c8919e6537
Last year, right before I sent xv6 to the printer, I changed the
SETGATE calls so that interrupts would be disabled on entry to
interrupt handlers, and I added the nlock++ / nlock-- in trap()
so that interrupts would stay disabled while the hw handlers
(but not the syscall handler) did their work. I did this because
the kernel was otherwise causing Bochs to triple-fault in SMP
mode, and time was short.
Robert observed yesterday that something was keeping the SMP
preemption user test from working. It turned out that when I
simplified the lapic code I swapped the order of two register
writes that I didn't realize were order dependent. I fixed that
and then since I had everything paged in kept going and tried
to figure out why you can't leave interrupts on during interrupt
handlers. There are a few issues.
First, there must be some way to keep interrupts from "stacking
up" and overflowing the stack. Keeping interrupts off the whole
time solves this problem -- even if the clock tick handler runs
long enough that the next clock tick is waiting when it finishes,
keeping interrupts off means that the handler runs all the way
through the "iret" before the next handler begins. This is not
really a problem unless you are putting too many prints in trap
-- if the OS is doing its job right, the handlers should run
quickly and not stack up.
Second, if xv6 had page faults, then it would be important to
keep interrupts disabled between the start of the interrupt and
the time that cr2 was read, to avoid a scenario like:
p1 page faults [cr2 set to faulting address]
p1 starts executing trapasm.S
clock interrupt, p1 preempted, p2 starts executing
p2 page faults [cr2 set to another faulting address]
p2 starts, finishes fault handler
p1 rescheduled, reads cr2, sees wrong fault address
Alternately p1 could be rescheduled on the other cpu, in which
case it would still see the wrong cr2. That said, I think cr2
is the only interrupt state that isn't pushed onto the interrupt
stack atomically at fault time, and xv6 doesn't care. (This isn't
entirely hypothetical -- I debugged this problem on Plan 9.)
Third, and this is the big one, it is not safe to call cpu()
unless interrupts are disabled. If interrupts are enabled then
there is no guarantee that, between the time cpu() looks up the
cpu id and the time that it the result gets used, the process
has not been rescheduled to the other cpu. For example, the
very commonly-used expression curproc[cpu()] (aka the macro cp)
can end up referring to the wrong proc: the code stores the
result of cpu() in %eax, gets rescheduled to the other cpu at
just the wrong instant, and then reads curproc[%eax].
We use curproc[cpu()] to get the current process a LOT. In that
particular case, if we arranged for the current curproc entry
to be addressed by %fs:0 and just use a different %fs on each
CPU, then we could safely get at curproc even with interrupts
disabled, since the read of %fs would be atomic with the read
of %fs:0. Alternately, we could have a curproc() function that
disables interrupts while computing curproc[cpu()]. I've done
that last one.
Even in the current kernel, with interrupts off on entry to trap,
interrupts are enabled inside release if there are no locks held.
Also, the scheduler's idle loop must be interruptible at times
so that the clock and disk interrupts (which might make processes
runnable) can be handled.
In addition to the rampant use of curproc[cpu()], this little
snippet from acquire is wrong on smp:
if(cpus[cpu()].nlock == 0)
cli();
cpus[cpu()].nlock++;
because if interrupts are off then we might call cpu(), get
rescheduled to a different cpu, look at cpus[oldcpu].nlock, and
wrongly decide not to disable interrupts on the new cpu. The
fix is to always call cli(). But this is wrong too:
if(holding(lock))
panic("acquire");
cli();
cpus[cpu()].nlock++;
because holding looks at cpu(). The fix is:
cli();
if(holding(lock))
panic("acquire");
cpus[cpu()].nlock++;
I've done that, and I changed cpu() to complain the first time
it gets called with interrupts disabled. (It gets called too
much to complain every time.)
I added new functions splhi and spllo that are like acquire and
release but without the locking:
void
splhi(void)
{
cli();
cpus[cpu()].nsplhi++;
}
void
spllo(void)
{
if(--cpus[cpu()].nsplhi == 0)
sti();
}
and I've used those to protect other sections of code that refer
to cpu() when interrupts would otherwise be disabled (basically
just curproc and setupsegs). I also use them in acquire/release
and got rid of nlock.
I'm not thrilled with the names, but I think the concept -- a
counted cli/sti -- is sound. Having them also replaces the
nlock++/nlock-- in trap.c and main.c, which is nice.
