f1a0a376ca
As pointed out by commit
de9b8f5dcb ("sched: Fix crash trying to dequeue/enqueue the idle thread")
init_idle() can and will be invoked more than once on the same idle
task. At boot time, it is invoked for the boot CPU thread by
sched_init(). Then smp_init() creates the threads for all the secondary
CPUs and invokes init_idle() on them.
As the hotplug machinery brings the secondaries to life, it will issue
calls to idle_thread_get(), which itself invokes init_idle() yet again.
In this case it's invoked twice more per secondary: at _cpu_up(), and at
bringup_cpu().
Given smp_init() already initializes the idle tasks for all *possible*
CPUs, no further initialization should be required. Now, removing
init_idle() from idle_thread_get() exposes some interesting expectations
with regards to the idle task's preempt_count: the secondary startup always
issues a preempt_disable(), requiring some reset of the preempt count to 0
between hot-unplug and hotplug, which is currently served by
idle_thread_get() -> idle_init().
Given the idle task is supposed to have preemption disabled once and never
see it re-enabled, it seems that what we actually want is to initialize its
preempt_count to PREEMPT_DISABLED and leave it there. Do that, and remove
init_idle() from idle_thread_get().
Secondary startups were patched via coccinelle:
@begone@
@@
-preempt_disable();
...
cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210512094636.2958515-1-valentin.schneider@arm.com
90 lines
2.3 KiB
C
90 lines
2.3 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __ASM_PREEMPT_H
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#define __ASM_PREEMPT_H
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#include <linux/thread_info.h>
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#define PREEMPT_NEED_RESCHED BIT(32)
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#define PREEMPT_ENABLED (PREEMPT_NEED_RESCHED)
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static inline int preempt_count(void)
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{
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return READ_ONCE(current_thread_info()->preempt.count);
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}
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static inline void preempt_count_set(u64 pc)
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{
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/* Preserve existing value of PREEMPT_NEED_RESCHED */
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WRITE_ONCE(current_thread_info()->preempt.count, pc);
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}
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#define init_task_preempt_count(p) do { \
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task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \
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} while (0)
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#define init_idle_preempt_count(p, cpu) do { \
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task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
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} while (0)
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static inline void set_preempt_need_resched(void)
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{
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current_thread_info()->preempt.need_resched = 0;
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}
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static inline void clear_preempt_need_resched(void)
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{
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current_thread_info()->preempt.need_resched = 1;
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}
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static inline bool test_preempt_need_resched(void)
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{
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return !current_thread_info()->preempt.need_resched;
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}
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static inline void __preempt_count_add(int val)
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{
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u32 pc = READ_ONCE(current_thread_info()->preempt.count);
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pc += val;
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WRITE_ONCE(current_thread_info()->preempt.count, pc);
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}
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static inline void __preempt_count_sub(int val)
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{
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u32 pc = READ_ONCE(current_thread_info()->preempt.count);
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pc -= val;
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WRITE_ONCE(current_thread_info()->preempt.count, pc);
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}
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static inline bool __preempt_count_dec_and_test(void)
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{
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struct thread_info *ti = current_thread_info();
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u64 pc = READ_ONCE(ti->preempt_count);
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/* Update only the count field, leaving need_resched unchanged */
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WRITE_ONCE(ti->preempt.count, --pc);
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/*
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* If we wrote back all zeroes, then we're preemptible and in
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* need of a reschedule. Otherwise, we need to reload the
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* preempt_count in case the need_resched flag was cleared by an
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* interrupt occurring between the non-atomic READ_ONCE/WRITE_ONCE
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* pair.
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*/
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return !pc || !READ_ONCE(ti->preempt_count);
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}
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static inline bool should_resched(int preempt_offset)
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{
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u64 pc = READ_ONCE(current_thread_info()->preempt_count);
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return pc == preempt_offset;
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}
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#ifdef CONFIG_PREEMPTION
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void preempt_schedule(void);
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#define __preempt_schedule() preempt_schedule()
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void preempt_schedule_notrace(void);
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#define __preempt_schedule_notrace() preempt_schedule_notrace()
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#endif /* CONFIG_PREEMPTION */
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#endif /* __ASM_PREEMPT_H */
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