Read: spinlock.c In this part of the assignment we will explore some of the interaction between interrupts and locking.
Make sure you understand what would happen if the kernel executed the following code snippet:
struct spinlock lk; initlock(&lk, "test lock"); acquire(&lk); acquire(&lk);(Feel free to use QEMU to find out.
acquire
is in spinlock.c
.)
An acquire
ensures interrupts are off
on the local processor using the cli
instruction
(via pushcli()
),
and interrupts remain off until the release
of the last lock held by that processor
(at which point they are enabled using sti
).
Let's see what happens if we turn on interrupts while
holding the ide
lock.
In iderw
in ide.c
, add a call
to sti()
after the acquire()
,
and a call to cli()
just before the release()
.
Rebuild the kernel and boot it in QEMU.
Chances are the kernel will panic soon after boot; try booting QEMU a few times
if it doesn't.
Turn in:
explain in a few sentences why the kernel panicked.
You may find it useful to look up the stack trace
(the sequence of %eip
values printed by panic
)
in the kernel.asm
listing.
sti()
and cli()
you added,
rebuild the kernel, and make sure it works again.
Now let's see what happens if we turn on interrupts
while holding the file_table_lock
.
This lock protects the table of file descriptors,
which the kernel modifies when an application opens or closes
a file.
In filealloc()
in file.c
, add
a call to
sti()
after the call to acquire()
,
and a cli()
just before each of the
release()
es.
You will also need to add
#include "x86.h"
at the top of the file after
the other #include
lines.
Rebuild the kernel and boot it in QEMU.
It will not panic.
Turn in:
explain in a few sentences why the kernel didn't panic.
Why do file_table_lock
and ide_lock
have
different behavior in this respect?
You do not need to understand anything about the details of the IDE hardware to answer this question, but you may find it helpful to look at which functions acquire each lock, and then at when those functions get called.
(There is a very small but non-zero chance that the kernel will panic
with the extra sti()
in filealloc()
.
If the kernel does panic, make doubly sure that
you removed the sti()
call from
iderw
. If it continues to panic and the
only extra sti()
is in filealloc()
,
then email the staff
and think about buying a lottery ticket.)
Turn in:
Why does release()
clear
lk->pcs[0]
and lk->cpu
before clearing lk->locked
?
Why not wait until after?
lock_acquire()
and sema_down
functions in pintos
source code.
The sema_down()
function in pintos
first disables
interrupts and
then re-enables interrupts (or sets them to their original status)
before returning. Whereas xv6 acquire()
function keeps interrupts
disabled. Also, sema_down()
blocks the thread, while
acquire()
busy waits.
Turn in:
Why does sema_down()
need to disable (and re-enable) interrupts?
Will this implementation work on a multiprocessor? If not, how will you
change it so that it works on multiprocessors too? Provide code (or pseudo-code).
xchg
to implement locks (or other forms of
synchronization). The concurrency on uni-processor systems
is due to pre-emption of threads by the timer interrupt.
Because the timer interrupt may pre-empt a thread at
any point in program execution, instructions of two
threads may interleave in any arbitrary order.
lock()
and
unlock()
primitives using interrupt enable and
disable mechanisms. Answer the associated questions.
lock(L) { cli(); //disable preemption while (L==0) continue; L = 0; sti(); //enable preemption } unlock(L) { L = 1; }Does this implementation of locks work on a uniprocessor? If not, why not?
lock(L) { int acquired = 0; while (!acquired) { cli(); if (L == 1) { acquired = 1; L = 0; } sti(); } } unlock(L) { L = 1; }Does this implementation of locks work on a uniprocessor? If not, why not?
struct pcq { void *ptr; struct spinlock lock; }; void* pcqread(struct pcq *q) { void *p; acquire(&q->lock); while(q->ptr == 0) sleep(q, &q->lock); p = q->ptr; q->ptr = 0; wakeup(q); /* wake pcqwrite */ release(&q->lock); return p; } void pcqwrite(struct pcq *q, void *p) { acquire(&q->lock); while(q->ptr != 0) sleep(q, &q->lock); q->ptr = p; wakeup(q); /* wake pcqread */ release(&q->lock); return p; }
Turn in:
Both producer (pcqwrite
) and consumer (pcqread
)
are sleeping on the same channel q
. Is this correct? Why or
why not? Should they sleep on different channels? For example, what happens
if the producer calls
wakeup(q)
? Can some unrelated part of the code call wakeup
a consumer thread?