Assembly

Assembly RSS

atomic increment is not working properly..

•

Join Date: Sep 2008

Posts: 1

Reputation: akmittal is an unknown quantity at this point

Solved Threads: 0

akmittal

Offline

Newbie Poster

atomic increment is not working properly..

Sep 15th, 2008

i am new to assembly programming,can anybody help me out to solve this problem -

i have a assembly code which does the atomic increment

 Help with Code Tags 
 assembly Syntax (Toggle Plain Text) 
Atomic_Add (volatile unsigned int *addr, int additive)
{
    register unsigned int previous;
 
    __asm__ volatile (
        "pushf;"
        "cli;"
        "movl   (%2), %0;"      /* previous = *addr; */
        "add    %0, %1;"        /* *addr = previous + additive; */
        "movl   %1, (%2);"
        "popf"
    : "=&r"(previous)
        : "r"(additive), "r"(addr) : "cc");
    return previous;
}
Atomic_Add (volatile unsigned int *addr, int additive)
{
    register unsigned int previous;

    __asm__ volatile (
        "pushf;"
        "cli;"
        "movl   (%2), %0;"      /* previous = *addr; */
        "add    %0, %1;"        /* *addr = previous + additive; */
        "movl   %1, (%2);"
        "popf"
    : "=&r"(previous)
        : "r"(additive), "r"(addr) : "cc");
    return previous;
}

here if addr is allocated on the stack(local variable) this function works fine and increment the addr variable correctly by additive, but if i addr is allocated on heap or is from gss, addr is not getting incremented correctly. and one more thing, if i merge the last two instruction the code works fine for all the scenarios.

 Help with Code Tags 
 assembly Syntax (Toggle Plain Text) 
Atomic_Add (volatile unsigned int *addr, int additive)
{
    register unsigned int previous;
 
    __asm__ volatile (
        "pushf;"
        "cli;"
        "movl   (%2), %0;"      /* previous = *addr; */
        "add    %1, (%2);"        /* *addr = previous + additive; */
        "popf"
    : "=&r"(previous)
        : "r"(additive), "r"(addr) : "cc");
    return previous;
}
Atomic_Add (volatile unsigned int *addr, int additive)
{
    register unsigned int previous;

    __asm__ volatile (
        "pushf;"
        "cli;"
        "movl   (%2), %0;"      /* previous = *addr; */
        "add    %1, (%2);"        /* *addr = previous + additive; */
        "popf"
    : "=&r"(previous)
        : "r"(additive), "r"(addr) : "cc");
    return previous;
}

Last edited by cscgal; Sep 15th, 2008 at 11:24 am. Reason: Added code tags

•

Join Date: Dec 2005

Posts: 5,850

Reputation: Salem has a reputation beyond repute

Solved Threads: 749

Salem

Offline

Void main'ers are DOOMed

Re: atomic increment is not working properly..

Sep 15th, 2008

So is the "not working properly" related to the "add" part of the problem or the "lock" part of the problem?

cli isn't going to do anything useful at all if you have multiple cores. AFAIK, it's a "per core" thing, so disabling it on one doesn't stop the other from messing with the variable.

The use case you quote of locals good, globals bad makes it seem like a cache coherency problem. Or it could be down to out-of-order execution of instructions meaning the popf happens beforre the memory write. IIRC, the instruction to stop this is 'sync'