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authorrsc <rsc>2007-09-27 21:25:37 +0000
committerrsc <rsc>2007-09-27 21:25:37 +0000
commitab08960f6402f5c7cbb7b6e81694a60b6abed4c8 (patch)
tree156a197126bdfb99bb5081b7fe78ee3e8528aa5b
parentf97f0d2b3d3afbad3ef154b047f1b0408fd7288b (diff)
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Final word on the locking fiasco?
Change pushcli / popcli so that they can never turn on interrupts unexpectedly. That is, if interrupts are on, then pushcli(); popcli(); turns them off and back on, but if they are off to begin with, then pushcli(); popcli(); is a no-op. I think our fundamental mistake was having a primitive (release and then popcli nee spllo) that could turn interrupts on at unexpected moments instead of being explicit about when we want to start allowing interrupts. With the new semantics, all the manual fiddling of ncli to force interrupts off in certain sections goes away. In return, we must explicitly mark the places where we want to enable interrupts unconditionally, by calling sti(). There is only one: inside the scheduler loop.
-rw-r--r--main.c25
-rw-r--r--proc.c9
-rw-r--r--proc.h4
-rw-r--r--spinlock.c14
-rw-r--r--trap.c5
5 files changed, 24 insertions, 33 deletions
diff --git a/main.c b/main.c
index b489231..275aa80 100644
--- a/main.c
+++ b/main.c
@@ -12,19 +12,13 @@ static void mpmain(void) __attribute__((noreturn));
int
main(void)
{
- int bcpu, i;
extern char edata[], end[];
// clear BSS
memset(edata, 0, end - edata);
- // pushcli() every processor during bootstrap.
- for(i=0; i<NCPU; i++)
- cpus[i].ncli = 1; // no interrupts during bootstrap
-
mp_init(); // collect info about this machine
- bcpu = mp_bcpu();
- lapic_init(bcpu);
+ lapic_init(mp_bcpu());
cprintf("\ncpu%d: starting xv6\n\n", cpu());
pinit(); // process table
@@ -38,19 +32,15 @@ main(void)
console_init(); // I/O devices & their interrupts
ide_init(); // disk
if(!ismp)
- timer_init(); // uniprocessor timer
+ timer_init(); // uniprocessor timer
userinit(); // first user process
+ bootothers(); // start other processors
- // Allocate scheduler stacks and boot the other CPUs.
- for(i=0; i<ncpu; i++)
- cpus[i].stack = kalloc(KSTACKSIZE);
- bootothers();
-
- // Switch to our scheduler stack and continue with mpmain.
- asm volatile("movl %0, %%esp" : : "r" (cpus[bcpu].stack+KSTACKSIZE));
+ // Finish setting up this processor in mpmain.
mpmain();
}
+// Bootstrap processor gets here after setting up the hardware.
// Additional processors start here.
static void
mpmain(void)
@@ -62,7 +52,6 @@ mpmain(void)
setupsegs(0);
cpuid(0, 0, 0, 0, 0); // memory barrier
cpus[cpu()].booted = 1;
- popcli();
scheduler();
}
@@ -73,6 +62,7 @@ bootothers(void)
extern uchar _binary_bootother_start[], _binary_bootother_size[];
uchar *code;
struct cpu *c;
+ char *stack;
// Write bootstrap code to unused memory at 0x7000.
code = (uchar*)0x7000;
@@ -83,7 +73,8 @@ bootothers(void)
continue;
// Fill in %esp, %eip and start code on cpu.
- *(void**)(code-4) = c->stack + KSTACKSIZE;
+ stack = kalloc(KSTACKSIZE);
+ *(void**)(code-4) = stack + KSTACKSIZE;
*(void**)(code-8) = mpmain;
lapic_startap(c->apicid, (uint)code);
diff --git a/proc.c b/proc.c
index b009892..808a15e 100644
--- a/proc.c
+++ b/proc.c
@@ -179,7 +179,6 @@ userinit(void)
}
// Return currently running process.
-// XXX comment better
struct proc*
curproc(void)
{
@@ -206,11 +205,13 @@ scheduler(void)
struct cpu *c;
int i;
+ c = &cpus[cpu()];
for(;;){
+ // Enable interrupts on this processor.
+ sti();
+
// Loop over process table looking for process to run.
acquire(&proc_table_lock);
-
- c = &cpus[cpu()];
for(i = 0; i < NPROC; i++){
p = &proc[i];
if(p->state != RUNNABLE)
@@ -229,8 +230,8 @@ scheduler(void)
c->curproc = 0;
setupsegs(0);
}
-
release(&proc_table_lock);
+
}
}
diff --git a/proc.h b/proc.h
index 36913c4..fa60452 100644
--- a/proc.h
+++ b/proc.h
@@ -56,9 +56,9 @@ struct cpu {
struct context context; // Switch here to enter scheduler
struct taskstate ts; // Used by x86 to find stack for interrupt
struct segdesc gdt[NSEGS]; // x86 global descriptor table
- char *stack;
volatile int booted; // Has the CPU started?
- int ncli; // Depth of pushcli nesting.
+ int ncli; // Depth of pushcli nesting.
+ int intena; // Were interrupts enabled before pushcli?
};
extern struct cpu cpus[NCPU];
diff --git a/spinlock.c b/spinlock.c
index bf02292..a1aa37d 100644
--- a/spinlock.c
+++ b/spinlock.c
@@ -88,15 +88,19 @@ holding(struct spinlock *lock)
}
-
-// XXX!
-// Better names? Better functions?
+// Pushcli/popcli are like cli/sti except that they are matched:
+// it takes two popcli to undo two pushcli. Also, if interrupts
+// are off, then pushcli, popcli leaves them off.
void
pushcli(void)
{
+ int eflags;
+
+ eflags = read_eflags();
cli();
- cpus[cpu()].ncli++;
+ if(cpus[cpu()].ncli++ == 0)
+ cpus[cpu()].intena = eflags & FL_IF;
}
void
@@ -106,7 +110,7 @@ popcli(void)
panic("popcli - interruptible");
if(--cpus[cpu()].ncli < 0)
panic("popcli");
- if(cpus[cpu()].ncli == 0)
+ if(cpus[cpu()].ncli == 0 && cpus[cpu()].intena)
sti();
}
diff --git a/trap.c b/trap.c
index 0acc94b..e38cd00 100644
--- a/trap.c
+++ b/trap.c
@@ -44,9 +44,6 @@ trap(struct trapframe *tf)
return;
}
- // No interrupts during interrupt handling.
- pushcli();
-
switch(tf->trapno){
case IRQ_OFFSET + IRQ_TIMER:
if(cpu() == 0){
@@ -84,8 +81,6 @@ trap(struct trapframe *tf)
cp->killed = 1;
}
- popcli();
-
// Force process exit if it has been killed and is in user space.
// (If it is still executing in the kernel, let it keep running
// until it gets to the regular system call return.)