| Message ID | 20210210034823.64867-1-songmuchun@bytedance.com |
|---|---|
| State | Accepted |
| Commit | 8a8109f303e25a27f92c1d8edd67d7cbbc60a4eb |
| Headers | show |
| Series | [v3] printk: fix deadlock when kernel panic | expand |
On (21/02/10 11:48), Muchun Song wrote: > 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: cf9b1106c81c ("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> -ss
On Wed 2021-02-10 11:48:23, Muchun Song wrote: > 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: cf9b1106c81c ("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> The patch is committed on printk/linux.git, branch for-5.12. Best Regards, Petr
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index a0e6f746de6c..2e9e3ed7d63e 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -45,6 +45,8 @@ struct printk_safe_seq_buf { static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq); static DEFINE_PER_CPU(int, printk_context); +static DEFINE_RAW_SPINLOCK(safe_read_lock); + #ifdef CONFIG_PRINTK_NMI static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq); #endif @@ -180,8 +182,6 @@ static void report_message_lost(struct printk_safe_seq_buf *s) */ static void __printk_safe_flush(struct irq_work *work) { - static raw_spinlock_t read_lock = - __RAW_SPIN_LOCK_INITIALIZER(read_lock); struct printk_safe_seq_buf *s = container_of(work, struct printk_safe_seq_buf, work); unsigned long flags; @@ -195,7 +195,7 @@ static void __printk_safe_flush(struct irq_work *work) * different CPUs. This is especially important when printing * a backtrace. */ - raw_spin_lock_irqsave(&read_lock, flags); + raw_spin_lock_irqsave(&safe_read_lock, flags); i = 0; more: @@ -232,7 +232,7 @@ static void __printk_safe_flush(struct irq_work *work) out: report_message_lost(s); - raw_spin_unlock_irqrestore(&read_lock, flags); + raw_spin_unlock_irqrestore(&safe_read_lock, flags); } /** @@ -278,6 +278,14 @@ void printk_safe_flush_on_panic(void) raw_spin_lock_init(&logbuf_lock); } + if (raw_spin_is_locked(&safe_read_lock)) { + if (num_online_cpus() > 1) + return; + + debug_locks_off(); + raw_spin_lock_init(&safe_read_lock); + } + printk_safe_flush(); }
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: cf9b1106c81c ("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> --- kernel/printk/printk_safe.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-)