arch/x86/kernel/process_32.c - maze/linux - Git at Google

 /*
  *  Copyright (C) 1995  Linus Torvalds
  *
  *  Pentium III FXSR, SSE support
  *	Gareth Hughes <gareth@valinux.com>, May 2000
  */

 /*
  * This file handles the architecture-dependent parts of process handling..
  */

 #include <linux/stackprotector.h>
 #include <linux/cpu.h>
 #include <linux/errno.h>
 #include <linux/sched.h>
 #include <linux/fs.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/elfcore.h>
 #include <linux/smp.h>
 #include <linux/stddef.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/user.h>
 #include <linux/interrupt.h>
 #include <linux/delay.h>
 #include <linux/reboot.h>
 #include <linux/init.h>
 #include <linux/mc146818rtc.h>
 #include <linux/module.h>
 #include <linux/kallsyms.h>
 #include <linux/ptrace.h>
 #include <linux/personality.h>
 #include <linux/tick.h>
 #include <linux/percpu.h>
 #include <linux/prctl.h>
 #include <linux/ftrace.h>
 #include <linux/uaccess.h>
 #include <linux/io.h>
 #include <linux/kdebug.h>

 #include <asm/pgtable.h>
 #include <asm/system.h>
 #include <asm/ldt.h>
 #include <asm/processor.h>
 #include <asm/i387.h>
 #include <asm/desc.h>
 #ifdef CONFIG_MATH_EMULATION
 #include <asm/math_emu.h>
 #endif

 #include <linux/err.h>

 #include <asm/tlbflush.h>
 #include <asm/cpu.h>
 #include <asm/idle.h>
 #include <asm/syscalls.h>
 #include <asm/debugreg.h>

 #include <trace/events/power.h>

 asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");

 /*
  * Return saved PC of a blocked thread.
  */
 unsigned long thread_saved_pc(struct task_struct *tsk)
 {
 	return ((unsigned long *)tsk->thread.sp)[3];
 }

 #ifndef CONFIG_SMP
 static inline void play_dead(void)
 {
 	BUG();
 }
 #endif

 /*
  * The idle thread. There's no useful work to be
  * done, so just try to conserve power and have a
  * low exit latency (ie sit in a loop waiting for
  * somebody to say that they'd like to reschedule)
  */
 void cpu_idle(void)
 {
 	int cpu = smp_processor_id();

 	/*
 	 * If we're the non-boot CPU, nothing set the stack canary up
 	 * for us.  CPU0 already has it initialized but no harm in
 	 * doing it again.  This is a good place for updating it, as
 	 * we wont ever return from this function (so the invalid
 	 * canaries already on the stack wont ever trigger).
 	 */
 	boot_init_stack_canary();

 	current_thread_info()->status |= TS_POLLING;

 	/* endless idle loop with no priority at all */
 	while (1) {
 		tick_nohz_stop_sched_tick(1);
 		while (!need_resched()) {

 			check_pgt_cache();
 			rmb();

 			if (cpu_is_offline(cpu))
 				play_dead();

 			local_irq_disable();
 			/* Don't trace irqs off for idle */
 			stop_critical_timings();
 			pm_idle();
 			start_critical_timings();

 			trace_power_end(smp_processor_id());
 		}
 		tick_nohz_restart_sched_tick();
 		preempt_enable_no_resched();
 		schedule();
 		preempt_disable();
 	}
 }

 void __show_regs(struct pt_regs *regs, int all)
 {
 	unsigned long cr0 = 0L, cr2 = 0L, cr3 = 0L, cr4 = 0L;
 	unsigned long d0, d1, d2, d3, d6, d7;
 	unsigned long sp;
 	unsigned short ss, gs;

 	if (user_mode_vm(regs)) {
 		sp = regs->sp;
 		ss = regs->ss & 0xffff;
 		gs = get_user_gs(regs);
 	} else {
 		sp = kernel_stack_pointer(regs);
 		savesegment(ss, ss);
 		savesegment(gs, gs);
 	}

 	show_regs_common();

 	printk(KERN_DEFAULT "EIP: %04x:[<%08lx>] EFLAGS: %08lx CPU: %d\n",
 			(u16)regs->cs, regs->ip, regs->flags,
 			smp_processor_id());
 	print_symbol("EIP is at %s\n", regs->ip);

