kill PF_BORROWED_MM in favour of PF_KTHREAD

Kill PF_BORROWED_MM.  Change use_mm/unuse_mm to not play with ->flags, and
do s/PF_BORROWED_MM/PF_KTHREAD/ for a couple of other users.

No functional changes yet.  But this allows us to do further
fixes/cleanups.

oom_kill/ptrace/etc often check "p->mm != NULL" to filter out the
kthreads, this is wrong because of use_mm().  The problem with
PF_BORROWED_MM is that we need task_lock() to avoid races.  With this
patch we can check PF_KTHREAD directly, or use a simple lockless helper:

	/* The result must not be dereferenced !!! */
	struct mm_struct *__get_task_mm(struct task_struct *tsk)
	{
		if (tsk->flags & PF_KTHREAD)
			return NULL;
		return tsk->mm;
	}

Note also ecard_task().  It runs with ->mm != NULL, but it's the kernel
thread without PF_BORROWED_MM.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index eec64a4adb9d..0560999eb1db 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1483,7 +1483,6 @@ static inline void put_task_struct(struct task_struct *t)
 #define PF_EXITING      0x00000004      /* getting shut down */
 #define PF_EXITPIDONE   0x00000008      /* pi exit done on shut down */
 #define PF_VCPU         0x00000010      /* I'm a virtual CPU */
-#define PF_KTHREAD      0x00000020      /* I am a kernel thread */
 #define PF_FORKNOEXEC   0x00000040      /* forked but didn't exec */
 #define PF_SUPERPRIV    0x00000100      /* used super-user privileges */
 #define PF_DUMPCORE     0x00000200      /* dumped core */
@@ -1497,7 +1496,7 @@ static inline void put_task_struct(struct task_struct *t)
 #define PF_KSWAPD       0x00040000      /* I am kswapd */
 #define PF_SWAPOFF      0x00080000      /* I am in swapoff */
 #define PF_LESS_THROTTLE 0x00100000     /* Throttle me less: I clean memory */
-#define PF_BORROWED_MM  0x00200000      /* I am a kthread doing use_mm */
+#define PF_KTHREAD      0x00200000      /* I am a kernel thread */
 #define PF_RANDOMIZE    0x00400000      /* randomize virtual address space */
 #define PF_SWAPWRITE    0x00800000      /* Allowed to write to swap */
 #define PF_SPREAD_PAGE  0x01000000      /* Spread page cache over cpuset */