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-rw-r--r--kernel/cpuset.c24
1 files changed, 19 insertions, 5 deletions
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 961d74044de..00e8f257551 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -166,9 +166,8 @@ static struct super_block *cpuset_sb = NULL;
* The hooks from fork and exit, cpuset_fork() and cpuset_exit(), don't
* (usually) grab cpuset_sem. These are the two most performance
* critical pieces of code here. The exception occurs on exit(),
- * if the last task using a cpuset exits, and the cpuset was marked
- * notify_on_release. In that case, the cpuset_sem is taken, the
- * path to the released cpuset calculated, and a usermode call made
+ * when a task in a notify_on_release cpuset exits. Then cpuset_sem
+ * is taken, and if the cpuset count is zero, a usermode call made
* to /sbin/cpuset_release_agent with the name of the cpuset (path
* relative to the root of cpuset file system) as the argument.
*
@@ -1404,6 +1403,18 @@ void cpuset_fork(struct task_struct *tsk)
*
* Description: Detach cpuset from @tsk and release it.
*
+ * Note that cpusets marked notify_on_release force every task
+ * in them to take the global cpuset_sem semaphore when exiting.
+ * This could impact scaling on very large systems. Be reluctant
+ * to use notify_on_release cpusets where very high task exit
+ * scaling is required on large systems.
+ *
+ * Don't even think about derefencing 'cs' after the cpuset use
+ * count goes to zero, except inside a critical section guarded
+ * by the cpuset_sem semaphore. If you don't hold cpuset_sem,
+ * then a zero cpuset use count is a license to any other task to
+ * nuke the cpuset immediately.
+ *
**/
void cpuset_exit(struct task_struct *tsk)
@@ -1415,10 +1426,13 @@ void cpuset_exit(struct task_struct *tsk)
tsk->cpuset = NULL;
task_unlock(tsk);
- if (atomic_dec_and_test(&cs->count)) {
+ if (notify_on_release(cs)) {
down(&cpuset_sem);
- check_for_release(cs);
+ if (atomic_dec_and_test(&cs->count))
+ check_for_release(cs);
up(&cpuset_sem);
+ } else {
+ atomic_dec(&cs->count);
}
}