 	printk(KERN_DEFAULT "EAX: %08lx EBX: %08lx ECX: %08lx EDX: %08lx\n",
 		regs->ax, regs->bx, regs->cx, regs->dx);
 	printk(KERN_DEFAULT "ESI: %08lx EDI: %08lx EBP: %08lx ESP: %08lx\n",
 		regs->si, regs->di, regs->bp, sp);
 	printk(KERN_DEFAULT " DS: %04x ES: %04x FS: %04x GS: %04x SS: %04x\n",
 	       (u16)regs->ds, (u16)regs->es, (u16)regs->fs, gs, ss);

 	if (!all)
 		return;

 	cr0 = read_cr0();
 	cr2 = read_cr2();
 	cr3 = read_cr3();
 	cr4 = read_cr4_safe();
 	printk(KERN_DEFAULT "CR0: %08lx CR2: %08lx CR3: %08lx CR4: %08lx\n",
 			cr0, cr2, cr3, cr4);

 	get_debugreg(d0, 0);
 	get_debugreg(d1, 1);
 	get_debugreg(d2, 2);
 	get_debugreg(d3, 3);
 	printk(KERN_DEFAULT "DR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
 			d0, d1, d2, d3);

 	get_debugreg(d6, 6);
 	get_debugreg(d7, 7);
 	printk(KERN_DEFAULT "DR6: %08lx DR7: %08lx\n",
 			d6, d7);
 }

 void release_thread(struct task_struct *dead_task)
 {
 	BUG_ON(dead_task->mm);
 	release_vm86_irqs(dead_task);
 }

 /*
  * This gets called before we allocate a new thread and copy
  * the current task into it.
  */
 void prepare_to_copy(struct task_struct *tsk)
 {
 	unlazy_fpu(tsk);
 }

 int copy_thread(unsigned long clone_flags, unsigned long sp,
 	unsigned long unused,
 	struct task_struct *p, struct pt_regs *regs)
 {
 	struct pt_regs *childregs;
 	struct task_struct *tsk;
 	int err;

 	childregs = task_pt_regs(p);
 	*childregs = *regs;
 	childregs->ax = 0;
 	childregs->sp = sp;

 	p->thread.sp = (unsigned long) childregs;
 	p->thread.sp0 = (unsigned long) (childregs+1);

 	p->thread.ip = (unsigned long) ret_from_fork;

 	task_user_gs(p) = get_user_gs(regs);

 	p->thread.io_bitmap_ptr = NULL;
 	tsk = current;
 	err = -ENOMEM;

 	memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));

 	if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
 		p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
 						IO_BITMAP_BYTES, GFP_KERNEL);
 		if (!p->thread.io_bitmap_ptr) {
 			p->thread.io_bitmap_max = 0;
 			return -ENOMEM;
 		}
 		set_tsk_thread_flag(p, TIF_IO_BITMAP);
 	}

 	err = 0;

 	/*
 	 * Set a new TLS for the child thread?
 	 */
 	if (clone_flags & CLONE_SETTLS)
 		err = do_set_thread_area(p, -1,
 			(struct user_desc __user *)childregs->si, 0);

 	if (err && p->thread.io_bitmap_ptr) {
 		kfree(p->thread.io_bitmap_ptr);
 		p->thread.io_bitmap_max = 0;
 	}
 	return err;
 }

 void
 start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
 {
 	set_user_gs(regs, 0);
 	regs->fs		= 0;
 	set_fs(USER_DS);
 	regs->ds		= __USER_DS;
 	regs->es		= __USER_DS;
 	regs->ss		= __USER_DS;
 	regs->cs		= __USER_CS;
 	regs->ip		= new_ip;
 	regs->sp		= new_sp;
 	/*
 	 * Free the old FP and other extended state
 	 */
 	free_thread_xstate(current);
 }
 EXPORT_SYMBOL_GPL(start_thread);


