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Subject: [PATCH 1/4] exit: Move read_unlock() up in mm_update_next_owner()
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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        ktkhai@virtuozzo.com
Date:   Thu, 26 Apr 2018 14:00:33 +0300
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All the places, we update task's mm, are made this
under task_lock(). These are exec_mmap(), exit_mm()
and kernel thread's use_mm() and unuse_mm().
The only exception is copy_mm(), which initializes
newborn task's mm, but we can't race with it, as
mm_update_next_owner() iterates tasks already linked
to task list.

get_task_struct() guarantees that task_struct can't free.
exec_mmap() and exit_mm() both call mm_update_next_owner():

	task_lock(current);
	current->mm = new_mm;
	task_unlock(current);
	mm_update_next_owner(mm);

and since mm_update_next_owner() checks and assigns mm under
task_lock() too, they can't miss they became a new mm owner.
So we can relax the lock and release tasklist_lock earlier.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 kernel/exit.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index c3c7ac560114..9fb7b699bdeb 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -464,18 +464,15 @@ void mm_update_next_owner(struct mm_struct *mm)
 	return;
 
 assign_new_owner:
-	BUG_ON(c == p);
 	get_task_struct(c);
+	read_unlock(&tasklist_lock);
+	BUG_ON(c == p);
+
 	/*
 	 * The task_lock protects c->mm from changing.
 	 * We always want mm->owner->mm == mm
 	 */
 	task_lock(c);
-	/*
-	 * Delay read_unlock() till we have the task_lock()
-	 * to ensure that c does not slip away underneath us
-	 */
-	read_unlock(&tasklist_lock);
 	if (c->mm != mm) {
 		task_unlock(c);
 		put_task_struct(c);