Re: Broken ARM atomic ops wrt memory barriers (was : [PATCH] Addcmpxchg support for ARMv6+ systems)

From: Mathieu Desnoyers
Date: Wed May 27 2009 - 14:41:15 EST

* Catalin Marinas (catalin.marinas@xxxxxxx) wrote:
> On Tue, 2009-05-26 at 21:22 -0400, Mathieu Desnoyers wrote:
> > So, my questions is : is ARMv7 weak memory ordering model as weak as
> > Alpha ?
> I'm not familiar with Alpha but ARM allows a weakly ordered memory
> system (starting with ARMv6), it's up to the processor implementer to
> decide how weak but within the ARM ARM restrictions (section A3.8.2).
> I think the main difference with Alpha is that ARM doesn't do
> speculative writes, only speculative reads. The write cannot become
> visible to other observers in the same shareability domain before the
> instruction occurs in program order. But because of the write buffer,
> there is no guarantee on the order of two writes becoming visible to
> other observers in the same shareability domain. The reads from normal
> memory can happen speculatively (with a few restrictions)
> Summarising from the ARM ARM, there are two terms used:
> Address dependency - an address dependency exists when the value
> returned by a read access is used to compute the virtual address
> of a subsequent read or write access.
> Control dependency - a control dependency exists when the data
> value returned by a read access is used to determine the
> condition code flags, and the values of the flags are used for
> condition code checking to determine the address of a subsequent
> read access.
> The (simplified) memory ordering restrictions of two explicit accesses
> (where multiple observers are present and in the same shareability
> domain):
> * If there is an address dependency then the two memory accesses
> are observed in program order by any observer
> * If the value returned by a read access is used as data written
> by a subsequent write access, then the two memory accesses are
> observed in program order
> * It is impossible for an observer of a memory location to observe
> a write access to that memory location if that location would
> not be written to in a sequential execution of a program
> Outside of these restrictions, the processor implementer can do whatever
> it makes the CPU faster. To ensure the relative ordering between memory
> accesses (either read or write), the software should have DMB
> instructions.

Just to make sure :

for the read seqlock, a smp_rmb() is present. I assume that given there
is no address nor control dependency (as stated above) between the
seqlock value reads and the data access, these barriers cannot be
downgraded to a smp read barrier depend. It's a shame to have to do two
full dmb for every sequence lock. Are there any plans on the ARM side to
eventually add faster read barriers ?

Basically, on arm, a seqlock fast path takes 11 cycles on UP. If we add
the two dmb, it now takes 73 cycles.


> --
> Catalin

Mathieu Desnoyers
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