 /*
  *	switch_to(x,yn) should switch tasks from x to y.
  *
  * We fsave/fwait so that an exception goes off at the right time
  * (as a call from the fsave or fwait in effect) rather than to
  * the wrong process. Lazy FP saving no longer makes any sense
  * with modern CPU's, and this simplifies a lot of things (SMP
  * and UP become the same).
  *
  * NOTE! We used to use the x86 hardware context switching. The
  * reason for not using it any more becomes apparent when you
  * try to recover gracefully from saved state that is no longer
  * valid (stale segment register values in particular). With the
  * hardware task-switch, there is no way to fix up bad state in
  * a reasonable manner.
  *
  * The fact that Intel documents the hardware task-switching to
  * be slow is a fairly red herring - this code is not noticeably
  * faster. However, there _is_ some room for improvement here,
  * so the performance issues may eventually be a valid point.
  * More important, however, is the fact that this allows us much
  * more flexibility.
  *
  * The return value (in %ax) will be the "prev" task after
  * the task-switch, and shows up in ret_from_fork in entry.S,
  * for example.
  */
 __notrace_funcgraph struct task_struct *
 __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 {
 	struct thread_struct *prev = &prev_p->thread,
 				 *next = &next_p->thread;
 	int cpu = smp_processor_id();
 	struct tss_struct *tss = &per_cpu(init_tss, cpu);
 	bool preload_fpu;

 	/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */

 	/*
 	 * If the task has used fpu the last 5 timeslices, just do a full
 	 * restore of the math state immediately to avoid the trap; the
 	 * chances of needing FPU soon are obviously high now
 	 */
 	preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter > 5;

 	__unlazy_fpu(prev_p);

 	/* we're going to use this soon, after a few expensive things */
 	if (preload_fpu)
 		prefetch(next->fpu.state);

 	/*
 	 * 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
 	 * before setting the new TLS descriptors avoids the situation
 	 * where we temporarily have non-reloadable segments in %fs
 	 * and %gs.  This could be an issue if the NMI handler ever
 	 * used %fs or %gs (it does not today), or if the kernel is
 	 * running inside of a hypervisor layer.
 	 */
 	lazy_save_gs(prev->gs);

 	/*
 	 * Load the per-thread Thread-Local Storage descriptor.
 	 */
 	load_TLS(next, cpu);

 	/*
 	 * Restore IOPL if needed.  In normal use, the flags restore
 	 * in the switch assembly will handle this.  But if the kernel
 	 * is running virtualized at a non-zero CPL, the popf will
 	 * not restore flags, so it must be done in a separate step.
 	 */
 	if (get_kernel_rpl() && unlikely(prev->iopl != next->iopl))
 		set_iopl_mask(next->iopl);

 	/*
 	 * Now maybe handle debug registers and/or IO bitmaps
 	 */
 	if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
 		     task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
 		__switch_to_xtra(prev_p, next_p, tss);

 	/* If we're going to preload the fpu context, make sure clts
 	   is run while we're batching the cpu state updates. */
 	if (preload_fpu)
 		clts();

 	/*
 	 * Leave lazy mode, flushing any hypercalls made here.
 	 * This must be done before restoring TLS segments so
 	 * the GDT and LDT are properly updated, and must be
 	 * done before math_state_restore, so the TS bit is up
 	 * to date.
 	 */
 	arch_end_context_switch(next_p);

 	if (preload_fpu)
 		__math_state_restore();

 	/*
 	 * Restore %gs if needed (which is common)
 	 */
 	if (prev->gs | next->gs)
 		lazy_load_gs(next->gs);

 	percpu_write(current_task, next_p);

 	return prev_p;
 }

 #define top_esp                (THREAD_SIZE - sizeof(unsigned long))
 #define top_ebp                (THREAD_SIZE - 2*sizeof(unsigned long))

 unsigned long get_wchan(struct task_struct *p)
 {
 	unsigned long bp, sp, ip;
 	unsigned long stack_page;
 	int count = 0;
 	if (!p || p == current || p->state == TASK_RUNNING)
 		return 0;
 	stack_page = (unsigned long)task_stack_page(p);
 	sp = p->thread.sp;
 	if (!stack_page || sp < stack_page || sp > top_esp+stack_page)
 		return 0;
 	/* include/asm-i386/system.h:switch_to() pushes bp last. */
 	bp = *(unsigned long *) sp;
 	do {
 		if (bp < stack_page || bp > top_ebp+stack_page)
 			return 0;
 		ip = *(unsigned long *) (bp+4);
 		if (!in_sched_functions(ip))
 			return ip;
 		bp = *(unsigned long *) bp;
 	} while (count++ < 16);
 	return 0;
 }
	/*
	* Copyright (C) 1995 Linus Torvalds
	*
	* Pentium III FXSR, SSE support
	* Gareth Hughes <gareth@valinux.com>, May 2000
	*/

