linux-kernel - Re: [RFC] LKMM: Add volatile

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Date:   Fri, 4 Jun 2021 13:44:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>, will@...nel.org,
        paulmck@...nel.org, stern@...land.harvard.edu,
        parri.andrea@...il.com, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com
Cc:     linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> +/**
> + * volatile_if() - Provide a control-dependency
> + *
> + * volatile_if(READ_ONCE(A))
> + *	WRITE_ONCE(B, 1);
> + *
> + * will ensure that the STORE to B happens after the LOAD of A. Normally a
> + * control dependency relies on a conditional branch having a data dependency
> + * on the LOAD and an architecture's inability to speculate STOREs. IOW, this
> + * provides a LOAD->STORE order.
> + *
> + * Due to optimizing compilers extra care is needed; as per the example above
> + * the LOAD must be 'volatile' qualified in order to ensure the compiler
> + * actually emits the load, such that the data-dependency to the conditional
> + * branch can be formed.
> + *
> + * Secondly, the compiler must be prohibited from lifting anything out of the
> + * selection statement, as this would obviously also break the ordering.
> + *
> + * Thirdly, and this is the tricky bit, architectures that allow the
> + * LOAD->STORE reorder must ensure the compiler actually emits the conditional
> + * branch instruction, this isn't possible in generic.
> + *
> + * See the volatile_cond() wrapper.
> + */
> +#define volatile_if(cond) if (volatile_cond(cond))

On naming (sorry Paul for forgetting that in the initial mail); while I
think using the volatile qualifier for the language feature (can we haz
plz, kthxbai) makes perfect sense, Paul felt that we might use a
'better' name for the kernel use, ctrl_dep_if() was proposed.

Let us pain bike sheds :-)