spinlock.c 5.19 KB
Newer Older
Wilke Pierre's avatar
Wilke Pierre committed
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
// 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++;
}

#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);
  }
  if(nbtries > MAXTRIES){
    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;
}