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printk: fix deadlock when kernel panic
commit8a8109f303
upstream. printk_safe_flush_on_panic() caused the following deadlock on our server: CPU0: CPU1: panic rcu_dump_cpu_stacks kdump_nmi_shootdown_cpus nmi_trigger_cpumask_backtrace register_nmi_handler(crash_nmi_callback) printk_safe_flush __printk_safe_flush raw_spin_lock_irqsave(&read_lock) // send NMI to other processors apic_send_IPI_allbutself(NMI_VECTOR) // NMI interrupt, dead loop crash_nmi_callback printk_safe_flush_on_panic printk_safe_flush __printk_safe_flush // deadlock raw_spin_lock_irqsave(&read_lock) DEADLOCK: read_lock is taken on CPU1 and will never get released. It happens when panic() stops a CPU by NMI while it has been in the middle of printk_safe_flush(). Handle the lock the same way as logbuf_lock. The printk_safe buffers are flushed only when both locks can be safely taken. It can avoid the deadlock _in this particular case_ at expense of losing contents of printk_safe buffers. Note: It would actually be safe to re-init the locks when all CPUs were stopped by NMI. But it would require passing this information from arch-specific code. It is not worth the complexity. Especially because logbuf_lock and printk_safe buffers have been obsoleted by the lockless ring buffer. Fixes:cf9b1106c8
("printk/nmi: flush NMI messages on the system panic") Signed-off-by: Muchun Song <songmuchun@bytedance.com> Reviewed-by: Petr Mladek <pmladek@suse.com> Cc: <stable@vger.kernel.org> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com> Link: https://lore.kernel.org/r/20210210034823.64867-1-songmuchun@bytedance.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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parent
8aad9180a0
commit
ba7ae3629d
1 changed files with 12 additions and 4 deletions
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@ -55,6 +55,8 @@ struct printk_safe_seq_buf {
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static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
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static DEFINE_PER_CPU(int, printk_context);
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static DEFINE_RAW_SPINLOCK(safe_read_lock);
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#ifdef CONFIG_PRINTK_NMI
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static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
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#endif
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@ -190,8 +192,6 @@ static void report_message_lost(struct printk_safe_seq_buf *s)
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*/
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static void __printk_safe_flush(struct irq_work *work)
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{
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static raw_spinlock_t read_lock =
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__RAW_SPIN_LOCK_INITIALIZER(read_lock);
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struct printk_safe_seq_buf *s =
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container_of(work, struct printk_safe_seq_buf, work);
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unsigned long flags;
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@ -205,7 +205,7 @@ static void __printk_safe_flush(struct irq_work *work)
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* different CPUs. This is especially important when printing
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* a backtrace.
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*/
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raw_spin_lock_irqsave(&read_lock, flags);
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raw_spin_lock_irqsave(&safe_read_lock, flags);
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i = 0;
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more:
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@ -242,7 +242,7 @@ static void __printk_safe_flush(struct irq_work *work)
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out:
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report_message_lost(s);
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raw_spin_unlock_irqrestore(&read_lock, flags);
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raw_spin_unlock_irqrestore(&safe_read_lock, flags);
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}
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/**
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@ -288,6 +288,14 @@ void printk_safe_flush_on_panic(void)
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raw_spin_lock_init(&logbuf_lock);
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}
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if (raw_spin_is_locked(&safe_read_lock)) {
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if (num_online_cpus() > 1)
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return;
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debug_locks_off();
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raw_spin_lock_init(&safe_read_lock);
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}
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printk_safe_flush();
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}
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