	/*
	* This file handles the architecture-dependent parts of process handling..
	*/

	#include <linux/stackprotector.h>
	#include <linux/cpu.h>
	#include <linux/errno.h>
	#include <linux/sched.h>
	#include <linux/fs.h>
	#include <linux/kernel.h>
	#include <linux/mm.h>
	#include <linux/elfcore.h>
	#include <linux/smp.h>
	#include <linux/stddef.h>
	#include <linux/slab.h>
	#include <linux/vmalloc.h>
	#include <linux/user.h>
	#include <linux/interrupt.h>
	#include <linux/delay.h>
	#include <linux/reboot.h>
	#include <linux/init.h>
	#include <linux/mc146818rtc.h>
	#include <linux/module.h>
	#include <linux/kallsyms.h>
	#include <linux/ptrace.h>
	#include <linux/personality.h>
	#include <linux/tick.h>
	#include <linux/percpu.h>
	#include <linux/prctl.h>
	#include <linux/ftrace.h>
	#include <linux/uaccess.h>
	#include <linux/io.h>
	#include <linux/kdebug.h>

	#include <asm/pgtable.h>
	#include <asm/system.h>
	#include <asm/ldt.h>
	#include <asm/processor.h>
	#include <asm/i387.h>
	#include <asm/desc.h>
	#ifdef CONFIG_MATH_EMULATION
	#include <asm/math_emu.h>
	#endif

	#include <linux/err.h>

	#include <asm/tlbflush.h>
	#include <asm/cpu.h>
	#include <asm/idle.h>
	#include <asm/syscalls.h>
	#include <asm/debugreg.h>

	#include <trace/events/power.h>

	asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");

	/*
	* Return saved PC of a blocked thread.
	*/
	unsigned long thread_saved_pc(struct task_struct *tsk)
	{
	return ((unsigned long *)tsk->thread.sp)[3];
	}

	#ifndef CONFIG_SMP
	static inline void play_dead(void)
	{
	BUG();
	}
	#endif

	/*
	* The idle thread. There's no useful work to be
	* done, so just try to conserve power and have a
	* low exit latency (ie sit in a loop waiting for
	* somebody to say that they'd like to reschedule)
	*/
	void cpu_idle(void)
	{
	int cpu = smp_processor_id();

	/*
	* If we're the non-boot CPU, nothing set the stack canary up
	* for us. CPU0 already has it initialized but no harm in
	* doing it again. This is a good place for updating it, as
	* we wont ever return from this function (so the invalid
	* canaries already on the stack wont ever trigger).
	*/
	boot_init_stack_canary();

	current_thread_info()->status \|= TS_POLLING;

	/* endless idle loop with no priority at all */
	while (1) {
	tick_nohz_stop_sched_tick(1);
	while (!need_resched()) {

	check_pgt_cache();
	rmb();

	if (cpu_is_offline(cpu))
	play_dead();

	local_irq_disable();
	/* Don't trace irqs off for idle */
	stop_critical_timings();
	pm_idle();
	start_critical_timings();

	trace_power_end(smp_processor_id());
	}
	tick_nohz_restart_sched_tick();
	preempt_enable_no_resched();
	schedule();
	preempt_disable();
	}
	}

	void __show_regs(struct pt_regs *regs, int all)
	{
	unsigned long cr0 = 0L, cr2 = 0L, cr3 = 0L, cr4 = 0L;
	unsigned long d0, d1, d2, d3, d6, d7;
	unsigned long sp;
	unsigned short ss, gs;

	if (user_mode_vm(regs)) {
	sp = regs->sp;
	ss = regs->ss & 0xffff;
	gs = get_user_gs(regs);
	} else {
	sp = kernel_stack_pointer(regs);
	savesegment(ss, ss);
	savesegment(gs, gs);
	}

	show_regs_common();

