linux-kernel - [PATCH 1/2] x86: Delay loading sp0 slightly on task switch

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Message-Id: <9fcaa47dd8487db59eed7a3911b6ae409476763e.1425692936.git.luto@amacapital.net>
Date:	Fri,  6 Mar 2015 17:50:18 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Borislav Petkov <bp@...en8.de>, Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: [PATCH 1/2] x86: Delay loading sp0 slightly on task switch

The change:

75182b1632a8 x86/asm/entry: Switch all C consumers of kernel_stack to this_cpu_sp0()

had the unintended side effect of changing the return value of
current_thread_info() during part of the context switch process.
Change it back.

This has no effect as far as I can tell -- it's just for
consistency.

Signed-off-by: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/kernel/process_32.c | 10 +++++-----
 arch/x86/kernel/process_64.c |  6 +++---
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index d3460af3d27a..0405cab6634d 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -256,11 +256,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	fpu = switch_fpu_prepare(prev_p, next_p, cpu);
 
 	/*
-	 * Reload esp0.
-	 */
-	load_sp0(tss, next);
-
-	/*
 	 * Save away %gs. No need to save %fs, as it was saved on the
 	 * stack on entry.  No need to save %es and %ds, as those are
 	 * always kernel segments while inside the kernel.  Doing this
@@ -310,6 +305,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	 */
 	arch_end_context_switch(next_p);
 
+	/*
+	 * Reload esp0.  This changes current_thread_info().
+	 */
+	load_sp0(tss, next);
+
 	this_cpu_write(kernel_stack,
 		  (unsigned long)task_stack_page(next_p) +
 		  THREAD_SIZE - KERNEL_STACK_OFFSET);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 2cd562f96c1f..1e393d27d701 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -283,9 +283,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 
 	fpu = switch_fpu_prepare(prev_p, next_p, cpu);
 
-	/* Reload esp0 and ss1. */
-	load_sp0(tss, next);
-
 	/* We must save %fs and %gs before load_TLS() because
 	 * %fs and %gs may be cleared by load_TLS().
 	 *
@@ -413,6 +410,9 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	task_thread_info(prev_p)->saved_preempt_count = this_cpu_read(__preempt_count);
 	this_cpu_write(__preempt_count, task_thread_info(next_p)->saved_preempt_count);
 
+	/* Reload esp0 and ss1.  This changes current_thread_info(). */
+	load_sp0(tss, next);
+
 	this_cpu_write(kernel_stack,
 		  (unsigned long)task_stack_page(next_p) +
 		  THREAD_SIZE - KERNEL_STACK_OFFSET);
-- 
2.1.0

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