linux-kernel - Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

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Date:	Mon, 25 May 2009 14:46:34 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Harald Welte <HaraldWelte@...tech.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

On Mon May 25 2009, Michael S. Zick wrote:
> On Mon May 25 2009, Michael S. Zick wrote:
> > On Sun May 24 2009, H. Peter Anvin wrote:
> > > Michael S. Zick wrote:
> > > > 
> > > > Note: I have seem to recall that newer gcc's optimizer presume 
> > > > that the flags register is preserved across asm - 
> > > > It didn't use to do that - but there is now a "cc" to deal with
> > > > that - Have not yet audited for that, but it is high on my list.
> > > > 
> > > 
> > > I am pretty sure that's false... if it was true we'd have failures all
> > > over the kernel.
> > > 
> > 
> > No information on the above (yet) - but you gotta love this one: ;)
> > 
> > Programmer authors code specifying that the subtraction be done
> > prior to the addition to avoid over-flow conditions;
> > 
> > GCC's optimizer, in its great wisdom, codes in the overflow case:
> > ( the case of finding the characters used/free in a ring buffer )
> > 
> > extern int diff_umask(int mask, int *cnt1, int *cnt2) 
> > { return (((mask - *cnt1) + *cnt2) & mask); }
> > 
> > /**
> >  * gcc -O2 -S -fomit-frame-pointer difftest.c
> >  *
> >          .file   "difftest.c"
> >         .text
> >         .p2align 4,,15
> > .globl diff_umask
> >         .type   diff_umask, @function
> > diff_umask:
> >         movl    12(%esp), %eax
> >         movl    4(%esp), %ecx
> >         movl    (%eax), %edx
> >         leal    (%ecx,%edx), %eax
> >         movl    8(%esp), %edx
> >         subl    (%edx), %eax
> >         andl    %ecx, %eax
> >         ret
> >         .size   diff_umask, .-diff_umask
> >         .ident  "GCC: (Debian 4.3.2-1.1) 4.3.2"
> >         .section        .note.GNU-stack,"",@progbits
> > */
> > 
> > Note: That is not the compiler version I am building my kernels with.
> > 
> 
> The compiler I am using (Gentoo 4.1.2) gets it correct:
> 
>         .file   "difftest.c"
>         .text
>         .p2align 4,,15
> .globl diff_umask
>         .type   diff_umask, @function
> diff_umask:
>         movl    4(%esp), %eax
>         movl    8(%esp), %edx
>         movl    %eax, %ecx
>         subl    (%edx), %ecx
>         movl    %ecx, %edx
>         movl    12(%esp), %ecx
>         addl    (%ecx), %edx
>         andl    %edx, %eax
>         ret
>         .size   diff_umask, .-diff_umask
>         .ident  "GCC: (GNU) 4.1.2 (Gentoo 4.1.2 p1.1)"
>         .section        .note.GNU-stack,"",@progbits
> 

Gentoo's current 4.3 gets it wrong also:

        .file   "difftest.c"
        .text
        .p2align 4,,15
.globl diff_umask
        .type   diff_umask, @function
diff_umask:
        movl    12(%esp), %eax
        movl    4(%esp), %ecx
        movl    (%eax), %edx
        leal    (%ecx,%edx), %eax
        movl    8(%esp), %edx
        subl    (%edx), %eax
        andl    %ecx, %eax
        ret
        .size   diff_umask, .-diff_umask
        .ident  "GCC: (Gentoo 4.3.2-r3 p1.6, pie-10.1.5) 4.3.2"
        .section        .note.GNU-stack,"",@progbits

= = = =

Might be time to put compiler version checking back into the
build system and/or re-test the sources that do have version
checking in them (hint: the boss's code).

Mike
> Mike
> > Don't blame me, I didn't write the compiler. ;)
> > 
> > Mike
> > > 	-hpa
> > > 
> > 
> > 
> > --


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