linux-kernel - Re: [PATCH] RISC-V: Break load reservations during switch

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Date:   Thu, 6 Jun 2019 02:05:18 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     linux-riscv@...ts.infradead.org,
        Paul Walmsley <paul.walmsley@...ive.com>, marco@...red.org,
        me@...losedp.com, joel@...g.id.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RISC-V: Break load reservations during switch_to

On Wed, Jun 05, 2019 at 04:17:35PM -0700, Palmer Dabbelt wrote:
>  	REG_S ra,  TASK_THREAD_RA_RA(a3)
> +	/*
> +	 * The Linux ABI allows programs to depend on load reservations being
> +	 * broken on context switches, but the ISA doesn't require that the
> +	 * hardware ever breaks a load reservation.  The only way to break a
> +	 * load reservation is with a store conditional, so we emit one here.
> +	 * Since nothing ever takes a load reservation on TASK_THREAD_RA_RA we
> +	 * know this will always fail, but just to be on the safe side this
> +	 * writes the same value that was unconditionally written by the
> +	 * previous instruction.
> +	 */
> +#if (TASK_THREAD_RA_RA != 0)

I don't think this check works as intended.  TASK_THREAD_RA_RA is a
parameterized macro, thus the above would never evaluate to 0. The
error message also is rather odd while we're at it.

> +#if (__riscv_xlen == 64)
> +	sc.d  x0, ra, 0(a3)
> +#else
> +	sc.w  x0, ra, 0(a3)
> +#endif

I'd rather add an macro ala REG_S to asm.h and distinguish between the
xlen variants there:

#define REG_SC		__REG_SEL(sc.d, sc.w)