93 lines
2.4 KiB
C
93 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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// Acquire the lock.
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// Loops (spins) until the lock is acquired.
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void acquire(struct spinlock *lk) {
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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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// On RISC-V, sync_lock_test_and_set turns into an atomic swap:
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// a5 = 1
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// s1 = &lk->locked
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// amoswap.w.aq a5, a5, (s1)
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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 strictly after the lock is acquired.
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// On RISC-V, this emits a fence instruction.
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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 release(struct spinlock *lk) {
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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 CPU 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 CPUs before the lock is released,
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// and that loads in the critical section occur strictly before
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// the lock is released.
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// On RISC-V, this emits a fence instruction.
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__sync_synchronize();
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// Release the lock, equivalent to lk->locked = 0.
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// This code doesn't use a C assignment, since the C standard
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// implies that an assignment might be implemented with
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// multiple store instructions.
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// On RISC-V, sync_lock_release turns into an atomic swap:
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// s1 = &lk->locked
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// amoswap.w zero, zero, (s1)
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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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// Interrupts must be off.
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int holding(struct spinlock *lk) {
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int r;
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r = (lk->locked && lk->cpu == mycpu());
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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()s to undo two push_off()s. Also, if interrupts
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// are initially off, then push_off, pop_off leaves them off.
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void push_off(void) {
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int old = intr_get();
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intr_off();
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if (mycpu()->noff == 0)
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mycpu()->intena = old;
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mycpu()->noff += 1;
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}
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void pop_off(void) {
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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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if (c->noff < 1)
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panic("pop_off");
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c->noff -= 1;
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if (c->noff == 0 && c->intena)
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intr_on();
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}
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