[PATCH] sched: Add missing memory barrier in switch_mm_cid

[Mathieu Desnoyers]

Many architectures' switch_mm() (e.g. arm64) do not have an smp_mb()
which the core scheduler code has depended upon since commit:

    commit 223baf9d17f25 ("sched: Fix performance regression introduced by mm_cid")

If switch_mm() doesn't call smp_mb(), sched_mm_cid_remote_clear() can
unset the actively used cid when it fails to observe active task after it
sets lazy_put.

There *is* a memory barrier between storing to rq->curr and _return to
userspace_ (as required by membarrier), but the rseq mm_cid has stricter
requirements: the barrier needs to be issued between store to rq->curr
and switch_mm_cid(), which happens earlier than:

- spin_unlock(),
- switch_to().

So it's fine when the architecture switch_mm happens to have that barrier
already, but less so when the architecture only provides the full barrier
in switch_to() or spin_unlock().

It is a bug in the rseq switch_mm_cid() implementation. All architectures
that don't have memory barriers in switch_mm(), but rather have the full
barrier either in finish_lock_switch() or switch_to() have them too late
for the needs of switch_mm_cid().

Introduce a new smp_mb__after_switch_mm(), defined as smp_mb() in the
generic barrier.h header, and use it in switch_mm_cid() for scheduler
transitions where switch_mm() is expected to provide a memory barrier.

Architectures can override smp_mb__after_switch_mm() if their
switch_mm() implementation provides an implicit memory barrier.
Override it with a no-op on x86 which implicitly provide this memory
barrier by writing to CR3.

Link: https://lore.kernel.org/lkml/20240305145335.2696125-1-yeoreum.yun@xxxxxxx/
Reported-by: levi.yun <yeoreum.yun@xxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.4.x
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: levi.yun <yeoreum.yun@xxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
---
 arch/x86/include/asm/barrier.h |  3 +++
 include/asm-generic/barrier.h  |  8 ++++++++
 kernel/sched/sched.h           | 20 ++++++++++++++------
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 35389b2af88e..0d5e54201eb2 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/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>
 
 /*
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 961f4d88f9ef..5a6c94d7a598 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -296,5 +296,13 @@ 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 */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2e5a95486a42..044d842c696c 100644
--- a/kernel/sched/sched.h
+++ b/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"
 
@@ -3481,13 +3483,19 @@ 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);
-- 
2.39.2