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Subject[PATCH] x86-32: don't switch to irq stack for a user-mode irq

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun Feb 19 11:35:34 2012 -0800

x86-32: 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>
---

This came up during the i387 work. It's not a bug, but I do believe that
we do stupid things on x86-32 when we get an interrupt in user mode.

I'm throwing this patch out here because I think it's the right thing to
do, but I won't commit it myself or push it any more than this.

arch/x86/kernel/irq_32.c | 11 +++--------
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);

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