![Thomas Gleixner](/assets/img/avatar_default.png)
Jack Ren and Eric Miao tracked down the following long standing problem in the NOHZ code: scheduler switch to idle task enable interrupts Window starts here ----> interrupt happens (does not set NEED_RESCHED) irq_exit() stops the tick ----> interrupt happens (does set NEED_RESCHED) return from schedule() cpu_idle(): preempt_disable(); Window ends here The interrupts can happen at any point inside the race window. The first interrupt stops the tick, the second one causes the scheduler to rerun and switch away from idle again and we end up with the tick disabled. The fact that it needs two interrupts where the first one does not set NEED_RESCHED and the second one does made the bug obscure and extremly hard to reproduce and analyse. Kudos to Jack and Eric. Solution: Limit the NOHZ functionality to the idle loop to make sure that we can not run into such a situation ever again. cpu_idle() { preempt_disable(); while(1) { tick_nohz_stop_sched_tick(1); <- tell NOHZ code that we are in the idle loop while (!need_resched()) halt(); tick_nohz_restart_sched_tick(); <- disables NOHZ mode preempt_enable_no_resched(); schedule(); preempt_disable(); } } In hindsight we should have done this forever, but ... /me grabs a large brown paperbag. Debugged-by: Jack Ren <jack.ren@marvell.com>, Debugged-by: eric miao <eric.y.miao@gmail.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
454 lines
9.2 KiB
C
454 lines
9.2 KiB
C
/*
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* Copyright (C) 2000 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
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* Copyright 2003 PathScale, Inc.
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* Licensed under the GPL
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*/
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#include <linux/stddef.h>
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#include <linux/err.h>
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#include <linux/hardirq.h>
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#include <linux/gfp.h>
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#include <linux/mm.h>
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#include <linux/personality.h>
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#include <linux/proc_fs.h>
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#include <linux/ptrace.h>
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#include <linux/random.h>
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#include <linux/sched.h>
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#include <linux/tick.h>
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#include <linux/threads.h>
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#include <asm/current.h>
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#include <asm/pgtable.h>
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#include <asm/uaccess.h>
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#include "as-layout.h"
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#include "kern_util.h"
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#include "os.h"
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#include "skas.h"
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#include "tlb.h"
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/*
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* This is a per-cpu array. A processor only modifies its entry and it only
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* cares about its entry, so it's OK if another processor is modifying its
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* entry.
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*/
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struct cpu_task cpu_tasks[NR_CPUS] = { [0 ... NR_CPUS - 1] = { -1, NULL } };
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static inline int external_pid(void)
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{
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/* FIXME: Need to look up userspace_pid by cpu */
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return userspace_pid[0];
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}
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int pid_to_processor_id(int pid)
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{
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int i;
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for (i = 0; i < ncpus; i++) {
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if (cpu_tasks[i].pid == pid)
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return i;
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}
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return -1;
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}
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void free_stack(unsigned long stack, int order)
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{
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free_pages(stack, order);
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}
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unsigned long alloc_stack(int order, int atomic)
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{
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unsigned long page;
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gfp_t flags = GFP_KERNEL;
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if (atomic)
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flags = GFP_ATOMIC;
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page = __get_free_pages(flags, order);
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return page;
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}
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int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
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{
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int pid;
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current->thread.request.u.thread.proc = fn;
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current->thread.request.u.thread.arg = arg;
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pid = do_fork(CLONE_VM | CLONE_UNTRACED | flags, 0,
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¤t->thread.regs, 0, NULL, NULL);
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return pid;
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}
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static inline void set_current(struct task_struct *task)
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{
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cpu_tasks[task_thread_info(task)->cpu] = ((struct cpu_task)
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{ external_pid(), task });
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}
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extern void arch_switch_to(struct task_struct *to);
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void *_switch_to(void *prev, void *next, void *last)
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{
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struct task_struct *from = prev;
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struct task_struct *to = next;
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to->thread.prev_sched = from;
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set_current(to);
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do {
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current->thread.saved_task = NULL;
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switch_threads(&from->thread.switch_buf,
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&to->thread.switch_buf);
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arch_switch_to(current);
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if (current->thread.saved_task)
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show_regs(&(current->thread.regs));
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to = current->thread.saved_task;
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from = current;
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} while (current->thread.saved_task);
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return current->thread.prev_sched;
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}
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void interrupt_end(void)
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{
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if (need_resched())
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schedule();
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if (test_tsk_thread_flag(current, TIF_SIGPENDING))
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do_signal();
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}
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void exit_thread(void)
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{
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}
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void *get_current(void)
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{
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return current;
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}
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/*
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* This is called magically, by its address being stuffed in a jmp_buf
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* and being longjmp-d to.
