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SubjectRe: [RFR] Store tearing
On Mon, Oct 29, 2018 at 10:23:07AM +0100, Arnd Bergmann wrote:
> On Mon, Oct 29, 2018 at 2:21 AM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> >
> > On Mon, Oct 29, 2018 at 12:10:03AM +0100, Andrea Parri wrote:
> > > Hopefully, with Paul's proper email address this time,
> > >
> > > Andrea
> > >
> > > On Mon, Oct 29, 2018 at 12:06:27AM +0100, Andrea Parri wrote:
> > > > Hi,
> > > >
> > > > memory-barriers.txt says:
> > > >
> > > > [on "store tearing"]
> > > >
> > > > "In fact, a recent bug (since fixed) caused GCC to incorrectly use
> > > > this optimization in a volatile store.".
> > > >
> > > > I was wondering if you could help me retrieve some reference/discussions
> > > > about this?
> >
> > This was quite some time ago, but it involved a 32-bit volatile store
> > of a constant such as 0x10001. The machine in question had a narrow
> > store-immediate instruction, so the compiler emitted a pair of 16-bit
> > store-immediate instructions. This bug was fixed, though only after
> > significant screaming and shouting.
>
> A related issue I remember was on ARMv5 (an architecture without
> unaligned access) where a function like )not sure if this specific
> one triggers it, but something like it did)
>
> struct my_registers {
> u32 a;
> u32 b;
> u32 c;
> } __attribute__((packed));
> #define __raw_writel(p, v) do { (volatile u32 __iomem *)(p) = (v); } while (0)
> void my_write_a(struct my_registers __iomem *r, u32 val)
> {
> __raw_writel(&r->a, val);
> }
>
> The above is undefined behavior because we cast from an unaligned
> data type to a 32-bit aligned type, and gcc resolved this by turning the
> intended 32-bit store into a set of 8 bit stores. We worked around this
> by changing __raw_writel() into a inline assembly that always uses a
> 32-bit store.

I had either missed or forgotten this one, nice example of store tearing!

Thanx, Paul

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