Re: [PATCH 1/2] sched/debug: Don't disable IRQ when acquiring sched_debug_lock

[Daniel Thompson]

On Sat, Mar 27, 2021 at 07:25:28PM -0400, Waiman Long wrote:
> The sched_debug_lock was used only in print_cpu().  The
> print_cpu() function has two callers - sched_debug_show() and
> sysrq_sched_debug_show(). Both of them are invoked by user action
> (sched_debug file and sysrq-t). As print_cpu() won't be called from
> interrupt context at all, there is no point in disabling IRQ when
> acquiring sched_debug_lock.

This looks like it introduces a deadlock risk if sysrq-t triggers from an
interrupt context. Has the behaviour of sysrq changed recently or will
tools like MAGIC_SYSRQ_SERIAL still trigger from interrupt context?


Daniel.


> 
> Besides, if the system has many cpus and the sched_debug_lock is somehow
> busy (e.g. parallel sysrq-t), the system may hit a hard lockup panic, like
> 
> [ 7809.796262] Kernel panic - not syncing: Hard LOCKUP
> [ 7809.796264] CPU: 13 PID: 79867 Comm: reproducer.sh Kdump: loaded Tainted: G          I      --------- -  - 4.18.0-301.el8.x86_64 #1
> [ 7809.796264] Hardware name: Dell Inc. PowerEdge R640/0W23H8, BIOS 1.4.9 06/29/2018
> [ 7809.796265] Call Trace:
> [ 7809.796265]  <NMI>
> [ 7809.796266]  dump_stack+0x5c/0x80
> [ 7809.796266]  panic+0xe7/0x2a9
> [ 7809.796267]  nmi_panic.cold.9+0xc/0xc
> [ 7809.796267]  watchdog_overflow_callback.cold.7+0x5c/0x70
> [ 7809.796268]  __perf_event_overflow+0x52/0xf0
> [ 7809.796268]  handle_pmi_common+0x204/0x2a0
> [ 7809.796269]  ? __set_pte_vaddr+0x32/0x50
> [ 7809.796269]  ? __native_set_fixmap+0x24/0x30
> [ 7809.796270]  ? ghes_copy_tofrom_phys+0xd3/0x1c0
> [ 7809.796271]  intel_pmu_handle_irq+0xbf/0x160
> [ 7809.796271]  perf_event_nmi_handler+0x2d/0x50
> [ 7809.796272]  nmi_handle+0x63/0x110
> [ 7809.796272]  default_do_nmi+0x49/0x100
> [ 7809.796273]  do_nmi+0x17e/0x1e0
> [ 7809.796273]  end_repeat_nmi+0x16/0x6f
> [ 7809.796274] RIP: 0010:native_queued_spin_lock_slowpath+0x5b/0x1d0
> [ 7809.796275] Code: 6d f0 0f ba 2f 08 0f 92 c0 0f b6 c0 c1 e0 08 89 c2 8b 07 30 e4 09 d0 a9 00 01 ff ff 75 47 85 c0 74 0e 8b 07 84 c0 74 08 f3 90 <8b> 07 84 c0 75 f8 b8 01 00 00 00 66 89 07 c3 8b 37 81 fe 00 01 00
> [ 7809.796276] RSP: 0018:ffffaa54cd887df8 EFLAGS: 00000002
> [ 7809.796277] RAX: 0000000000000101 RBX: 0000000000000246 RCX: 0000000000000000
> [ 7809.796278] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff936b66d0
> [ 7809.796278] RBP: ffffffff9301fb40 R08: 0000000000000004 R09: 000000000000004f
> [ 7809.796279] R10: 0000000000000000 R11: ffffaa54cd887cc0 R12: ffff907fd0a29ec0
> [ 7809.796280] R13: 0000000000000000 R14: ffffffff926ab7c0 R15: 0000000000000000
> [ 7809.796280]  ? native_queued_spin_lock_slowpath+0x5b/0x1d0
> [ 7809.796281]  ? native_queued_spin_lock_slowpath+0x5b/0x1d0
> [ 7809.796281]  </NMI>
> [ 7809.796282]  _raw_spin_lock_irqsave+0x32/0x40
> [ 7809.796283]  print_cpu+0x261/0x7c0
> [ 7809.796283]  sysrq_sched_debug_show+0x34/0x50
> [ 7809.796284]  sysrq_handle_showstate+0xc/0x20
> [ 7809.796284]  __handle_sysrq.cold.11+0x48/0xfb
> [ 7809.796285]  write_sysrq_trigger+0x2b/0x30
> [ 7809.796285]  proc_reg_write+0x39/0x60
> [ 7809.796286]  vfs_write+0xa5/0x1a0
> [ 7809.796286]  ksys_write+0x4f/0xb0
> [ 7809.796287]  do_syscall_64+0x5b/0x1a0
> [ 7809.796287]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> [ 7809.796288] RIP: 0033:0x7fabe4ceb648
> 
> It is not to disable IRQ for so long that a hard lockup panic is
> triggered. Fix that by not disabling IRQ when taking sched_debug_lock.
> 
> Fixes: efe25c2c7b3a ("sched: Reinstate group names in /proc/sched_debug")
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/sched/debug.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 486f403a778b..c4ae8a0853a1 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -666,7 +666,6 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
>  static void print_cpu(struct seq_file *m, int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
> -	unsigned long flags;
>  
>  #ifdef CONFIG_X86
>  	{
> @@ -717,13 +716,13 @@ do {									\
>  	}
>  #undef P
>  
> -	spin_lock_irqsave(&sched_debug_lock, flags);
> +	spin_lock(&sched_debug_lock);
>  	print_cfs_stats(m, cpu);
>  	print_rt_stats(m, cpu);
>  	print_dl_stats(m, cpu);
>  
>  	print_rq(m, rq, cpu);
> -	spin_unlock_irqrestore(&sched_debug_lock, flags);
> +	spin_unlock(&sched_debug_lock);
>  	SEQ_printf(m, "\n");
>  }
>  
> -- 
> 2.18.1
>