	printk(KERN_DEFAULT "EIP: %04x:[<%08lx>] EFLAGS: %08lx CPU: %d\n",
	(u16)regs->cs, regs->ip, regs->flags,
	smp_processor_id());
	print_symbol("EIP is at %s\n", regs->ip);

	printk(KERN_DEFAULT "EAX: %08lx EBX: %08lx ECX: %08lx EDX: %08lx\n",
	regs->ax, regs->bx, regs->cx, regs->dx);
	printk(KERN_DEFAULT "ESI: %08lx EDI: %08lx EBP: %08lx ESP: %08lx\n",
	regs->si, regs->di, regs->bp, sp);
	printk(KERN_DEFAULT " DS: %04x ES: %04x FS: %04x GS: %04x SS: %04x\n",
	(u16)regs->ds, (u16)regs->es, (u16)regs->fs, gs, ss);

	if (!all)
	return;

	cr0 = read_cr0();
	cr2 = read_cr2();
	cr3 = read_cr3();
	cr4 = read_cr4_safe();
	printk(KERN_DEFAULT "CR0: %08lx CR2: %08lx CR3: %08lx CR4: %08lx\n",
	cr0, cr2, cr3, cr4);

	get_debugreg(d0, 0);
	get_debugreg(d1, 1);
	get_debugreg(d2, 2);
	get_debugreg(d3, 3);
	printk(KERN_DEFAULT "DR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
	d0, d1, d2, d3);

	get_debugreg(d6, 6);
	get_debugreg(d7, 7);
	printk(KERN_DEFAULT "DR6: %08lx DR7: %08lx\n",
	d6, d7);
	}

	void release_thread(struct task_struct *dead_task)
	{
	BUG_ON(dead_task->mm);
	release_vm86_irqs(dead_task);
	}

	/*
	* This gets called before we allocate a new thread and copy
	* the current task into it.
	*/
	void prepare_to_copy(struct task_struct *tsk)
	{
	unlazy_fpu(tsk);
	}

	int copy_thread(unsigned long clone_flags, unsigned long sp,
	unsigned long unused,
	struct task_struct p, struct pt_regs regs)
	{
	struct pt_regs *childregs;
	struct task_struct *tsk;
	int err;

	childregs = task_pt_regs(p);
	childregs = regs;
	childregs->ax = 0;
	childregs->sp = sp;

	p->thread.sp = (unsigned long) childregs;
	p->thread.sp0 = (unsigned long) (childregs+1);

	p->thread.ip = (unsigned long) ret_from_fork;

	task_user_gs(p) = get_user_gs(regs);

	p->thread.io_bitmap_ptr = NULL;
	tsk = current;
	err = -ENOMEM;

	memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));

	if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
	p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
	IO_BITMAP_BYTES, GFP_KERNEL);
	if (!p->thread.io_bitmap_ptr) {
	p->thread.io_bitmap_max = 0;
	return -ENOMEM;
	}
	set_tsk_thread_flag(p, TIF_IO_BITMAP);
	}

	err = 0;

	/*
	* Set a new TLS for the child thread?
	*/
	if (clone_flags & CLONE_SETTLS)
	err = do_set_thread_area(p, -1,
	(struct user_desc __user *)childregs->si, 0);

	if (err && p->thread.io_bitmap_ptr) {
	kfree(p->thread.io_bitmap_ptr);
	p->thread.io_bitmap_max = 0;
	}
	return err;
	}

	void
	start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
	{
	set_user_gs(regs, 0);
	regs->fs = 0;
	set_fs(USER_DS);
	regs->ds = __USER_DS;
	regs->es = __USER_DS;
	regs->ss = __USER_DS;
	regs->cs = __USER_CS;
	regs->ip = new_ip;
	regs->sp = new_sp;
	/*
	* Free the old FP and other extended state
	*/
	free_thread_xstate(current);
	}
	EXPORT_SYMBOL_GPL(start_thread);


