Re: [PATCH 1/3] printk/nbcon: Block printk kthreads when any CPU is in an emergency context

[kernel test robot <lkp@intel.com>]

Hi Petr,

kernel test robot noticed the following build warnings:

[auto build test WARNING on soc/for-next]
[also build test WARNING on linus/master v6.17 next-20250929]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Petr-Mladek/printk-nbcon-Block-printk-kthreads-when-any-CPU-is-in-an-emergency-context/20250926-205414
base:   https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc.git for-next
patch link:    https://lore.kernel.org/r/20250926124912.243464-2-pmladek%40suse.com
patch subject: [PATCH 1/3] printk/nbcon: Block printk kthreads when any CPU is in an emergency context
config: arc-randconfig-r131-20251001 (https://download.01.org/0day-ci/archive/20251001/202510010320.jV84a9vM-lkp@intel.com/config)
compiler: arc-linux-gcc (GCC) 8.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251001/202510010320.jV84a9vM-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202510010320.jV84a9vM-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
>> kernel/printk/nbcon.c:121:10: sparse: sparse: symbol 'nbcon_cpu_emergency_cnt' was not declared. Should it be static?

vim +/nbcon_cpu_emergency_cnt +121 kernel/printk/nbcon.c

     4	
     5	#include <linux/atomic.h>
     6	#include <linux/bug.h>
     7	#include <linux/console.h>
     8	#include <linux/delay.h>
     9	#include <linux/errno.h>
    10	#include <linux/export.h>
    11	#include <linux/init.h>
    12	#include <linux/irqflags.h>
    13	#include <linux/kthread.h>
    14	#include <linux/minmax.h>
    15	#include <linux/percpu.h>
    16	#include <linux/preempt.h>
    17	#include <linux/slab.h>
    18	#include <linux/smp.h>
    19	#include <linux/stddef.h>
    20	#include <linux/string.h>
    21	#include <linux/types.h>
    22	#include "internal.h"
    23	#include "printk_ringbuffer.h"
    24	/*
    25	 * Printk console printing implementation for consoles which does not depend
    26	 * on the legacy style console_lock mechanism.
    27	 *
    28	 * The state of the console is maintained in the "nbcon_state" atomic
    29	 * variable.
    30	 *
    31	 * The console is locked when:
    32	 *
    33	 *   - The 'prio' field contains the priority of the context that owns the
    34	 *     console. Only higher priority contexts are allowed to take over the
    35	 *     lock. A value of 0 (NBCON_PRIO_NONE) means the console is not locked.
    36	 *
    37	 *   - The 'cpu' field denotes on which CPU the console is locked. It is used
    38	 *     to prevent busy waiting on the same CPU. Also it informs the lock owner
    39	 *     that it has lost the lock in a more complex scenario when the lock was
    40	 *     taken over by a higher priority context, released, and taken on another
    41	 *     CPU with the same priority as the interrupted owner.
    42	 *
    43	 * The acquire mechanism uses a few more fields:
    44	 *
    45	 *   - The 'req_prio' field is used by the handover approach to make the
    46	 *     current owner aware that there is a context with a higher priority
    47	 *     waiting for the friendly handover.
    48	 *
    49	 *   - The 'unsafe' field allows to take over the console in a safe way in the
    50	 *     middle of emitting a message. The field is set only when accessing some
    51	 *     shared resources or when the console device is manipulated. It can be
    52	 *     cleared, for example, after emitting one character when the console
    53	 *     device is in a consistent state.
    54	 *
    55	 *   - The 'unsafe_takeover' field is set when a hostile takeover took the
    56	 *     console in an unsafe state. The console will stay in the unsafe state
    57	 *     until re-initialized.
    58	 *
    59	 * The acquire mechanism uses three approaches:
    60	 *
    61	 *   1) Direct acquire when the console is not owned or is owned by a lower
    62	 *      priority context and is in a safe state.
    63	 *
    64	 *   2) Friendly handover mechanism uses a request/grant handshake. It is used
    65	 *      when the current owner has lower priority and the console is in an
    66	 *      unsafe state.
    67	 *
    68	 *      The requesting context:
    69	 *
    70	 *        a) Sets its priority into the 'req_prio' field.
    71	 *
    72	 *        b) Waits (with a timeout) for the owning context to unlock the
    73	 *           console.
    74	 *
    75	 *        c) Takes the lock and clears the 'req_prio' field.
    76	 *
    77	 *      The owning context:
    78	 *
    79	 *        a) Observes the 'req_prio' field set on exit from the unsafe
    80	 *           console state.
    81	 *
    82	 *        b) Gives up console ownership by clearing the 'prio' field.
    83	 *
    84	 *   3) Unsafe hostile takeover allows to take over the lock even when the
    85	 *      console is an unsafe state. It is used only in panic() by the final
    86	 *      attempt to flush consoles in a try and hope mode.
    87	 *
    88	 *      Note that separate record buffers are used in panic(). As a result,
    89	 *      the messages can be read and formatted without any risk even after
    90	 *      using the hostile takeover in unsafe state.
    91	 *
    92	 * The release function simply clears the 'prio' field.
    93	 *
    94	 * All operations on @console::nbcon_state are atomic cmpxchg based to
    95	 * handle concurrency.
    96	 *
    97	 * The acquire/release functions implement only minimal policies:
    98	 *
    99	 *   - Preference for higher priority contexts.
   100	 *   - Protection of the panic CPU.
   101	 *
   102	 * All other policy decisions must be made at the call sites:
   103	 *
   104	 *   - What is marked as an unsafe section.
   105	 *   - Whether to spin-wait if there is already an owner and the console is
   106	 *     in an unsafe state.
   107	 *   - Whether to attempt an unsafe hostile takeover.
   108	 *
   109	 * The design allows to implement the well known:
   110	 *
   111	 *     acquire()
   112	 *     output_one_printk_record()
   113	 *     release()
   114	 *
   115	 * The output of one printk record might be interrupted with a higher priority
   116	 * context. The new owner is supposed to reprint the entire interrupted record
   117	 * from scratch.
   118	 */
   119	
   120	/* Counter of active nbcon emergency contexts. */
 > 121	atomic_t nbcon_cpu_emergency_cnt;
   122	

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