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

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Message-Id: <200905251403.11786.lkml@morethan.org>
Date:	Mon, 25 May 2009 14:03:09 -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 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.

Don't blame me, I didn't write the compiler. ;)

Mike
> 	-hpa
> 


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