linux-kernel - Re: Documentation/memory-barriers.txt: Is "stores are not speculated" correct?

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Message-ID: <20210426035043.GW975577@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 25 Apr 2021 20:50:43 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     szyhb810501.student@...a.com
Cc:     stern <stern@...land.harvard.edu>,
        "parri.andrea" <parri.andrea@...il.com>, will <will@...nel.org>,
        peterz <peterz@...radead.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Documentation/memory-barriers.txt: Is "stores are not
 speculated" correct?

On Mon, Apr 26, 2021 at 10:23:09AM +0800, szyhb810501.student@...a.com wrote:
> 
> Hello everyone, I have a question."Documentation/memory-barriers.txt"
> says:However, stores are not speculated.  This means that ordering -is-
> providedfor load-store control dependencies, as in the following example:
	q = READ_ONCE(a);
	if (q) {
		WRITE_ONCE(b, 1);
	}
> Is "stores are not speculated" correct? I
> think store instructions can be executed speculatively.
> "https://stackoverflow.com/questions/64141366/can-a-speculatively-executed-cpu-branch-contain-opcodes-that-access-ram"
> says:Store instructions can also be executed speculatively thanks to the
> store buffer. The actual execution of a store just writes the address and
> data into the store buffer.Commit to L1d cache happens some time after
> the store instruction retires from the ROB, i.e. when the store is known
> to be non-speculative, the associated store-buffer entry "graduates"
> and becomes eligible to commit to cache and become globally visible.

>From the viewpoint of other CPUs, the store hasn't really happened
until it finds its way into a cacheline.  As you yourself note above,
if the store is still in the store buffer, it might be squashed when
speculation fails.

So Documentation/memory-barriers.txt and that stackoverflow entry are
not really in conflict, but are instead using words a bit differently
from each other.  The stackoverflow entry is considering a store to have
in some sense happened during a time when it might later be squashed.
In contrast, the Documentation/memory-barriers.txt document only considers
a store to have completed once it is visible outside of the CPU executing
that store.

So from a stackoverflow viewpoint, stores can be speculated, but until
they are finalized, they must be hidden from other CPUs.

>From a Documentation/memory-barriers.txt viewpoint, stores don't complete
until they update their cachelines, and stores may not be speculated.
Some of the actions that lead up to the completion of a store may be
speculated, but not the completion of the store itself.

Different words, but same effect.  Welcome to our world!  ;-)

							Thanx, Paul