From: Nicholas Piggin Date: Mon, 4 Nov 2024 14:23:18 +0000 (-0300) Subject: lazy tlb: fix hotplug exit race with MMU_LAZY_TLB_SHOOTDOWN X-Git-Tag: v6.14-rc1~77^2~271 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=21641bd9a7a7ce0360106a5a8e5b89a4fc74529d;p=linux-2.6-block.git lazy tlb: fix hotplug exit race with MMU_LAZY_TLB_SHOOTDOWN CPU unplug first calls __cpu_disable(), and that's where powerpc calls cleanup_cpu_mmu_context(), which clears this CPU from mm_cpumask() of all mms in the system. However this CPU may still be using a lazy tlb mm, and its mm_cpumask bit will be cleared from it. The CPU does not switch away from the lazy tlb mm until arch_cpu_idle_dead() calls idle_task_exit(). If that user mm exits in this window, it will not be subject to the lazy tlb mm shootdown and may be freed while in use as a lazy mm by the CPU that is being unplugged. cleanup_cpu_mmu_context() could be moved later, but it looks better to move the lazy tlb mm switching earlier. The problem with doing the lazy mm switching in idle_task_exit() is explained in commit bf2c59fce4074 ("sched/core: Fix illegal RCU from offline CPUs"), which added a wart to switch away from the mm but leave it set in active_mm to be cleaned up later. So instead, switch away from the lazy tlb mm at sched_cpu_wait_empty(), which is the last hotplug state before teardown (CPUHP_AP_SCHED_WAIT_EMPTY). This CPU will never switch to a user thread from this point, so it has no chance to pick up a new lazy tlb mm. This removes the lazy tlb mm handling wart in CPU unplug. With this, idle_task_exit() is not needed anymore and can be cleaned up. This leaves the prototype alone, to be cleaned after this change. herton: took the suggestions from https://lore.kernel.org/all/87jzvyprsw.ffs@tglx/ and made adjustments on the initial patch proposed by Nicholas. Link: https://lkml.kernel.org/r/20230524060455.147699-1-npiggin@gmail.com Link: https://lore.kernel.org/all/20230525205253.E2FAEC433EF@smtp.kernel.org/ Link: https://lkml.kernel.org/r/20241104142318.3295663-1-herton@redhat.com Fixes: 2655421ae69f ("lazy tlb: shoot lazies, non-refcounting lazy tlb mm reference handling scheme") Signed-off-by: Nicholas Piggin Signed-off-by: Herton R. Krzesinski Suggested-by: Thomas Gleixner Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Michael Ellerman Signed-off-by: Andrew Morton --- diff --git a/include/linux/sched/hotplug.h b/include/linux/sched/hotplug.h index 412cdaba33eb..17e04859b9a4 100644 --- a/include/linux/sched/hotplug.h +++ b/include/linux/sched/hotplug.h @@ -18,10 +18,6 @@ extern int sched_cpu_dying(unsigned int cpu); # define sched_cpu_dying NULL #endif -#ifdef CONFIG_HOTPLUG_CPU -extern void idle_task_exit(void); -#else static inline void idle_task_exit(void) {} -#endif #endif /* _LINUX_SCHED_HOTPLUG_H */ diff --git a/kernel/cpu.c b/kernel/cpu.c index b605334f8ee6..7f3bf759cbdf 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -905,12 +905,13 @@ static int finish_cpu(unsigned int cpu) struct mm_struct *mm = idle->active_mm; /* - * idle_task_exit() will have switched to &init_mm, now - * clean up any remaining active_mm state. + * sched_force_init_mm() ensured the use of &init_mm, + * drop that refcount now that the CPU has stopped. */ - if (mm != &init_mm) - idle->active_mm = &init_mm; + WARN_ON(mm != &init_mm); + idle->active_mm = NULL; mmdrop_lazy_tlb(mm); + return 0; } diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 755ae4659b64..6fbf4c373a50 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7930,19 +7930,26 @@ void sched_setnuma(struct task_struct *p, int nid) #ifdef CONFIG_HOTPLUG_CPU /* - * Ensure that the idle task is using init_mm right before its CPU goes - * offline. + * Invoked on the outgoing CPU in context of the CPU hotplug thread + * after ensuring that there are no user space tasks left on the CPU. + * + * If there is a lazy mm in use on the hotplug thread, drop it and + * switch to init_mm. + * + * The reference count on init_mm is dropped in finish_cpu(). */ -void idle_task_exit(void) +static void sched_force_init_mm(void) { struct mm_struct *mm = current->active_mm; - BUG_ON(cpu_online(smp_processor_id())); - BUG_ON(current != this_rq()->idle); - if (mm != &init_mm) { - switch_mm(mm, &init_mm, current); + mmgrab_lazy_tlb(&init_mm); + local_irq_disable(); + current->active_mm = &init_mm; + switch_mm_irqs_off(mm, &init_mm, current); + local_irq_enable(); finish_arch_post_lock_switch(); + mmdrop_lazy_tlb(mm); } /* finish_cpu(), as ran on the BP, will clean up the active_mm state */ @@ -8344,6 +8351,7 @@ int sched_cpu_starting(unsigned int cpu) int sched_cpu_wait_empty(unsigned int cpu) { balance_hotplug_wait(); + sched_force_init_mm(); return 0; }