kernel/spinlock.c

1// Mutual exclusion spin locks.23#include "types.h"4#include "param.h"5#include "memlayout.h"6#include "spinlock.h"7#include "riscv.h"8#include "proc.h"9#include "defs.h"1011void12initlock(struct spinlock *lk, char *name)13{14  lk->name = name;15  lk->locked = 0;16  lk->cpu = 0;17}1819// Acquire the lock.20// Loops (spins) until the lock is acquired.21void22acquire(struct spinlock *lk)23{24  push_off(); // disable interrupts to avoid deadlock.25  if(holding(lk))26    panic("acquire");2728  // On RISC-V, sync_lock_test_and_set turns into an atomic swap:29  //   a5 = 130  //   s1 = &lk->locked31  //   amoswap.w.aq a5, a5, (s1)32  while(__sync_lock_test_and_set(&lk->locked, 1) != 0)33    ;3435  // Tell the C compiler and the processor to not move loads or stores36  // past this point, to ensure that the critical section's memory37  // references happen strictly after the lock is acquired.38  // On RISC-V, this emits a fence instruction.39  __sync_synchronize();4041  // Record info about lock acquisition for holding() and debugging.42  lk->cpu = mycpu();43}4445// Release the lock.46void47release(struct spinlock *lk)48{49  if(!holding(lk))50    panic("release");5152  lk->cpu = 0;5354  // Tell the C compiler and the CPU to not move loads or stores55  // past this point, to ensure that all the stores in the critical56  // section are visible to other CPUs before the lock is released,57  // and that loads in the critical section occur strictly before58  // the lock is released.59  // On RISC-V, this emits a fence instruction.60  __sync_synchronize();6162  // Release the lock, equivalent to lk->locked = 0.63  // This code doesn't use a C assignment, since the C standard64  // implies that an assignment might be implemented with65  // multiple store instructions.66  // On RISC-V, sync_lock_release turns into an atomic swap:67  //   s1 = &lk->locked68  //   amoswap.w zero, zero, (s1)69  __sync_lock_release(&lk->locked);7071  pop_off();72}7374// Check whether this cpu is holding the lock.75// Interrupts must be off.76int77holding(struct spinlock *lk)78{79  int r;80  r = (lk->locked && lk->cpu == mycpu());81  return r;82}8384// push_off/pop_off are like intr_off()/intr_on() except that they are matched:85// it takes two pop_off()s to undo two push_off()s.  Also, if interrupts86// are initially off, then push_off, pop_off leaves them off.8788void89push_off(void)90{91  int old = intr_get();9293  // disable interrupts to prevent an involuntary context94  // switch while using mycpu().95  intr_off();9697  if(mycpu()->noff == 0)98    mycpu()->intena = old;99  mycpu()->noff += 1;100}101102void103pop_off(void)104{105  struct cpu *c = mycpu();106  if(intr_get())107    panic("pop_off - interruptible");108  if(c->noff < 1)109    panic("pop_off");110  c->noff -= 1;111  if(c->noff == 0 && c->intena)112    intr_on();113}114