Subj : Re: What is the real costs of LOCK on x86 multiprocesor machine?
To   : comp.programming.threads
From : David Schwartz
Date : Sat Jul 30 2005 02:19 pm


"chris noonan" <usenet@leapheap.co.uk> wrote in message 
news:1122727886.839675.223500@g14g2000cwa.googlegroups.com...

> I vaguely remember encountering the LOCK prefix
> in one or two places in Microsoft OS code disassembly
> (uniprocessor installation). I assumed it was simply
> a mistake, something like new routines being hacked
> in quickly without them taking the trouble to add
> the offsets of the LOCK prefixes to the list to be
> NOP'ed out.

    By the way, the way I do this in applications is by using an illegal 
instruction. I catch the fault and replace the illegal instruction with 
either a LOCK or  NOP at run time.

static void sigill(int, struct siginfo *info, void *extra)
{ // Dynamically patch a 'ud2' to a 'lock nop' or a 'nop nop'
   // By: David J. Schwartz
   // Copyright (C) WebMaster Incorporated, 2003-4

 // Begin with a full CPU sync (purge unmodified code)
 __asm__ __volatile__("cpuid" : : : "ax", "bx", "cx", "dx");

 while(AtomicExchange(&sigspin, 1)!=0)
 { // relax the CPU
  __asm__ __volatile__("rep; nop");
 }

 ucontext_t *uc=(ucontext_t *) extra;
 if(uc==NULL) {char *j=NULL; *j++;}
 else
 {
  unsigned char *instr=(unsigned char *) uc->uc_mcontext.gregs[14]; // EIP
  if(instr==NULL) {char *j=NULL; *j++;}
  else
  {
   if(instr[0]!=0x90)
   { // instruction wasn't already patched
    if (   (instr[0]!=0x0f) || // must be a ud2
           ((instr[1]!=0x0b)&&(instr[1]!=0xb9)) )
     { char *j=NULL; *j++; }
    else
    {
     mprotect( (void *) (   ((int) instr) & ~(PAGESIZE-1)    ),
      PAGESIZE, PROT_WRITE | PROT_EXEC | PROT_READ);
     instr[0]=0x90; // first byte is always a nop
     instr[1]=(IsUP) ? 0x90 : 0xf0;
     mprotect( (void *) (   ((int) instr) & ~(PAGESIZE-1)    ),
      PAGESIZE, PROT_EXEC | PROT_READ);
    }
   }
  }

  // release the lock
  AtomicExchange(&sigspin, 0);

  // Reflush the CPU
  __asm__ __volatile__("cpuid" : : : "ax", "bx", "cx", "dx");
 }
}

    DS

.