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Message-ID: <1273707708.15428.4.camel@mulgrave.site>
Date: Wed, 12 May 2010 18:41:48 -0500
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Jamie Lokier <jamie@...reable.org>,
"Shilimkar, Santosh" <santosh.shilimkar@...com>,
"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
Nicolas Pitre <nico@...vell.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"James E.J. Bottomley" <jejb@...isc-linux.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Saeed Bishara <saeed@...vell.com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: Re: Rampant ext3/4 corruption on 2.6.34-rc7 with VIVT ARM (Marvell
88f5182)
On Thu, 2010-05-13 at 08:47 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2010-05-12 at 23:21 +0100, Jamie Lokier wrote:
> > Shilimkar, Santosh wrote:
> > > There was a memory write barrier missing before the DMA descriptors
> > > are handed over to DMA controller.
> >
> > On that note, are the cache flush functions implicit memory barriers?
Not exactly ... they *should* be stream ordered with respect to accesses
to the memory they're flushing (which isn't the same thing, and no-one
ever went broke overestimating the stupidity of chip designers, but if a
flush instruction needs explicit ordering, I'd expect that to be built
into the arch layer).
> (Adding Fujita on CC)
>
> That's a very good question. The generic inline implementation of
> dma_sync_* is:
>
> static inline void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
> size_t size,
> enum dma_data_direction dir)
> {
> struct dma_map_ops *ops = get_dma_ops(dev);
>
> BUG_ON(!valid_dma_direction(dir));
> if (ops->sync_single_for_cpu)
> ops->sync_single_for_cpu(dev, addr, size, dir);
> debug_dma_sync_single_for_cpu(dev, addr, size, dir);
> }
>
> Which means that for coherent architectures that do not implement
> the ops->sync_* hooks, we are probably missing a barrier here...
>
> Thus if the above is expected to be a memory barrier, it's broken on
> cache coherent powerpc for example. On non-coherent powerpc, we do cache
> flushes and those are implicit barriers.
Can you explain this a little more. On a cache coherent machine, the
sync is a nop ... why would you want a nop to be any type of barrier?
James
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