Re: [PATCH v3] printk: fix deadlock when kernel panic
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(_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(_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
> Reviewed-by: Petr Mladek
> Cc:
The patch is committed on printk/linux.git, branch for-5.12.
Best Regards,
Petr
Re: [PATCH v3] printk: fix deadlock when kernel panic
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(_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(_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
> Reviewed-by: Petr Mladek
> Cc:
Acked-by: Sergey Senozhatsky
-ss
[PATCH v3] printk: fix deadlock when kernel panic
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(_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(_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
Reviewed-by: Petr Mladek
Cc:
---
kernel/printk/printk_safe.c | 16
1 file changed, 12 insertions(+), 4 deletions(-)
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(_lock, flags);
+ raw_spin_lock_irqsave(_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(_lock, flags);
+ raw_spin_unlock_irqrestore(_read_lock, flags);
}
/**
@@ -278,6 +278,14 @@ void printk_safe_flush_on_panic(void)
raw_spin_lock_init(_lock);
}
+ if (raw_spin_is_locked(_read_lock)) {
+ if (num_online_cpus() > 1)
+ return;
+
+ debug_locks_off();
+ raw_spin_lock_init(_read_lock);
+ }
+
printk_safe_flush();
}
--
2.11.0
