[PATCH] Linux 2.6.x VM86 interrupt emulation fixes

Patch solves VM86 interrupt emulation deadlock on SMP systems.  The VM86
interrupt emulation has been heavily tested and works well on UP systems
after last update, but it seems to deadlock when we have used it on SMP/HT
boxes now.

It seems, that disable_irq() cannot be called from interrupts, because it
waits until disabled interrupt handler finishes
(/kernel/irq/manage.c:synchronize_irq():while(IRQ_INPROGRESS);).  This
blocks one CPU after another.  Solved by use disable_irq_nosync.

There is the second problem.  If IRQ source is fast, it is possible, that
interrupt is sometimes processed and re-enabled by the second CPU, before
it is disabled by the first one, but negative IRQ disable depths are not
allowed.  The spinlocking and disabling IRQs over call to
disable_irq_nosync/enable_irq is the only solution found reliable till now.

Signed-off-by: Michal Sojka <sojkam1@control.felk.cvut.cz>
Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/vm86.c

@@ -717,12 +717,12 @@ static irqreturn_t irq_handler(int intno, void *dev_id, struct pt_regs * regs)
 	irqbits |= irq_bit;
 	if (vm86_irqs[intno].sig)
 		send_sig(vm86_irqs[intno].sig, vm86_irqs[intno].tsk, 1);
-	spin_unlock_irqrestore(&irqbits_lock, flags);
 	/*
 	 * IRQ will be re-enabled when user asks for the irq (whether
 	 * polling or as a result of the signal)
 	 */
-	disable_irq(intno);
+	disable_irq_nosync(intno);
+	spin_unlock_irqrestore(&irqbits_lock, flags);
 	return IRQ_HANDLED;
 
 out:
@@ -754,17 +754,20 @@ static inline int get_and_reset_irq(int irqnumber)
 {
 	int bit;
 	unsigned long flags;
+	int ret = 0;
 
 	if (invalid_vm86_irq(irqnumber)) return 0;
 	if (vm86_irqs[irqnumber].tsk != current) return 0;
 	spin_lock_irqsave(&irqbits_lock, flags);	
 	bit = irqbits & (1 << irqnumber);
 	irqbits &= ~bit;
+	if (bit) {
+		enable_irq(irqnumber);
+		ret = 1;
+	}
+
 	spin_unlock_irqrestore(&irqbits_lock, flags);	
-	if (!bit)
-		return 0;
-	enable_irq(irqnumber);
-	return 1;
+	return ret;
 }