Final note: it's still not safe to enable interrupts in
the middle of trap() between lapic_eoi and returning
to user space. I don't understand why, but we get a
fault on pop %es because 0x10 is a bad segment
descriptor (!) and then the fault faults trying to go into
a new interrupt because 0x8 is a bad segment descriptor too!
Triple fault. I haven't debugged this yet.
149 lines
4.2 KiB
C
149 lines
4.2 KiB
C
struct buf;
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struct context;
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struct file;
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struct inode;
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struct pipe;
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struct proc;
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struct spinlock;
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struct stat;
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// bio.c
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void binit(void);
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struct buf* bread(uint, uint);
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void brelse(struct buf*);
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void bwrite(struct buf*);
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// console.c
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void console_init(void);
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void cprintf(char*, ...);
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void console_intr(int(*)(void));
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void panic(char*) __attribute__((noreturn));
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// exec.c
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int exec(char*, char**);
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// file.c
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struct file* filealloc(void);
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void fileclose(struct file*);
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struct file* filedup(struct file*);
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void fileinit(void);
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int fileread(struct file*, char*, int n);
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int filestat(struct file*, struct stat*);
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int filewrite(struct file*, char*, int n);
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// fs.c
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int dirlink(struct inode*, char*, uint);
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struct inode* dirlookup(struct inode*, char*, uint*);
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struct inode* ialloc(uint, short);
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struct inode* idup(struct inode*);
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void iinit(void);
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void ilock(struct inode*);
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void iput(struct inode*);
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void iunlock(struct inode*);
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void iunlockput(struct inode*);
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void iupdate(struct inode*);
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int namecmp(const char*, const char*);
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struct inode* namei(char*);
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struct inode* nameiparent(char*, char*);
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int readi(struct inode*, char*, uint, uint);
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void stati(struct inode*, struct stat*);
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int writei(struct inode*, char*, uint, uint);
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// ide.c
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void ide_init(void);
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void ide_intr(void);
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void ide_rw(struct buf *);
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// ioapic.c
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void ioapic_enable(int irq, int cpu);
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extern uchar ioapic_id;
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void ioapic_init(void);
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// kalloc.c
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char* kalloc(int);
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void kfree(char*, int);
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void kinit(void);
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// kbd.c
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void kbd_intr(void);
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// lapic.c
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int cpu(void);
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extern volatile uint* lapic;
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void lapic_eoi(void);
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void lapic_init(int);
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void lapic_startap(uchar, uint);
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// mp.c
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extern int ismp;
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int mp_bcpu(void);
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void mp_init(void);
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void mp_startthem(void);
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// picirq.c
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void pic_enable(int);
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void pic_init(void);
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// pipe.c
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int pipealloc(struct file**, struct file**);
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void pipeclose(struct pipe*, int);
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int piperead(struct pipe*, char*, int);
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int pipewrite(struct pipe*, char*, int);
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// proc.c
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struct proc* copyproc(struct proc*);
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struct proc* curproc();
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void exit(void);
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int growproc(int);
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int kill(int);
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void pinit(void);
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void procdump(void);
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void scheduler(void) __attribute__((noreturn));
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void setupsegs(struct proc*);
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void sleep(void*, struct spinlock*);
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void userinit(void);
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int wait(void);
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void wakeup(void*);
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void yield(void);
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// swtch.S
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void swtch(struct context*, struct context*);
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// spinlock.c
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void acquire(struct spinlock*);
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void getcallerpcs(void*, uint*);
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int holding(struct spinlock*);
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void initlock(struct spinlock*, char*);
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void release(struct spinlock*);
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void splhi();
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void spllo();
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// string.c
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int memcmp(const void*, const void*, uint);
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void* memmove(void*, const void*, uint);
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void* memset(void*, int, uint);
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char* safestrcpy(char*, const char*, int);
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int strlen(const char*);
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int strncmp(const char*, const char*, uint);
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char* strncpy(char*, const char*, int);
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// syscall.c
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int argint(int, int*);
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int argptr(int, char**, int);
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int argstr(int, char**);
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int fetchint(struct proc*, uint, int*);
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int fetchstr(struct proc*, uint, char**);
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void syscall(void);
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// timer.c
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void timer_init(void);
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// trap.c
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void idtinit(void);
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extern int ticks;
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void tvinit(void);
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extern struct spinlock tickslock;
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// number of elements in fixed-size array
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#define NELEM(x) (sizeof(x)/sizeof((x)[0]))
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