1 // Mutual exclusion spin locks. 2 3 #include "types.h" 4 #include "defs.h" 5 #include "param.h" 6 #include "x86.h" 7 #include "mmu.h" 8 #include "proc.h" 9 #include "spinlock.h" 10 11 extern int use_console_lock; 12 13 void 14 initlock(struct spinlock *lock, char *name) 15 { 16 lock->name = name; 17 lock->locked = 0; 18 lock->cpu = 0xffffffff; 19 } 20 21 // Acquire the lock. 22 // Loops (spins) until the lock is acquired. 23 // Holding a lock for a long time may cause 24 // other CPUs to waste time spinning to acquire it. 25 void 26 acquire(struct spinlock *lock) 27 { 28 pushcli(); 29 if(holding(lock)) 30 panic("acquire"); 31 32 // The xchg is atomic. 33 // It also serializes, so that reads after acquire are not 34 // reordered before it. 35 while(xchg(&lock->locked, 1) == 1) 36 ; 37 38 // Record info about lock acquisition for debugging. 39 // The +10 is only so that we can tell the difference 40 // between forgetting to initialize lock->cpu 41 // and holding a lock on cpu 0. 42 lock->cpu = cpu() + 10; 43 getcallerpcs(&lock, lock->pcs); 44 } 45 46 // Release the lock. 47 void 48 release(struct spinlock *lock) 49 { 50 if(!holding(lock)) 51 panic("release"); 52 53 lock->pcs[0] = 0; 54 lock->cpu = 0xffffffff; 55 56 // The xchg serializes, so that reads before release are 57 // not reordered after it. (This reordering would be allowed 58 // by the Intel manuals, but does not happen on current 59 // Intel processors. The xchg being asm volatile also keeps 60 // gcc from delaying the above assignments.) 61 xchg(&lock->locked, 0); 62 63 popcli(); 64 } 65 66 // Record the current call stack in pcs[] by following the %ebp chain. 67 void 68 getcallerpcs(void *v, uint pcs[]) 69 { 70 uint *ebp; 71 int i; 72 73 ebp = (uint*)v - 2; 74 for(i = 0; i < 10; i++){ 75 if(ebp == 0 || ebp == (uint*)0xffffffff) 76 break; 77 pcs[i] = ebp[1]; // saved %eip 78 ebp = (uint*)ebp[0]; // saved %ebp 79 } 80 for(; i < 10; i++) 81 pcs[i] = 0; 82 } 83 84 // Check whether this cpu is holding the lock. 85 int 86 holding(struct spinlock *lock) 87 { 88 return lock->locked && lock->cpu == cpu() + 10; 89 } 90 91 92 // Pushcli/popcli are like cli/sti except that they are matched: 93 // it takes two popcli to undo two pushcli. Also, if interrupts 94 // are off, then pushcli, popcli leaves them off. 95 96 void 97 pushcli(void) 98 { 99 int eflags; 100 101 eflags = read_eflags(); 102 cli(); 103 if(cpus[cpu()].ncli++ == 0) 104 cpus[cpu()].intena = eflags & FL_IF; 105 } 106 107 void 108 popcli(void) 109 { 110 if(read_eflags()&FL_IF) 111 panic("popcli - interruptible"); 112 if(--cpus[cpu()].ncli < 0) 113 panic("popcli"); 114 if(cpus[cpu()].ncli == 0 && cpus[cpu()].intena) 115 sti(); 116 } 117 118