linux-kernel - [tip:irq/core] x86-32/irq: Don' t switch to irq stack for a user-mode irq

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Date:	Mon, 20 Feb 2012 03:47:57 -0800
From:	tip-bot for Linus Torvalds <torvalds@...ux-foundation.org>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu
Subject: [tip:irq/core] x86-32/irq: Don'
 t switch to irq stack for a user-mode irq

Commit-ID:  986cb48c5a4de0085db94d343b4e7dcf54355ec1
Gitweb:     http://git.kernel.org/tip/986cb48c5a4de0085db94d343b4e7dcf54355ec1
Author:     Linus Torvalds <torvalds@...ux-foundation.org>
AuthorDate: Sun, 19 Feb 2012 11:46:36 -0800
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Mon, 20 Feb 2012 09:30:18 +0100

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@...ux-foundation.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1202191139260.10000@i5.linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 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 40fc861..58b7f27 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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