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Freezer: avoid freezing kernel threads prematurely

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Kernel threads should not have TIF_FREEZE set when user space processes are
being frozen, since otherwise some of them might be frozen prematurely.
To prevent this from happening we can (1) make exit_mm() unset TIF_FREEZE
unconditionally just after clearing tsk->mm and (2) make try_to_freeze_tasks()
check if p->mm is different from zero and PF_BORROWED_MM is unset in p->flags
when user space processes are to be frozen.

Namely, when user space processes are being frozen, we only should set
TIF_FREEZE for tasks that have p->mm different from NULL and don't have
PF_BORROWED_MM set in p->flags.  For this reason task_lock() must be used to
prevent try_to_freeze_tasks() from racing with use_mm()/unuse_mm(), in which
p->mm and p->flags.PF_BORROWED_MM are changed under task_lock(p).  Also, we
need to prevent the following scenario from happening:

* daemonize() is called by a task spawned from a user space code path
* freezer checks if the task has p->mm set and the result is positive
* task enters exit_mm() and clears its TIF_FREEZE
* freezer sets TIF_FREEZE for the task
* task calls try_to_freeze() and goes to the refrigerator, which is wrong at
  that point

This requires us to acquire task_lock(p) before p->flags.PF_BORROWED_MM and
p->mm are examined and release it after TIF_FREEZE is set for p (or it turns
out that TIF_FREEZE should not be set).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Rafael J. Wysocki 2007-07-19 01:47:33 -07:00 committed by Linus Torvalds
parent b1457bcc3a
commit 0c1eecfb34
3 changed files with 41 additions and 35 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
include/linux/freezer.h
View file

@ -25,7 +25,7 @@ static inline int freezing(struct task_struct *p)
/*
* Request that a process be frozen
*/
static inline void freeze(struct task_struct *p)
static inline void set_freeze_flag(struct task_struct *p)
{
set_tsk_thread_flag(p, TIF_FREEZE);
}
@ -33,7 +33,7 @@ static inline void freeze(struct task_struct *p)
/*
* Sometimes we may need to cancel the previous 'freeze' request
*/
static inline void do_not_freeze(struct task_struct *p)
static inline void clear_freeze_flag(struct task_struct *p)
{
clear_tsk_thread_flag(p, TIF_FREEZE);
}
@ -56,7 +56,7 @@ static inline int thaw_process(struct task_struct *p)
wake_up_process(p);
return 1;
}
clear_tsk_thread_flag(p, TIF_FREEZE);
clear_freeze_flag(p);
task_unlock(p);
return 0;
}
@ -129,7 +129,8 @@ static inline void set_freezable(void)
#else
static inline int frozen(struct task_struct *p) { return 0; }
static inline int freezing(struct task_struct *p) { return 0; }
static inline void freeze(struct task_struct *p) { BUG(); }
static inline void set_freeze_flag(struct task_struct *p) {}
static inline void clear_freeze_flag(struct task_struct *p) {}
static inline int thaw_process(struct task_struct *p) { return 1; }
static inline void refrigerator(void) {}

3
kernel/exit.c
View file

@ -45,6 +45,7 @@
#include <linux/resource.h>
#include <linux/blkdev.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/freezer.h>
#include <asm/uaccess.h>
#include <asm/unistd.h>
@ -594,6 +595,8 @@ static void exit_mm(struct task_struct * tsk)
tsk->mm = NULL;
up_read(&mm->mmap_sem);
enter_lazy_tlb(mm, current);
/* We don't want this task to be frozen prematurely */
clear_freeze_flag(tsk);
task_unlock(tsk);
mmput(mm);
}

64
kernel/power/process.c
View file

@ -40,7 +40,7 @@ static inline void frozen_process(void)
current->flags |= PF_FROZEN;
wmb();
}
clear_tsk_thread_flag(current, TIF_FREEZE);
clear_freeze_flag(current);
}
/* Refrigerator is place where frozen processes are stored :-). */
@ -75,17 +75,16 @@ void refrigerator(void)
current->state = save;
}
static inline void freeze_process(struct task_struct *p)
static void freeze_task(struct task_struct *p)
{
unsigned long flags;
if (!freezing(p)) {
rmb();
if (!frozen(p)) {
set_freeze_flag(p);
if (p->state == TASK_STOPPED)
force_sig_specific(SIGSTOP, p);
freeze(p);
spin_lock_irqsave(&p->sighand->siglock, flags);
signal_wake_up(p, p->state == TASK_STOPPED);
spin_unlock_irqrestore(&p->sighand->siglock, flags);
@ -99,18 +98,13 @@ static void cancel_freezing(struct task_struct *p)
if (freezing(p)) {
pr_debug(" clean up: %s\n", p->comm);
do_not_freeze(p);
clear_freeze_flag(p);
spin_lock_irqsave(&p->sighand->siglock, flags);
recalc_sigpending_and_wake(p);
spin_unlock_irqrestore(&p->sighand->siglock, flags);
}
}
static inline int is_user_space(struct task_struct *p)
{
return p->mm && !(p->flags & PF_BORROWED_MM);
}
static unsigned int try_to_freeze_tasks(int freeze_user_space)
{
struct task_struct *g, *p;
@ -122,20 +116,34 @@ static unsigned int try_to_freeze_tasks(int freeze_user_space)
todo = 0;
read_lock(&tasklist_lock);
do_each_thread(g, p) {
if (!freezeable(p))
if (frozen(p) || !freezeable(p))
continue;
if (frozen(p))
continue;
if (p->state == TASK_TRACED && frozen(p->parent)) {
cancel_freezing(p);
continue;
if (freeze_user_space) {
if (p->state == TASK_TRACED &&
frozen(p->parent)) {
cancel_freezing(p);
continue;
}
/*
* Kernel threads should not have TIF_FREEZE set
* at this point, so we must ensure that either
* p->mm is not NULL *and* PF_BORROWED_MM is
* unset, or TIF_FRREZE is left unset.
* The task_lock() is necessary to prevent races
* with exit_mm() or use_mm()/unuse_mm() from
* occuring.
*/
task_lock(p);
if (!p->mm || (p->flags & PF_BORROWED_MM)) {
task_unlock(p);
continue;
}
freeze_task(p);
task_unlock(p);
} else {
freeze_task(p);
}
if (freeze_user_space && !is_user_space(p))
continue;
freeze_process(p);
if (!freezer_should_skip(p))
todo++;
} while_each_thread(g, p);
@ -152,22 +160,16 @@ static unsigned int try_to_freeze_tasks(int freeze_user_space)
* but it cleans up leftover PF_FREEZE requests.
*/
printk("\n");
printk(KERN_ERR "Stopping %s timed out after %d seconds "
printk(KERN_ERR "Freezing of %s timed out after %d seconds "
"(%d tasks refusing to freeze):\n",
freeze_user_space ? "user space processes" :
"kernel threads",
freeze_user_space ? "user space " : "tasks ",
TIMEOUT / HZ, todo);
show_state();
read_lock(&tasklist_lock);
do_each_thread(g, p) {
if (freeze_user_space && !is_user_space(p))
continue;
task_lock(p);
if (freezeable(p) && !frozen(p) &&
!freezer_should_skip(p))
if (freezing(p) && !freezer_should_skip(p))
printk(KERN_ERR " %s\n", p->comm);
cancel_freezing(p);
task_unlock(p);
} while_each_thread(g, p);
@ -211,7 +213,7 @@ static void thaw_tasks(int thaw_user_space)
if (!freezeable(p))
continue;
if (is_user_space(p) == !thaw_user_space)
if (!p->mm == thaw_user_space)
continue;
thaw_process(p);