127 lines
2.7 KiB
C
127 lines
2.7 KiB
C
// Mutual exclusion spin locks.
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#include "types.h"
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#include "defs.h"
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#include "param.h"
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#include "x86.h"
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#include "memlayout.h"
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#include "mmu.h"
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#include "proc.h"
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#include "spinlock.h"
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void
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initlock(struct spinlock *lk, char *name)
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{
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lk->name = name;
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lk->locked = 0;
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lk->cpu = 0;
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}
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// Acquire the lock.
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// Loops (spins) until the lock is acquired.
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// Holding a lock for a long time may cause
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// other CPUs to waste time spinning to acquire it.
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void
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acquire(struct spinlock *lk)
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{
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pushcli(); // disable interrupts to avoid deadlock.
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if(holding(lk))
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panic("acquire");
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// The xchg is atomic.
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while(xchg(&lk->locked, 1) != 0)
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;
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// Tell the C compiler and the processor to not move loads or stores
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// past this point, to ensure that the critical section's memory
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// references happen after the lock is acquired.
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__sync_synchronize();
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// Record info about lock acquisition for debugging.
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lk->cpu = mycpu();
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getcallerpcs(&lk, lk->pcs);
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}
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// Release the lock.
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void
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release(struct spinlock *lk)
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{
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if(!holding(lk))
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panic("release");
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lk->pcs[0] = 0;
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lk->cpu = 0;
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// Tell the C compiler and the processor to not move loads or stores
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// past this point, to ensure that all the stores in the critical
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// section are visible to other cores before the lock is released.
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// Both the C compiler and the hardware may re-order loads and
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// stores; __sync_synchronize() tells them both not to.
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__sync_synchronize();
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// Release the lock, equivalent to lk->locked = 0.
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// This code can't use a C assignment, since it might
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// not be atomic. A real OS would use C atomics here.
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asm volatile("movl $0, %0" : "+m" (lk->locked) : );
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popcli();
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}
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// Record the current call stack in pcs[] by following the %ebp chain.
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void
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getcallerpcs(void *v, uint pcs[])
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{
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uint *ebp;
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int i;
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ebp = (uint*)v - 2;
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for(i = 0; i < 10; i++){
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if(ebp == 0 || ebp < (uint*)KERNBASE || ebp == (uint*)0xffffffff)
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break;
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pcs[i] = ebp[1]; // saved %eip
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ebp = (uint*)ebp[0]; // saved %ebp
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}
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for(; i < 10; i++)
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pcs[i] = 0;
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}
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// Check whether this cpu is holding the lock.
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int
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holding(struct spinlock *lock)
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{
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int r;
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pushcli();
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r = lock->locked && lock->cpu == mycpu();
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popcli();
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return r;
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}
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// Pushcli/popcli are like cli/sti except that they are matched:
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// it takes two popcli to undo two pushcli. Also, if interrupts
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// are off, then pushcli, popcli leaves them off.
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void
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pushcli(void)
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{
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int eflags;
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eflags = readeflags();
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cli();
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if(mycpu()->ncli == 0)
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mycpu()->intena = eflags & FL_IF;
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mycpu()->ncli += 1;
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}
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void
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popcli(void)
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{
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if(readeflags()&FL_IF)
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panic("popcli - interruptible");
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if(--mycpu()->ncli < 0)
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panic("popcli");
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if(mycpu()->ncli == 0 && mycpu()->intena)
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sti();
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}
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