	/*
	* switch_to(x,yn) should switch tasks from x to y.
	*
	* We fsave/fwait so that an exception goes off at the right time
	* (as a call from the fsave or fwait in effect) rather than to
	* the wrong process. Lazy FP saving no longer makes any sense
	* with modern CPU's, and this simplifies a lot of things (SMP
	* and UP become the same).
	*
	* NOTE! We used to use the x86 hardware context switching. The
	* reason for not using it any more becomes apparent when you
	* try to recover gracefully from saved state that is no longer
	* valid (stale segment register values in particular). With the
	* hardware task-switch, there is no way to fix up bad state in
	* a reasonable manner.
	*
	* The fact that Intel documents the hardware task-switching to
	* be slow is a fairly red herring - this code is not noticeably
	* faster. However, there _is_ some room for improvement here,
	* so the performance issues may eventually be a valid point.
	* More important, however, is the fact that this allows us much
	* more flexibility.
	*
	* The return value (in %ax) will be the "prev" task after
	* the task-switch, and shows up in ret_from_fork in entry.S,
	* for example.
	*/
	__notrace_funcgraph struct task_struct *
	__switch_to(struct task_struct prev_p, struct task_struct next_p)
	{
	struct thread_struct *prev = &prev_p->thread,
	*next = &next_p->thread;
	int cpu = smp_processor_id();
	struct tss_struct *tss = &per_cpu(init_tss, cpu);
	bool preload_fpu;

	/* never put a printk in __switch_to... printk() calls wake_up() indirectly /

	/*
	* If the task has used fpu the last 5 timeslices, just do a full
	* restore of the math state immediately to avoid the trap; the
	* chances of needing FPU soon are obviously high now
	*/
	preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter > 5;

	__unlazy_fpu(prev_p);

	/* we're going to use this soon, after a few expensive things */
	if (preload_fpu)
	prefetch(next->fpu.state);

	/*
	* 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
	* before setting the new TLS descriptors avoids the situation
	* where we temporarily have non-reloadable segments in %fs
	* and %gs. This could be an issue if the NMI handler ever
	* used %fs or %gs (it does not today), or if the kernel is
	* running inside of a hypervisor layer.
	*/
	lazy_save_gs(prev->gs);

	/*
	* Load the per-thread Thread-Local Storage descriptor.
	*/
	load_TLS(next, cpu);

	/*
	* Restore IOPL if needed. In normal use, the flags restore
	* in the switch assembly will handle this. But if the kernel
	* is running virtualized at a non-zero CPL, the popf will
	* not restore flags, so it must be done in a separate step.
	*/
	if (get_kernel_rpl() && unlikely(prev->iopl != next->iopl))
	set_iopl_mask(next->iopl);

	/*
	* Now maybe handle debug registers and/or IO bitmaps
	*/
	if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV \|\|
	task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
	__switch_to_xtra(prev_p, next_p, tss);

	/* If we're going to preload the fpu context, make sure clts
	is run while we're batching the cpu state updates. */
	if (preload_fpu)
	clts();

	/*
	* Leave lazy mode, flushing any hypercalls made here.
	* This must be done before restoring TLS segments so
	* the GDT and LDT are properly updated, and must be
	* done before math_state_restore, so the TS bit is up
	* to date.
	*/
	arch_end_context_switch(next_p);

	if (preload_fpu)
	__math_state_restore();

	/*
	* Restore %gs if needed (which is common)
	*/
	if (prev->gs \| next->gs)
	lazy_load_gs(next->gs);

	percpu_write(current_task, next_p);

	return prev_p;
	}

	#define top_esp (THREAD_SIZE - sizeof(unsigned long))
	#define top_ebp (THREAD_SIZE - 2*sizeof(unsigned long))

	unsigned long get_wchan(struct task_struct *p)
	{
	unsigned long bp, sp, ip;
	unsigned long stack_page;
	int count = 0;
	if (!p \|\| p == current \|\| p->state == TASK_RUNNING)
	return 0;
	stack_page = (unsigned long)task_stack_page(p);
	sp = p->thread.sp;
	if (!stack_page \|\| sp < stack_page \|\| sp > top_esp+stack_page)
	return 0;
	/* include/asm-i386/system.h:switch_to() pushes bp last. */
	bp = (unsigned long ) sp;
	do {
	if (bp < stack_page \|\| bp > top_ebp+stack_page)
	return 0;
	ip = (unsigned long ) (bp+4);
	if (!in_sched_functions(ip))
	return ip;
	bp = (unsigned long ) bp;
	} while (count++ < 16);
	return 0;
	}