from other CPUs
> must propagate to all other CPUs after the acquire operation "? 
> I was reading about acquire before posting this and trying to understand,
> and this was my conclusion that it should provide this, but I can easily be wrong
> on this. 
> 
> Andrea, Peter, could you please comment?

Short version:  I am not convinced by the above sentence, and I suggest
to remove it (as done in

  http://lkml.kernel.org/r/20190128142910.GA7232@andrea ).

---
To elaborate:  I think that we should first discuss the meaning of that
"[...] after the acquire operation (does)",  because there is no notion
of "ACQUIRE (or more generally, load) propagation" in the LKMM:

Stores propagate (after being executed) to other CPUs.  Loads _execute_
(possibly multiple times /speculatively, but this is irrelevant for the
discussion below).

A detailed, but still informal, description of these concepts is in:

  tools/memory-model/Documentation/explanation.txt

(c.f., in particular, section "AN OPERATIONAL MODEL"); I can illustrate
them with an example:

	{ initially: x=0, y=0; }

	CPU0			CPU1
	--------------------------------------
	LOAD-ACQUIRE x=0	LOAD y=1
	STORE y=1

In this scenario,

  a) CPU0's "LOAD-ACQUIRE x=0" executes before CPU0's "STORE y=1"
     executes (this is guaranteed by the ACQUIRE),

  b) CPU0's "STORE y=1" executes before "STORE y=1" propagates to
     CPU1 (a store cannot be propagated before being executed),

  c) CPU0's "STORE y=1" propagates to CPU1 before CPU1's "LOAD y=1"
     executes (since CPU1 "sees the store"). 

The example also illustrates the following property:

  ACQUIRE guarantees that po-later stores on the same CPU must
  propagate to all other CPUs after the acquire _executes_.

(combine (a) and (b) ).

OTOH, please notice that:

  ACQUIRE does _NOT_ guarantee that all propagated stores from
  other CPUs (to the CPU executing the ACQUIRE) must propagate
  to all other CPUs after the acquire operation _executes_.

In fact, we've already seen how full barriers can be used to break such
"guarantee"; for example, in

	{ initially: x=0, y=0; }

	CPU0			CPU1			...
	---------------------------------------------------
	STORE x=1		LOAD x=1	
				FULL-BARRIER
				LOAD-ACQUIRE y=0

the full barrier forces CPU0's "STORE x=1" (seen by/propagated to CPU1)
to be propagated to all CPUs _before_ "LOAD-ACQUIRE y=0" is executed.

Does this make sense?


> > Is ACQUIRE strictly stronger than control dependency?
> 
> In my understanding yes.

+1 (or we have a problem)


>
> > It generally looks so unless there is something very subtle that I am
> > missing. If so, should we replace it with just "RELEASE ordering +
> > ACQUIRE ordering on success"? Looks simpler with less magic trickery.
> 
> I was just trying to mention all the applicable orderings/guarantees. 
> I can remove "control dependency" part if it is easier for people to understand
> (the main goal of documentation).

This sounds like a good idea; thank you, Dmitry, for pointing this out.

  Andrea


> 
> Best Regards,
> Elena.