spinlock.c 5.59 KB
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// Mutual exclusion spin locks.

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "proc.h"
#include "defs.h"

#define NLOCK 1000

static int nlock;
static struct spinlock *locks[NLOCK];

// assumes locks are not freed
void
initlock(struct spinlock *lk, char *name)
{
  lk->name = name;
  lk->locked = 0;
  lk->cpu = 0;
  lk->nts = 0;
  lk->n = 0;
  if(nlock >= NLOCK)
    panic("initlock");
  locks[nlock] = lk;
  nlock++;
}

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void dump_locks(void){
  printf_no_lock("LID\tLOCKED\tCPU\tPID\tNAME\t\tPC\n");
  for(int i = 0; i < nlock; i++){
    if(locks[i]->locked)
      printf_no_lock("%d\t%d\t%d\t%d\t%s\t\t%p\n",
                     i,
                     locks[i]->locked,
                     locks[i]->cpu - cpus,
                     locks[i]->pid,
                     locks[i]->name,
                     locks[i]->pc
        );
  }
}

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#define MAXTRIES 100000

// Acquire the lock.
// Loops (spins) until the lock is acquired.
void
acquire(struct spinlock *lk)
{
  uint64 ra;
  asm volatile("mv %0, ra" : "=r" (ra));
  ra -= 4;
  push_off(); // disable interrupts to avoid deadlock.
  if(holding(lk)){
    printf_no_lock("requesting %s (%p) but already have it\n", lk->name, lk);
    printf_no_lock("process %d (CPU %d) took it at pc=%p \n", lk->pid,
                   lk->cpu - cpus,
                   lk->pc);
    printf_no_lock("I am myself at pc=%p in pid=%d on CPU %d\n",
                   ra,
                   myproc() ? myproc()->pid : -1,
                   cpuid());
    procdump();
    panic("acquire");
  }

  __sync_fetch_and_add(&(lk->n), 1);

  // On RISC-V, sync_lock_test_and_set turns into an atomic swap:
  //   a5 = 1
  //   s1 = &lk->locked
  //   amoswap.w.aq a5, a5, (s1)
  int nbtries = 0;
  int warned = 0;
  while(__sync_lock_test_and_set(&lk->locked, 1) != 0) {
    nbtries++;
    if(nbtries > MAXTRIES && !warned){
      printf_no_lock("CPU %d: Blocked while acquiring %s (%p)\n", cpuid(), lk->name, lk);
      printf_no_lock("process %d (CPU %d) took it at pc=%p \n", lk->pid,
                     lk->cpu - cpus,
                     lk->pc);
      printf_no_lock("I am myself at pc=%p in pid=%d on CPU %d\n",
                     ra,
                     myproc() ? myproc()->pid : -1,
                     cpuid());
      procdump();
      warned = 1;
    }
     __sync_fetch_and_add(&lk->nts, 1);
  }
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  if(warned){
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    printf_no_lock("CPU %d: Finally acquired %s (%p) after %d tries\n", cpuid(), lk->name, lk, nbtries);
  }

  // Tell the C compiler and the processor to not move loads or stores
  // past this point, to ensure that the critical section's memory
  // references happen strictly after the lock is acquired.
  // On RISC-V, this emits a fence instruction.
  __sync_synchronize();

  // Record info about lock acquisition for holding() and debugging.
  lk->cpu = mycpu();
  lk->pc = ra;
  lk->pid = myproc() ? myproc()->pid : -1;
}

// Release the lock.
void
release(struct spinlock *lk)
{
  if(!holding(lk))
    panic("release");

  lk->cpu = 0;

  // Tell the C compiler and the CPU to not move loads or stores
  // past this point, to ensure that all the stores in the critical
  // section are visible to other CPUs before the lock is released,
  // and that loads in the critical section occur strictly before
  // the lock is released.
  // On RISC-V, this emits a fence instruction.
  __sync_synchronize();

  // Release the lock, equivalent to lk->locked = 0.
  // This code doesn't use a C assignment, since the C standard
  // implies that an assignment might be implemented with
  // multiple store instructions.
  // On RISC-V, sync_lock_release turns into an atomic swap:
  //   s1 = &lk->locked
  //   amoswap.w zero, zero, (s1)
  __sync_lock_release(&lk->locked);

  pop_off();
}

// Check whether this cpu is holding the lock.
// Must be called with interrupts off.
int
holding(struct spinlock *lk)
{
  int r;
  r = (lk->locked && lk->cpu == mycpu());
  return r;
}

// push_off/pop_off are like intr_off()/intr_on() except that they are matched:
// it takes two pop_off()s to undo two push_off()s.  Also, if interrupts
// are initially off, then push_off, pop_off leaves them off.

void
push_off(void)
{
  int old = intr_get();
  if(old)
    intr_off();
  if(mycpu()->noff == 0)
    mycpu()->intena = old;
  mycpu()->noff += 1;
}

void
pop_off(void)
{
  if(intr_get())
    panic("pop_off - interruptible");
  struct cpu *c = mycpu();
  if(c->noff < 1)
    panic("pop_off");
  c->noff -= 1;
  if(c->noff == 0 && c->intena)
    intr_on();
}

void
print_lock(struct spinlock *lk)
{
  if(lk->n > 0) 
    printf("lock: %s: #test-and-set %d #acquire() %d\n", lk->name, lk->nts, lk->n);
}

uint64
sys_ntas(void)
{
  int zero = 0;
  int tot = 0;
  
  if (argint(0, &zero) < 0) {
    return -1;
  }
  if(zero == 0) {
    for(int i = 0; i < NLOCK; i++) {
      if(locks[i] == 0)
        break;
      locks[i]->nts = 0;
      locks[i]->n = 0;
    }
    return 0;
  }

  printf("=== lock kmem/bcache stats\n");
  for(int i = 0; i < NLOCK; i++) {
    if(locks[i] == 0)
      break;
    if(strncmp(locks[i]->name, "bcache", strlen("bcache")) == 0 ||
       strncmp(locks[i]->name, "kmem", strlen("kmem")) == 0) {
      tot += locks[i]->nts;
      print_lock(locks[i]);
    }
  }

  printf("=== top 5 contended locks:\n");
  int last = 100000000;
  // stupid way to compute top 5 contended locks
  for(int t= 0; t < 5; t++) {
    int top = 0;
    for(int i = 0; i < NLOCK; i++) {
      if(locks[i] == 0)
        break;
      if(locks[i]->nts > locks[top]->nts && locks[i]->nts < last) {
        top = i;
      }
    }
    print_lock(locks[top]);
    last = locks[top]->nts;
  }
  return tot;
}