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SubjectRe: BUG: Global FPU corruption in 2.2
On Thu, Apr 19, 2001 at 11:05:03AM -0500, Victor Zandy wrote:
>
> We have found that one of our programs can cause system-wide
> corruption of the x86 FPU under 2.2.16 and 2.2.17.
....

> We see this problem on dual 550MHz Xeons with 1GB RAM.

Hm, I started to wonder if this is not somewhat related to a recent
report I got. "The victim" was running 2.2.19 (basically) on an SMP
Alpha UP2000+ with two 800 MHz processors. He managed to reduce the
problem to a rather small test case and I attach sources, Makefile and
a "loop.sh" driver as a shar archive if you want to have a closer look.

This "loop.sh" simply fires triplets of "harry" process in a loop.
The guy hit by this gets apparently random floating point exceptions
starting with roughly sixth process and later intervals between bombs
will vary. I have also 'strace' outputs from failing processes but
they are not telling very much. 'gdb' is also not very illuminating:

Program received signal SIGFPE, Arithmetic exception.
0x1200010a8 in vadd_ (a=0x11fff21e4, ia=0x120003294, b=0x11fff7004,
ib=0x120003294, c=0x11fffbe20, ic=0x120003294, n=0x11ffffc70) at vadd.f:99
99 C(CI) = A(AI) + B(BI)
Current language: auto; currently fortran
(gdb) p *ia
$10 = 1
(gdb) p *ib
$11 = 1
(gdb) p *ic
$12 = 1
(gdb) p *n
Cannot access memory at address 0x4
(gdb) p *(0x11ffffc70)
$13 = 1024
(gdb) info locals
n = (PTR TO -> ( integer )) 0x4
__g77_expr_0 = 10


He tells me that he is getting that on two different machines he has
around.

The trouble is that I tried to repeat that with different hardware,
kernels, compilers and libraries and I failed even on SMP; but I got an
access to a box with only 667 MHz processors. OTOH he is running
right now 2.4.3-ac9 plus Andrea Arcangeli patches for rw semaphores
on Alpha and he reports that the problem went away (and, hopefuly,
nothing else will crop out :-).

Anybody can offer an insight what that may really be? It may be,
of course, totally unrelated to this report from Victor Zandy.

Michal
michal@harddata.com

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