On Mon 2021-03-01 14:20:32, gregkh@linuxfoundation.org wrote:

The patch below does not apply to the 4.9-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

Please, find below the backport for stable-4.9:

From bfc246e5ed94300431bc8d74b67c74b6f66e75ff Mon Sep 17 00:00:00 2001

From: Muchun Song songmuchun@bytedance.com Date: Wed, 10 Feb 2021 11:48:23 +0800 Subject: [PATCH] printk: fix deadlock when kernel panic

printk_nmi_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_nmi_flush __printk_nmi_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_nmi_flush_on_panic printk_nmi_flush __printk_nmi_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_nmi_flush().

Handle the lock the same way as logbuf_lock. The printk_nmi 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_nmi 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_nmi buffers have been obsoleted by the lockless ring buffer.

This is a backport of the commit 8a8109f303e25a27f92c ("printk: fix deadlock when kernel panic") for the stable 4.9 kernel.

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 Signed-off-by: Petr Mladek pmladek@suse.com Link: https://lore.kernel.org/r/20210210034823.64867-1-songmuchun@bytedance.com --- kernel/printk/nmi.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c index 2c3e7f024c15..7a50b405ad28 100644 --- a/kernel/printk/nmi.c +++ b/kernel/printk/nmi.c @@ -52,6 +52,8 @@ struct nmi_seq_buf { }; static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);

+static DEFINE_RAW_SPINLOCK(nmi_read_lock); + /* * Safe printk() for NMI context. It uses a per-CPU buffer to * store the message. NMIs are not nested, so there is always only @@ -134,8 +136,6 @@ static void printk_nmi_flush_seq_line(struct nmi_seq_buf *s, */ static void __printk_nmi_flush(struct irq_work *work) { - static raw_spinlock_t read_lock = - __RAW_SPIN_LOCK_INITIALIZER(read_lock); struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work); unsigned long flags; size_t len, size; @@ -148,7 +148,7 @@ static void __printk_nmi_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(&nmi_read_lock, flags);

i = 0; more: @@ -197,7 +197,7 @@ static void __printk_nmi_flush(struct irq_work *work) goto more;

out: - raw_spin_unlock_irqrestore(&read_lock, flags); + raw_spin_unlock_irqrestore(&nmi_read_lock, flags); }

/** @@ -239,6 +239,14 @@ void printk_nmi_flush_on_panic(void) raw_spin_lock_init(&logbuf_lock); }

+ if (in_nmi() && raw_spin_is_locked(&nmi_read_lock)) { + if (num_online_cpus() > 1) + return; + + debug_locks_off(); + raw_spin_lock_init(&nmi_read_lock); + } + printk_nmi_flush(); }