From fe90f3967bdb3e13f133e5f44025e15f943a99c5 Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Mon, 15 Apr 2024 11:21:13 -0400 Subject: sched: Add missing memory barrier in switch_mm_cid 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 Signed-off-by: Mathieu Desnoyers Signed-off-by: Ingo Molnar Reviewed-by: Catalin Marinas # for arm64 Acked-by: Dave Hansen # for x86 Cc: # 6.4.x Cc: Linus Torvalds Link: https://lore.kernel.org/r/20240415152114.59122-2-mathieu.desnoyers@efficios.com --- kernel/sched/sched.h | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) (limited to 'kernel/sched') diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index d2242679239e..ae50f212775e 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -79,6 +79,8 @@ # include #endif +#include + #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); -- cgit v1.2.3-58-ga151