Allow holding to be called with interrupts enabled (for Mark Morrissey).

Start of an experiment to remove the use of gs for cpu local variables.

xx

Update comment a bit.

use asm() for lock release, not a C assignment

comment about sched() saving/restoring cpu->intena

Remove trailing white space with:
for f in *.{h,c}; do sed -i .sed 's/[[:blank:]]*$//' $f; done
(Thanks to Nicolás Wolovick)

separate atomic test-and-set from memory barrier.
* use xchg only for its atomicness.
* use __sync_synchronize() for both CPU and compiler barrier.

Revert "Introduce and use sleeplocks instead of BUSY flags"
My changes have a race with re-used bufs and the code doesn't seem to get shorter
Keep the changes that fixed ip->off race

This reverts co

Revert "Introduce and use sleeplocks instead of BUSY flags"
My changes have a race with re-used bufs and the code doesn't seem to get shorter
Keep the changes that fixed ip->off race

This reverts commit 3a5fa7ed9020eaf8ab843a16d26db7393b2ec072.

Conflicts:

defs.h
file.c
file.h

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Introduce and use sleeplocks instead of BUSY flags
Remove I_BUSY, B_BUSY, and intrans defs and usages
One spinlock per buf to avoid ugly loop in bget
fix race in filewrite (don't update f->off after

Introduce and use sleeplocks instead of BUSY flags
Remove I_BUSY, B_BUSY, and intrans defs and usages
One spinlock per buf to avoid ugly loop in bget
fix race in filewrite (don't update f->off after releasing lock)

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Map kernel high
Very important to give qemu memory through PHYSTOP :(

make new code like old code

Variable declarations at top of function,
separate from initialization.

Use == 0 instead of ! for checking pointers.

Consistent spacing around {, *, casts.

Declare 0-p

make new code like old code

Variable declarations at top of function,
separate from initialization.

Use == 0 instead of ! for checking pointers.

Consistent spacing around {, *, casts.

Declare 0-parameter functions as (void) not ().

Integer valued functions return -1 on failure, 0 on success.

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change some comments, maybe more informative
delete most comments from bootother.S (since copy of bootasm.S)
ksegment() -> seginit()
move more stuff from main() to mainc()

nits

Initial version of single-cpu xv6 with page tables

assorted fixes:
* rename c/cp to cpu/proc
* rename cpu.context to cpu.scheduler
* fix some comments
* formatting for printout

spinlock: rename parameter lock -> lk

Some proc cleanup, moving some of copyproc into allocproc.

Also, an experiment: use "thread-local" storage for c and cp
instead of the #define macro for curproc[cpu()].

be consistent: no underscores in function names

include explicitly initialized globals (int x = 0;) in cross-refs,
also thanks to greg price.

document lock->locked=0 vs xchg(&lock->locked, 0)

Incorporate new understanding of/with Intel SMP spec.

Dropped cmpxchg in favor of xchg, to match lecture notes.

Use xchg to release lock, for future protection and to
keep gcc from acting clever.

Re: why cpuid() in locking code?

rtm wrote:
> Why does acquire() call cpuid()? Why does release() call cpuid()?

The cpuid in acquire is redundant with the cmpxchg, as you said.
I have removed the c

Re: why cpuid() in locking code?

rtm wrote:
> Why does acquire() call cpuid()? Why does release() call cpuid()?

The cpuid in acquire is redundant with the cmpxchg, as you said.
I have removed the cpuid from acquire.

The cpuid in release is actually doing something important,
but not on the hardware. It keeps gcc from reordering the
lock->locked assignment above the other two during optimization.
(Not that current gcc -O2 would choose to do that, but it is allowed to.)
I have replaced the cpuid in release with a "gcc barrier" that
keeps gcc from moving things around but has no hardware effect.

On a related note, I don't think the cpuid in mpmain is necessary,
for the same reason that the cpuid wasn't needed in release.

As to the question of whether

acquire();
x = protected;
release();

might read protected after release(), I still haven't convinced
myself whether it can. I'll put the cpuid back into release if
we determine that it can.

Russ

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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

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.

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rename splhi/spllo to pushcli/popcli