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https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson
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21641bd9a7
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 commitbf2c59fce4("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:2655421ae6("lazy tlb: shoot lazies, non-refcounting lazy tlb mm reference handling scheme") Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Herton R. Krzesinski <herton@redhat.com> Suggested-by: Thomas Gleixner <tglx@linutronix.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
24 lines
591 B
C
24 lines
591 B
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_SCHED_HOTPLUG_H
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#define _LINUX_SCHED_HOTPLUG_H
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/*
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* Scheduler interfaces for hotplug CPU support:
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*/
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extern int sched_cpu_starting(unsigned int cpu);
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extern int sched_cpu_activate(unsigned int cpu);
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extern int sched_cpu_deactivate(unsigned int cpu);
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#ifdef CONFIG_HOTPLUG_CPU
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extern int sched_cpu_wait_empty(unsigned int cpu);
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extern int sched_cpu_dying(unsigned int cpu);
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#else
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# define sched_cpu_wait_empty NULL
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# define sched_cpu_dying NULL
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#endif
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static inline void idle_task_exit(void) {}
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#endif /* _LINUX_SCHED_HOTPLUG_H */
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