109 lines
2.4 KiB
C
109 lines
2.4 KiB
C
// Mutual exclusion spin locks.
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#include "types.h"
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#include "param.h"
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#include "memlayout.h"
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#include "spinlock.h"
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#include "riscv.h"
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#include "proc.h"
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#include "defs.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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push_off(); // 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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while(__sync_lock_test_and_set(&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 holding() and debugging.
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lk->cpu = mycpu();
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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->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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__sync_lock_release(&lk->locked);
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pop_off();
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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 *lk)
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{
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int r;
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push_off();
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r = lk->locked && lk->cpu == mycpu();
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pop_off();
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return r;
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}
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// push_off/pop_off are like intr_off()/intr_on() except that they are matched:
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// it takes two pop_off to undo two push_off. Also, if interrupts
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// are initially off, then push_off, pop_off leaves them off.
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void
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push_off(void)
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{
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struct cpu *c = mycpu();
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int old = intr_get();
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intr_off();
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if(c->noff == 0)
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c->intena = old;
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c->noff += 1;
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}
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void
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pop_off(void)
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{
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struct cpu *c = mycpu();
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if(intr_get())
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panic("pop_off - interruptible");
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c->noff -= 1;
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if(c->noff < 0)
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panic("pop_off");
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if(c->noff == 0 && c->intena)
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intr_on();
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}
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