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authorRafael J. Wysocki <rjw@sisk.pl>2007-07-19 01:47:33 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-07-19 10:04:42 -0700
commit0c1eecfb345401629aa57c9d3b077273e56c45a7 (patch)
tree522d7090966c6e70f3147c30e3308781e0309938 /kernel/exit.c
parentb1457bcc3a00a0446c7f6e2f22fd24b6d8d0a309 (diff)
Freezer: avoid freezing kernel threads prematurely
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>
Diffstat (limited to 'kernel/exit.c')
-rw-r--r--kernel/exit.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/kernel/exit.c b/kernel/exit.c
index e8af8d0c248..464c2b172f0 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -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);
}