linux-kernel - Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

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Date:   Wed, 29 Sep 2021 14:28:37 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     will@...nel.org, paulmck@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.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,
        linux-toolchains@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

* Mathieu Desnoyers:

> + * 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, architectures that allow the LOAD->STORE reorder must ensure
> + * the compiler actually emits the conditional branch instruction.

If you need a specific instruction emitted, you need a compiler
intrinsic or inline assembly.

So something like this:

#define control_dep(x)                          \
  ({                                            \
    __typeof(x) x__ = (x);                      \
    __asm__("test $0, %0\n\t"                   \
            "jnz 1f\n\t"                        \
            "1:"                                \
            :: "r"(x__) : "cc");                \
  })

with an appropriate instruction sequence for each architecture.

I don't think it's possible to piggy-back this on something else.

Thanks,
Florian