Merge tag 'sched_urgent_for_v6.9_rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler fix from Borislav Petkov:

 - Add a missing memory barrier in the concurrency ID mm switching

* tag 'sched_urgent_for_v6.9_rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched: Add missing memory barrier in switch_mm_cid

Author: torvalds

arch/x86/include/asm/barrier.h

@@ -79,6 +79,9 @@ do {									\
 #define __smp_mb__before_atomic()	do { } while (0)
 #define __smp_mb__after_atomic()	do { } while (0)
 
+/* Writing to CR3 provides a full memory barrier in switch_mm(). */
+#define smp_mb__after_switch_mm()	do { } while (0)
+
 #include <asm-generic/barrier.h>
 
 #endif /* _ASM_X86_BARRIER_H */

include/asm-generic/barrier.h

@@ -294,5 +294,13 @@ do {									\
 #define io_stop_wc() do { } while (0)
 #endif
 
+/*
+ * Architectures that guarantee an implicit smp_mb() in switch_mm()
+ * can override smp_mb__after_switch_mm.
+ */
+#ifndef smp_mb__after_switch_mm
+# define smp_mb__after_switch_mm()	smp_mb()
+#endif
+
 #endif /* !__ASSEMBLY__ */
 #endif /* __ASM_GENERIC_BARRIER_H */

kernel/sched/sched.h

@@ -79,6 +79,8 @@
 # include <asm/paravirt_api_clock.h>
 #endif
 
+#include <asm/barrier.h>
+
 #include "cpupri.h"
 #include "cpudeadline.h"
 
@@ -3445,13 +3447,19 @@ static inline void switch_mm_cid(struct rq *rq,
 		 * between rq->curr store and load of {prev,next}->mm->pcpu_cid[cpu].
 		 * Provide it here.
 		 */
-		if (!prev->mm)                          // from kernel
+		if (!prev->mm) {                        // from kernel
 			smp_mb();
-		/*
-		 * user -> user transition guarantees a memory barrier through
-		 * switch_mm() when current->mm changes. If current->mm is
-		 * unchanged, no barrier is needed.
-		 */
+		} else {				// from user
+			/*
+			 * user->user transition relies on an implicit
+			 * memory barrier in switch_mm() when
+			 * current->mm changes. If the architecture
+			 * switch_mm() does not have an implicit memory
+			 * barrier, it is emitted here.  If current->mm
+			 * is unchanged, no barrier is needed.
+			 */
+			smp_mb__after_switch_mm();
+		}
 	}
 	if (prev->mm_cid_active) {
 		mm_cid_snapshot_time(rq, prev->mm);