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Re: Bug in Linux? fcr31 not being saved-restored

To: Carsten Langgaard <carstenl@mips.com>
Subject: Re: Bug in Linux? fcr31 not being saved-restored
From: Jun Sun <jsun@mvista.com>
Date: Mon, 17 Jun 2002 12:30:48 -0700
Cc: Louis Hamilton <hamilton@redhat.com>, linux-mips@oss.sgi.com, sandcraft-elinux-project@redhat.com
References: <3D0BD42E.20602@redhat.com> <3D0D7F98.566B3176@mips.com>
Sender: owner-linux-mips@oss.sgi.com
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:0.9.2.1) Gecko/20010901
Carsten Langgaard wrote:

This is one of the bugs, among others, we have fixed.
I'm not sure, if Ralf have integrated the patches we send him yet.



Carsten,

Do you remember the cause and the fix? It appears to me the first ctc1 instruction should trap into kernel and mark current process as fpu owner, and should not cause fcr31 corruption.

Or somehow the ctc1 does not trap into kernel?

Jun


/Carsten

Louis Hamilton wrote:


We have a customer here testing a 2.4.16 mips kernel on an embedded
Linux RM7000/SR71000 based system who has written a test that they
believe has uncovered a bug in Linux.  The FPU control register appears
to not get saved and restored.  I've reproduced the problem described
below and find the results consistent with their description.  The
problem occurs on both the RM7000 and SR71000 cpus.

It looks like save_fp_context and restore_fp_context are not being
called since the kernel save-restore logic thinks the process is not
using floating point math.  If you do some fp math before calling the
test routine below, it seems to works fine.

Is this a known caveat?  A true bug?  Or a contorted corner case
unlikely to be seen under typical end-user usage (see customer's
last paragraph :-) ?   If true bug, recommended remedy?

TIA,
Louis

Louis Hamilton
hamilton@redhat.com

------ customer reports the following: ---------
We found a bug in Linux.  A ^C (control-C) typed into a shell (or a
running program, it doesn't matter), causes the FCR (floating-point
control register) to be corrupted in another, unrelated process.  This
is repeatable behavior.

This can be reproduced with the following short assembly language
program that loops forever, waiting for the FCR to change.

       .align 2
       .globl mips_float_debug_loop
mips_float_debug_loop:
       li      $9, 0xF000F02F
       ctc1    $9, $31         # set FCR to some non-zero value
       nop
1:      cfc1    $8, $31         # get FCR
       beq     $8, $9, 1b      # spin, waiting for FCR to change
       nop
       or      $2, $0, $8
       jr    $31
       nop

You can call this function from a short C program and the return value
is the (corrupted) FCR, which turns out to alwyas be: 0x00000002.

Run the above loop in one window (connected to the board using telnet)
and then in another window (connected to the same board) type ^C.

I'm surprised this bug hasn't been encountered by other MIPS vendors.

<end>


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