[patch for-3.2-rc3] cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask

[David Rientjes <rientjes@google.com>]

c0ff7453bb5c ("cpuset,mm: fix no node to alloc memory when changing
cpuset's mems") adds get_mems_allowed() to prevent the set of allowed
nodes from changing for a thread.  This causes any update to a set of
allowed nodes to stall until put_mems_allowed() is called.

This stall is unncessary, however, if at least one node remains unchanged
in the update to the set of allowed nodes.  This was addressed by
89e8a244b97e ("cpusets: avoid looping when storing to mems_allowed if one
node remains set"), but it's still possible that an empty nodemask may be
read from a mempolicy because the old nodemask may be remapped to the new
nodemask during rebind.  To prevent this, only avoid the stall if there
is no mempolicy for the thread being changed.

This is a temporary solution until all reads from mempolicy nodemasks can
be guaranteed to not be empty without the get_mems_allowed()
synchronization.

Also moves the check for nodemask intersection inside task_lock() so that
tsk->mems_allowed cannot change.

Reported-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: David Rientjes <rientjes@google.com>
---
 kernel/cpuset.c |   17 +++++++++++------
 1 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -949,7 +949,7 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from,
 static void cpuset_change_task_nodemask(struct task_struct *tsk,
 					nodemask_t *newmems)
 {
-	bool masks_disjoint = !nodes_intersects(*newmems, tsk->mems_allowed);
+	bool need_loop;
 
 repeat:
 	/*
@@ -962,6 +962,14 @@ repeat:
 		return;
 
 	task_lock(tsk);
+	/*
+	 * Determine if a loop is necessary if another thread is doing
+	 * get_mems_allowed().  If at least one node remains unchanged and
+	 * tsk does not have a mempolicy, then an empty nodemask will not be
+	 * possible when mems_allowed is larger than a word.
+	 */
+	need_loop = tsk->mempolicy ||
+			!nodes_intersects(*newmems, tsk->mems_allowed);
 	nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
 	mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP1);
 
@@ -981,12 +989,9 @@ repeat:
 
 	/*
 	 * Allocation of memory is very fast, we needn't sleep when waiting
-	 * for the read-side.  No wait is necessary, however, if at least one
-	 * node remains unchanged and tsk has a mempolicy that could store an
-	 * empty nodemask.
+	 * for the read-side.
 	 */
-	while (masks_disjoint && tsk->mempolicy &&
-			ACCESS_ONCE(tsk->mems_allowed_change_disable)) {
+	while (need_loop && ACCESS_ONCE(tsk->mems_allowed_change_disable)) {
 		task_unlock(tsk);
 		if (!task_curr(tsk))
 			yield();

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