x86-32/irq: Don't switch to irq stack for a user-mode irq

If the irq happens in user mode, our kernel stack is empty
(apart from the pt_regs themselves, of course), so there's no
need or advantage to switch.

And it really doesn't save any stack space, quite the reverse:
it means that a nested interrupt cannot switch irq stacks. So
instead of saving kernel stack space, it actually causes the
potential for *more* stack usage.

Also simplify the preemption count copy when we do switch
stacks: just copy the whole preemption count, rather than just
the softirq parts of it.  There is no advantage to the partial
copy: it is more effort to get a less correct result.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1202191139260.10000@i5.linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 3 insertions, 8 deletions

diff --git a/arch/x86/kernel/irq_32.c b/arch/x86/kernel/irq_32.c
index 40fc86161d92..58b7f27cb3e9 100644
--- a/arch/x86/kernel/irq_32.c
+++ b/arch/x86/kernel/irq_32.c
@@ -100,13 +100,8 @@ execute_on_irq_stack(int overflow, struct irq_desc *desc, int irq)
         irqctx->tinfo.task = curctx->tinfo.task;
         irqctx->tinfo.previous_esp = current_stack_pointer;
 
-        /*
-         * Copy the softirq bits in preempt_count so that the
-         * softirq checks work in the hardirq context.
-         */
-        irqctx->tinfo.preempt_count =
-                (irqctx->tinfo.preempt_count & ~SOFTIRQ_MASK) |
-                (curctx->tinfo.preempt_count & SOFTIRQ_MASK);
+        /* Copy the preempt_count so that the [soft]irq checks work. */
+        irqctx->tinfo.preempt_count = curctx->tinfo.preempt_count;
 
         if (unlikely(overflow))
                 call_on_stack(print_stack_overflow, isp);
@@ -196,7 +191,7 @@ bool handle_irq(unsigned irq, struct pt_regs *regs)
         if (unlikely(!desc))
                 return false;
 
-        if (!execute_on_irq_stack(overflow, desc, irq)) {
+        if (user_mode_vm(regs) || !execute_on_irq_stack(overflow, desc, irq)) {
                 if (unlikely(overflow))
                         print_stack_overflow();
                 desc->handle_irq(irq, desc);