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.

1 files changed, 0 insertions, 5 deletions

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.)