[PATCH v4 1/4] mm: reduce the rcu lock duration

[Miaohe Lin <linmiaohe@huawei.com>]

Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct")
extends the period of the rcu_read_lock until after the permissions checks
are done to prevent the task pointed to from changing from under us. But
the task_struct refcount is also taken at that time, the reference to task
is guaranteed to be stable. So it's unnecessary to extend the period of
the rcu_read_lock. Release the rcu lock after task refcount is successfully
grabbed to reduce the rcu holding time.

Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
---
 mm/mempolicy.c | 3 +--
 mm/migrate.c   | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 0b4ba3ee810e..2dad094177bf 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1609,6 +1609,7 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
 		goto out;
 	}
 	get_task_struct(task);
+	rcu_read_unlock();
 
 	err = -EINVAL;
 
@@ -1617,11 +1618,9 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
 	 * Use the regular "ptrace_may_access()" checks.
 	 */
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
-		rcu_read_unlock();
 		err = -EPERM;
 		goto out_put;
 	}
-	rcu_read_unlock();
 
 	task_nodes = cpuset_mems_allowed(task);
 	/* Is the user allowed to access the target nodes? */
diff --git a/mm/migrate.c b/mm/migrate.c
index e51588e95f57..e88ebb88fa6f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1902,17 +1902,16 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
 		return ERR_PTR(-ESRCH);
 	}
 	get_task_struct(task);
+	rcu_read_unlock();
 
 	/*
 	 * Check if this process has the right to modify the specified
 	 * process. Use the regular "ptrace_may_access()" checks.
 	 */
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
-		rcu_read_unlock();
 		mm = ERR_PTR(-EPERM);
 		goto out;
 	}
-	rcu_read_unlock();
 
 	mm = ERR_PTR(security_task_movememory(task));
 	if (IS_ERR(mm))
-- 
2.23.0