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*/
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void new_thread_handler(void)
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{
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int (*fn)(void *), n;
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void *arg;
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if (current->thread.prev_sched != NULL)
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schedule_tail(current->thread.prev_sched);
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current->thread.prev_sched = NULL;
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fn = current->thread.request.u.thread.proc;
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arg = current->thread.request.u.thread.arg;
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/*
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* The return value is 1 if the kernel thread execs a process,
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* 0 if it just exits
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*/
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n = run_kernel_thread(fn, arg, ¤t->thread.exec_buf);
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if (n == 1) {
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/* Handle any immediate reschedules or signals */
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interrupt_end();
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userspace(¤t->thread.regs.regs);
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}
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else do_exit(0);
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}
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/* Called magically, see new_thread_handler above */
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void fork_handler(void)
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{
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force_flush_all();
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schedule_tail(current->thread.prev_sched);
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/*
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* XXX: if interrupt_end() calls schedule, this call to
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* arch_switch_to isn't needed. We could want to apply this to
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* improve performance. -bb
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*/
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arch_switch_to(current);
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current->thread.prev_sched = NULL;
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/* Handle any immediate reschedules or signals */
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interrupt_end();
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userspace(¤t->thread.regs.regs);
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}
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int copy_thread(int nr, unsigned long clone_flags, unsigned long sp,
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unsigned long stack_top, struct task_struct * p,
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struct pt_regs *regs)
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{
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void (*handler)(void);
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int ret = 0;
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p->thread = (struct thread_struct) INIT_THREAD;
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if (current->thread.forking) {
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memcpy(&p->thread.regs.regs, ®s->regs,
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sizeof(p->thread.regs.regs));
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REGS_SET_SYSCALL_RETURN(p->thread.regs.regs.gp, 0);
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if (sp != 0)
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REGS_SP(p->thread.regs.regs.gp) = sp;
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handler = fork_handler;
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arch_copy_thread(¤t->thread.arch, &p->thread.arch);
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}
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else {
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get_safe_registers(p->thread.regs.regs.gp);
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p->thread.request.u.thread = current->thread.request.u.thread;
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handler = new_thread_handler;
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}
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new_thread(task_stack_page(p), &p->thread.switch_buf, handler);
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if (current->thread.forking) {
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clear_flushed_tls(p);
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/*
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* Set a new TLS for the child thread?
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*/
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if (clone_flags & CLONE_SETTLS)
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ret = arch_copy_tls(p);
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}
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return ret;
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}
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void initial_thread_cb(void (*proc)(void *), void *arg)
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{
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int save_kmalloc_ok = kmalloc_ok;
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kmalloc_ok = 0;
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initial_thread_cb_skas(proc, arg);
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kmalloc_ok = save_kmalloc_ok;
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}
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void default_idle(void)
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{
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unsigned long long nsecs;
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while (1) {
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/* endless idle loop with no priority at all */
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/*
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* although we are an idle CPU, we do not want to
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* get into the scheduler unnecessarily.
