[PATCH v3 05/35] sched: add cpumask_find_and_set() and use it in __mm_cid_get()

[Yury Norov]

__mm_cid_get() uses __mm_cid_try_get() helper to atomically acquire a
bit in mm cid mask. Now that we have atomic find_and_set_bit(), we can
easily extend it to cpumasks and use in the scheduler code.

cpumask_find_and_set() considers cid mask as a volatile region of memory,
as it actually is in this case. So, if it's changed while search is in
progress, KCSAN wouldn't fire warning on it.

CC: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---
 include/linux/cpumask.h | 12 ++++++++++++
 kernel/sched/sched.h    | 14 +++++---------
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index cfb545841a2c..c2acced8be4e 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -271,6 +271,18 @@ unsigned int cpumask_next_and(int n, const struct cpumask *src1p,
 		small_cpumask_bits, n + 1);
 }
 
+/**
+ * cpumask_find_and_set - find the first unset cpu in a cpumask and
+ *			  set it atomically
+ * @srcp: the cpumask pointer
+ *
+ * Return: >= nr_cpu_ids if nothing is found.
+ */
+static inline unsigned int cpumask_find_and_set(volatile struct cpumask *srcp)
+{
+	return find_and_set_bit(cpumask_bits(srcp), small_cpumask_bits);
+}
+
 /**
  * for_each_cpu - iterate over every cpu in a mask
  * @cpu: the (optionally unsigned) integer iterator
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2e5a95486a42..2ce9112de89b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3347,23 +3347,19 @@ static inline void mm_cid_put(struct mm_struct *mm)
 
 static inline int __mm_cid_try_get(struct mm_struct *mm)
 {
-	struct cpumask *cpumask;
-	int cid;
+	struct cpumask *cpumask = mm_cidmask(mm);
+	int cid = nr_cpu_ids;
 
-	cpumask = mm_cidmask(mm);
 	/*
 	 * Retry finding first zero bit if the mask is temporarily
 	 * filled. This only happens during concurrent remote-clear
 	 * which owns a cid without holding a rq lock.
 	 */
-	for (;;) {
-		cid = cpumask_first_zero(cpumask);
-		if (cid < nr_cpu_ids)
-			break;
+	while (cid >= nr_cpu_ids) {
+		cid = cpumask_find_and_set(cpumask);
 		cpu_relax();
 	}
-	if (cpumask_test_and_set_cpu(cid, cpumask))
-		return -1;
+
 	return cid;
 }
 
-- 
2.40.1