[linux-next, x86_64] no backtrace after "printk/nmi: generic solution for safe printk in NMI"

[linux-next, x86_64] no backtrace after "printk/nmi: generic solution for safe printk in NMI"

[Sergey Senozhatsky]

Hello Petr,

seem that commit b927968830676373caf4241e80d8b447133f84b2
	Author: Petr Mladek <pmladek@suse.com>
	Date:   Thu Feb 25 13:00:35 2016 +1100

	    printk/nmi: generic solution for safe printk in NMI
    
	    printk() takes some locks and could not be used a safe way in NMI context.
    
	    The chance of a deadlock is real especially when printing stacks from all
	    CPUs.  This particular problem has been addressed on x86 by the commit
	    a9edc8809328 ("x86/nmi: Perform a safe NMI stack trace on all CPUs").
    
	    The patchset brings two big advantages.  First, it makes the NMI
	    backtraces safe on all architectures for free.  Second, it makes all NMI
	    messages almost safe on all architectures (the temporary buffer is
	    limited.  We still should keep the number of messages in NMI context at
	    minimum).
	[..]


makes my x86_64 boxen unhappy, I see no CPU backtraces and no panic messages
on HARDLOCKUPs (CONFIG_BOOTPARAM_HARDLOCKUP_PANIC_VALUE=1).

does it work for you?

	-ss

Re: [linux-next, x86_64] no backtrace after "printk/nmi: generic solution for safe printk in NMI"

[Sergey Senozhatsky]

On (02/25/16 18:46), Sergey Senozhatsky wrote:
> Hello Petr,
> 
> seem that commit b927968830676373caf4241e80d8b447133f84b2
> 	Author: Petr Mladek <pmladek@suse.com>
> 	Date:   Thu Feb 25 13:00:35 2016 +1100
> 
> 	    printk/nmi: generic solution for safe printk in NMI
>     
> 	    printk() takes some locks and could not be used a safe way in NMI context.
>     
> 	    The chance of a deadlock is real especially when printing stacks from all
> 	    CPUs.  This particular problem has been addressed on x86 by the commit
> 	    a9edc8809328 ("x86/nmi: Perform a safe NMI stack trace on all CPUs").
>     
> 	    The patchset brings two big advantages.  First, it makes the NMI
> 	    backtraces safe on all architectures for free.  Second, it makes all NMI
> 	    messages almost safe on all architectures (the temporary buffer is
> 	    limited.  We still should keep the number of messages in NMI context at
> 	    minimum).
> 	[..]
> 
> 
> makes my x86_64 boxen unhappy, I see no CPU backtraces and no panic messages
> on HARDLOCKUPs (CONFIG_BOOTPARAM_HARDLOCKUP_PANIC_VALUE=1).
> 
> does it work for you?

just for note,

I reverted (next-20160225) the following commits:

bdcdb76696402e1a0821db6bcccb2b4de4049f27
7ad17383fa428d1c89d2be95d00806f33df69dfd
054a26c149a5add6d27d154f2025ee2605d9f25d
c2dbc8fd415835a71b0dee0f3f3a353d116f8731
3fd57f62ac2a8de6b402bf5f6aeb3a7d1e3f11a3
3ff0033c1e878b8bd8fce6efcd1b432b24f1d321

-- still no HARDLOCKUP panic backtraces on a console.

reverting of

b927968830676373caf4241e80d8b447133f84b2  (plus MIPS config conflict resolution)

-- fixed the problem for me. I've tested several times.


the sample code that I use for testing is very simple (well.. but it
does the job):

---

       u64 s, e;
       unsigned long flags;

       local_irq_save(flags);
       s = local_clock() >> 31UL;
       pr_err(">>>>>>>>>>>>>>>>>START_THE_TEST\n");
       while (1) {
               e = local_clock() >> 31UL;
               if (e - s > %%YOUR_WATCHDOG_THRESHOLD%%)
                       goto out;
       }
out:
       pr_err(">>>>>>>>>>>>>>>>>END_THE_TEST\n");
       local_irq_restore(flags);

---



most likely I'll be able to reply only tomorrow (in case if there will be any questions).


	-ss

Re: [linux-next, x86_64] no backtrace after "printk/nmi: generic solution for safe printk in NMI"

[Sergey Senozhatsky]

On (02/25/16 19:31), Sergey Senozhatsky wrote:
[..]
> most likely I'll be able to reply only tomorrow (in case if there will be any questions).


solely for the record

lkml.kernel.org/r/1456457840-1059-1-git-send-email-sergey.senozhatsky@gmail.com

	-ss