1 // Mutual exclusion spin locks.
2
3 #include "types.h"
4 #include "defs.h"
5 #include "param.h"
6 #include "x86.h"
7 #include "memlayout.h"
8 #include "mmu.h"
9 #include "proc.h"
10 #include "spinlock.h"
11
12 void
initlock(struct spinlock * lk,char * name)13 initlock(struct spinlock *lk, char *name)
14 {
15 lk->name = name;
16 lk->locked = 0;
17 lk->cpu = 0;
18 }
19
20 // Acquire the lock.
21 // Loops (spins) until the lock is acquired.
22 // Holding a lock for a long time may cause
23 // other CPUs to waste time spinning to acquire it.
24 void
acquire(struct spinlock * lk)25 acquire(struct spinlock *lk)
26 {
27 pushcli(); // disable interrupts to avoid deadlock.
28 if(holding(lk))
29 panic("acquire");
30
31 // The xchg is atomic.
32 while(xchg(&lk->locked, 1) != 0)
33 ;
34
35 // Tell the C compiler and the processor to not move loads or stores
36 // past this point, to ensure that the critical section's memory
37 // references happen after the lock is acquired.
38 __sync_synchronize();
39
40 // Record info about lock acquisition for debugging.
41 lk->cpu = mycpu();
42 getcallerpcs(&lk, lk->pcs);
43 }
44
45 // Release the lock.
46 void
release(struct spinlock * lk)47 release(struct spinlock *lk)
48 {
49 if(!holding(lk))
50 panic("release");
51
52 lk->pcs[0] = 0;
53 lk->cpu = 0;
54
55 // Tell the C compiler and the processor to not move loads or stores
56 // past this point, to ensure that all the stores in the critical
57 // section are visible to other cores before the lock is released.
58 // Both the C compiler and the hardware may re-order loads and
59 // stores; __sync_synchronize() tells them both not to.
60 __sync_synchronize();
61
62 // Release the lock, equivalent to lk->locked = 0.
63 // This code can't use a C assignment, since it might
64 // not be atomic. A real OS would use C atomics here.
65 asm volatile("movl $0, %0" : "+m" (lk->locked) : );
66
67 popcli();
68 }
69
70 // Record the current call stack in pcs[] by following the %ebp chain.
71 void
getcallerpcs(void * v,uint pcs[])72 getcallerpcs(void *v, uint pcs[])
73 {
74 uint *ebp;
75 int i;
76
77 ebp = (uint*)v - 2;
78 for(i = 0; i < 10; i++){
79 if(ebp == 0 || ebp < (uint*)KERNBASE || ebp == (uint*)0xffffffff)
80 break;
81 pcs[i] = ebp[1]; // saved %eip
82 ebp = (uint*)ebp[0]; // saved %ebp
83 }
84 for(; i < 10; i++)
85 pcs[i] = 0;
86 }
87
88 // Check whether this cpu is holding the lock.
89 int
holding(struct spinlock * lock)90 holding(struct spinlock *lock)
91 {
92 int r;
93 pushcli();
94 r = lock->locked && lock->cpu == mycpu();
95 popcli();
96 return r;
97 }
98
99
100 // Pushcli/popcli are like cli/sti except that they are matched:
101 // it takes two popcli to undo two pushcli. Also, if interrupts
102 // are off, then pushcli, popcli leaves them off.
103
104 void
pushcli(void)105 pushcli(void)
106 {
107 int eflags;
108
109 eflags = readeflags();
110 cli();
111 if(mycpu()->ncli == 0)
112 mycpu()->intena = eflags & FL_IF;
113 mycpu()->ncli += 1;
114 }
115
116 void
popcli(void)117 popcli(void)
118 {
119 if(readeflags()&FL_IF)
120 panic("popcli - interruptible");
121 if(--mycpu()->ncli < 0)
122 panic("popcli");
123 if(mycpu()->ncli == 0 && mycpu()->intena)
124 sti();
125 }
126
127