On Mon 2021-03-01 14:20:32, gregkh@xxxxxxxxxxxxxxxxxxx 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@xxxxxxxxxxxxxxx>.
Please, find below the backport for stable-4.9:
>From bfc246e5ed94300431bc8d74b67c74b6f66e75ff Mon Sep 17 00:00:00 2001
From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lore.kernel.org/r/20210210034823.64867-1-songmuchun@xxxxxxxxxxxxx
---
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();
}
--
2.16.4
