Re: Opteron Rev E has a bug ... a locked instruction doesn't act as a read-acquire barrier (confirmed)

[Arkadiusz Miskiewicz]

On Wednesday 06 August 2008, Wahlig, Elsie wrote:
> Your issue may be one that has been seen on 1st generation
> AMD Opteron processor's with cpuid family 0Fh, cpuid model's
> < 40h with the code sequence that performs a read-modify write
> operation after acquiring a semaphore.

Matches my hardware

cpu family      : 15
model           : 33

>
> The memory read ordering between a semaphore operation and a
> subsequent read-modify-write instruction (an instruction which
> uses the same memory location as both a source and destination)
> may allow the read-modify-write instruction to operate on the
> memory location ahead of the completion of the semaphore
> operation and an erratum may occur.

I wonder why there was no official errata about this?


> If you think your software is encountering this code sequence,
> a work-around should be implemented by adding an LFENCE
> instruction right after the semaphore, after a cpuid check.
> The workaround's applied to OpenSolaris at
> http://mail.opensolaris.org/pipermail/onnv-notify/2006-October/009080.ht
> ml
> and Google performance tools tool at
> http://google-perftools.googlecode.com/svn-history/r48/trunk/src/base/at
> omicops-internals-x86.cc
> are suitable examples.
> A list of the model numbers this issue may occur on is at
> http://products.amd.com/en-us/downloads/AMD_Opteron_First_Generation_Ref
> erence_101607.pdf.

Would be better to fix the bug on kernel level if this is possible. Just 
someone with the knowledge needs to do this. Anyone interested?

> Mikael Pettersson writes:
> ... snip ...
>
> > I investigated the Solaris track, but I've found no detailed
> > explanation of the alleged bug. I've asked the Sun engineer
> > who committed the fix for an explanation, but so far there's
> > been no reply.
> >
> > Anyway, here's what I've found out.
> >
> > It's Solaris bug # 6323525.
> >
> > They call it "Mutex primitives don't work as expected."
> >
> > if (number_of_cores() < 2) then don't have bug if (family ==
> > 0xf && Model < 0x40) then have bug if
> > (rdmsr(MSR_BU_CFG/*0xC0011023*/) & 2) then bug is masked
> > lock: // mutex_lock, spin_lock, etc
> > ...
> > lock; cmpxchg ..
> > jnz fail
> > ret; nop; nop; nop // patched to "lfence; ret" if bug The
> > workaround is to place a fencing instruction (lfence) between
> > the mutex operation and the subsequent read-modify-write instruction.
> > (This provides the necessary load memory barrier.)
> >
> > There's no change to the unlock code.
> >
> > Anyone know who to contact @ AMD about confirming or denying this?
> >
> > /Mikael



-- 
Arkadiusz MiÅkiewicz        PLD/Linux Team
arekm / maven.pl            http://ftp.pld-linux.org/
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