Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute

From: Arnd Bergmann
Date: Tue Jun 21 2011 - 07:25:33 EST

On Tuesday 21 June 2011, Nicolas Pitre wrote:
> On Mon, 20 Jun 2011, Arnd Bergmann wrote:

> This example is flawed. The DMA API documentation already forbids DMA to
> the stack because of cache line sharing issues. If you declare your
> buffer outside of the function body, the compiler can't optimize away
> the buffer store anymore, and this example works as expected without any
> memory clobber.

Ok, another example, even simpler:

int f(int *dma_buf, volatile int *mmio_reg)
(void) *mmio_reg; /* wait for DMA to complete */
return *dma_buf;

gcc-4.4, 4.5 and 4.6 all turn this into:

ldr r0, [r0, #0]
ldr r3, [r1, #0]
bx lr

which means that the dma_buf variable is dereferenced before the
volatile mmio_reg variable, which opens up a race: An interrupt may have
signalled us that a DMA is in progress, so we read a MMIO register from
the device (this is guaranteed to flush the DMA on PCI and similar buses).
If we read the dma_buf before we read the mmio register, the data we get
back may be stale.

Adding a barrier() between the two turns the assembly into the expected

ldr r3, [r1, #0]
ldr r0, [r0, #0]
bx lr

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