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From: Mark Rutland <mark.rutland@arm.com>
To: Palmer Dabbelt <palmer@sifive.com>
Cc: linux-riscv@lists.infradead.org,
	Paul Walmsley <paul.walmsley@sifive.com>,
	marco@decred.org, me@carlosedp.com, joel@sing.id.au,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] RISC-V: Break load reservations during switch_to
Date: Wed, 19 Jun 2019 08:36:01 +0100	[thread overview]
Message-ID: <20190619073600.GA29918@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190607222222.15300-1-palmer@sifive.com>

On Fri, Jun 07, 2019 at 03:22:22PM -0700, Palmer Dabbelt wrote:
> The comment describes why in detail.  This was found because QEMU never
> gives up load reservations, the issue is unlikely to manifest on real
> hardware.
> 
> Thanks to Carlos Eduardo for finding the bug!

> @@ -330,6 +330,17 @@ ENTRY(__switch_to)
>  	add   a3, a0, a4
>  	add   a4, a1, a4
>  	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.
> +	 */

I suspect that you need to do the same as 32-bit ARM, and clear this in
your exception return path, rather than in __switch_to, since handlers
for interrupts and other exceptions could leave a dangling reservation.

For ARM, the architecture permits a store-exclusive to succeed even if
the address differed from the load-exclusive. I don't know if the same
applies here, but regardless I believe the case above applies if an IRQ
is taken from kernel context, since the handler can manipulate the same
variable as the interrupted code.

Thanks,
Mark.

WARNING: multiple messages have this Message-ID (diff)
From: Mark Rutland <mark.rutland@arm.com>
To: Palmer Dabbelt <palmer@sifive.com>
Cc: me@carlosedp.com, linux-kernel@vger.kernel.org, joel@sing.id.au,
	Paul Walmsley <paul.walmsley@sifive.com>,
	linux-riscv@lists.infradead.org, marco@decred.org
Subject: Re: [PATCH v2] RISC-V: Break load reservations during switch_to
Date: Wed, 19 Jun 2019 08:36:01 +0100	[thread overview]
Message-ID: <20190619073600.GA29918@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190607222222.15300-1-palmer@sifive.com>

On Fri, Jun 07, 2019 at 03:22:22PM -0700, Palmer Dabbelt wrote:
> The comment describes why in detail.  This was found because QEMU never
> gives up load reservations, the issue is unlikely to manifest on real
> hardware.
> 
> Thanks to Carlos Eduardo for finding the bug!

> @@ -330,6 +330,17 @@ ENTRY(__switch_to)
>  	add   a3, a0, a4
>  	add   a4, a1, a4
>  	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.
> +	 */

I suspect that you need to do the same as 32-bit ARM, and clear this in
your exception return path, rather than in __switch_to, since handlers
for interrupts and other exceptions could leave a dangling reservation.

For ARM, the architecture permits a store-exclusive to succeed even if
the address differed from the load-exclusive. I don't know if the same
applies here, but regardless I believe the case above applies if an IRQ
is taken from kernel context, since the handler can manipulate the same
variable as the interrupted code.

Thanks,
Mark.

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linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

  parent reply	other threads:[~2019-06-19  7:36 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-07 22:22 [PATCH v2] RISC-V: Break load reservations during switch_to Palmer Dabbelt
2019-06-07 22:22 ` Palmer Dabbelt
2019-06-08  8:19 ` Christoph Hellwig
2019-06-08  8:19   ` Christoph Hellwig
2019-06-16 17:54 ` Joel Sing
2019-06-16 17:54   ` Joel Sing
2019-06-17  3:09   ` Palmer Dabbelt
2019-06-17  3:09     ` Palmer Dabbelt
2019-06-19  7:36 ` Mark Rutland [this message]
2019-06-19  7:36   ` Mark Rutland
2019-06-21  1:50   ` Palmer Dabbelt
2019-06-21  1:50     ` Palmer Dabbelt

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