This is a note to let you know that I've just added the patch titled
sched: Add missing memory barrier in switch_mm_cid
to the 6.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-add-missing-memory-barrier-in-switch_mm_cid.patch
and it can be found in the queue-6.8 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From fe90f3967bdb3e13f133e5f44025e15f943a99c5 Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Date: Mon, 15 Apr 2024 11:21:13 -0400
Subject: sched: Add missing memory barrier in switch_mm_cid
From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
commit fe90f3967bdb3e13f133e5f44025e15f943a99c5 upstream.
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.
Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
Reported-by: levi.yun <yeoreum.yun@xxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx> # for arm64
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> # for x86
Cc: <stable@xxxxxxxxxxxxxxx> # 6.4.x
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240415152114.59122-2-mathieu.desnoyers@xxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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(-)
--- 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>
#endif /* _ASM_X86_BARRIER_H */
--- 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 */
--- 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"
@@ -3445,13 +3447,19 @@ static inline void switch_mm_cid(struct
* 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);
Patches currently in stable-queue which might be from mathieu.desnoyers@xxxxxxxxxxxx are
queue-6.8/sched-add-missing-memory-barrier-in-switch_mm_cid.patch