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*/
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if (need_resched())
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schedule();
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tick_nohz_stop_sched_tick(1);
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nsecs = disable_timer();
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idle_sleep(nsecs);
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tick_nohz_restart_sched_tick();
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}
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}
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void cpu_idle(void)
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{
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cpu_tasks[current_thread_info()->cpu].pid = os_getpid();
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default_idle();
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}
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int __cant_sleep(void) {
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return in_atomic() || irqs_disabled() || in_interrupt();
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/* Is in_interrupt() really needed? */
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}
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int user_context(unsigned long sp)
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{
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unsigned long stack;
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stack = sp & (PAGE_MASK << CONFIG_KERNEL_STACK_ORDER);
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return stack != (unsigned long) current_thread_info();
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}
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extern exitcall_t __uml_exitcall_begin, __uml_exitcall_end;
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void do_uml_exitcalls(void)
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{
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exitcall_t *call;
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call = &__uml_exitcall_end;
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while (--call >= &__uml_exitcall_begin)
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(*call)();
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}
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char *uml_strdup(const char *string)
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{
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return kstrdup(string, GFP_KERNEL);
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}
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int copy_to_user_proc(void __user *to, void *from, int size)
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{
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return copy_to_user(to, from, size);
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}
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int copy_from_user_proc(void *to, void __user *from, int size)
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{
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return copy_from_user(to, from, size);
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}
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int clear_user_proc(void __user *buf, int size)
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{
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return clear_user(buf, size);
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}
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int strlen_user_proc(char __user *str)
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{
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return strlen_user(str);
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}
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int smp_sigio_handler(void)
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{
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#ifdef CONFIG_SMP
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int cpu = current_thread_info()->cpu;
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IPI_handler(cpu);
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if (cpu != 0)
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return 1;
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#endif
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return 0;
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}
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int cpu(void)
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{
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return current_thread_info()->cpu;
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}
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static atomic_t using_sysemu = ATOMIC_INIT(0);
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int sysemu_supported;
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void set_using_sysemu(int value)
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{
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if (value > sysemu_supported)
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return;
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atomic_set(&using_sysemu, value);
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}
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int get_using_sysemu(void)
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{
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return atomic_read(&using_sysemu);
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}
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static int proc_read_sysemu(char *buf, char **start, off_t offset, int size,int *eof, void *data)
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{
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if (snprintf(buf, size, "%d\n", get_using_sysemu()) < size)
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/* No overflow */
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*eof = 1;
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return strlen(buf);
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}
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static int proc_write_sysemu(struct file *file,const char __user *buf, unsigned long count,void *data)
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{
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char tmp[2];
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if (copy_from_user(tmp, buf, 1))
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return -EFAULT;
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if (tmp[0] >= '0' && tmp[0] <= '2')
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set_using_sysemu(tmp[0] - '0');
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/* We use the first char, but pretend to write everything */
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return count;
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}
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int __init make_proc_sysemu(void)
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{
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struct proc_dir_entry *ent;
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if (!sysemu_supported)
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return 0;
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ent = create_proc_entry("sysemu", 0600, NULL);
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if (ent == NULL)
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{
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printk(KERN_WARNING "Failed to register /proc/sysemu\n");
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return 0;
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}
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ent->read_proc = proc_read_sysemu;
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ent->write_proc = proc_write_sysemu;
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return 0;
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}
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late_initcall(make_proc_sysemu);
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int singlestepping(void * t)
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{
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struct task_struct *task = t ? t : current;
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if (!(task->ptrace & PT_DTRACE))
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return 0;
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if (task->thread.singlestep_syscall)
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return 1;
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return 2;
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}
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/*
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* Only x86 and x86_64 have an arch_align_stack().
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* All other arches have "#define arch_align_stack(x) (x)"
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* in their asm/system.h
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* As this is included in UML from asm-um/system-generic.h,
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* we can use it to behave as the subarch does.
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*/
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#ifndef arch_align_stack
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unsigned long arch_align_stack(unsigned long sp)
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{
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if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
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sp -= get_random_int() % 8192;
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return sp & ~0xf;
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}
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#endif
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unsigned long get_wchan(struct task_struct *p)
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{
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unsigned long stack_page, sp, ip;
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bool seen_sched = 0;
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if ((p == NULL) || (p == current) || (p->state == TASK_RUNNING))
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return 0;
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stack_page = (unsigned long) task_stack_page(p);
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/* Bail if the process has no kernel stack for some reason */
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if (stack_page == 0)
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return 0;
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sp = p->thread.switch_buf->JB_SP;
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/*
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* Bail if the stack pointer is below the bottom of the kernel
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* stack for some reason
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*/
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if (sp < stack_page)
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return 0;
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while (sp < stack_page + THREAD_SIZE) {
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ip = *((unsigned long *) sp);
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if (in_sched_functions(ip))
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/* Ignore everything until we're above the scheduler */
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seen_sched = 1;
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else if (kernel_text_address(ip) && seen_sched)
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return ip;
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sp += sizeof(unsigned long);
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}
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return 0;
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}
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int elf_core_copy_fpregs(struct task_struct *t, elf_fpregset_t *fpu)
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{
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int cpu = current_thread_info()->cpu;
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return save_fp_registers(userspace_pid[cpu], (unsigned long *) fpu);
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}
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