printk: add functions to prefer direct printing

Once kthread printing is available, console printing will no longer
occur in the context of the printk caller. However, there are some
special contexts where it is desirable for the printk caller to
directly print out kernel messages. Using pr_flush() to wait for
threaded printers is only possible if the caller is in a sleepable
context and the kthreads are active. That is not always the case.

Introduce printk_prefer_direct_enter() and printk_prefer_direct_exit()
functions to explicitly (and globally) activate/deactivate preferred
direct console printing. The term "direct console printing" refers to
printing to all enabled consoles from the context of the printk
caller. The term "prefer" is used because this type of printing is
only best effort. If the console is currently locked or other
printers are already actively printing, the printk caller will need
to rely on the other contexts to handle the printing.

This preferred direct printing is how all printing has been handled
until now (unless it was explicitly deferred).

When kthread printing is introduced, there may be some unanticipated
problems due to kthreads being unable to flush important messages.
In order to minimize such risks, preferred direct printing is
activated for the primary important messages when the system
experiences general types of major errors. These are:

 - emergency reboot/shutdown
 - cpu and rcu stalls
 - hard and soft lockups
 - hung tasks
 - warn
 - sysrq

Note that since kthread printing does not yet exist, no behavior
changes result from this commit. This is only implementing the
counter and marking the various places where preferred direct
printing is active.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org> # for RCU
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220421212250.565456-13-john.ogness@linutronix.de

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index bbfd004449b5b81cb6ec1bad174415f5b12410a8..2884cd638d64b6466583cf7a6b14de011b540442 100644 (file)
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -578,6 +578,7 @@ void __handle_sysrq(int key, bool check_mask)
 
        rcu_sysrq_start();
        rcu_read_lock();
+       printk_prefer_direct_enter();
        /*
         * Raise the apparent loglevel to maximum so that the sysrq header
         * is shown to provide the user with positive feedback.  We do not
@@ -619,6 +620,7 @@ void __handle_sysrq(int key, bool check_mask)
                pr_cont("\n");
                console_loglevel = orig_log_level;
        }
+       printk_prefer_direct_exit();
        rcu_read_unlock();
        rcu_sysrq_end();
 

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 091fba7283e142af2c740e6b8cf36f080491dd9d..cd26aab0ab2a094d7fbb276cfa95f8486e9363b5 100644 (file)
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -170,6 +170,9 @@ extern void __printk_safe_exit(void);
 #define printk_deferred_enter __printk_safe_enter
 #define printk_deferred_exit __printk_safe_exit
 
+extern void printk_prefer_direct_enter(void);
+extern void printk_prefer_direct_exit(void);
+
 extern bool pr_flush(int timeout_ms, bool reset_on_progress);
 
 /*
@@ -222,6 +225,14 @@ static inline void printk_deferred_exit(void)
 {
 }
 
+static inline void printk_prefer_direct_enter(void)
+{
+}
+
+static inline void printk_prefer_direct_exit(void)
+{
+}
+
 static inline bool pr_flush(int timeout_ms, bool reset_on_progress)
 {
        return true;

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 52501e5f765547e0fa83824c5777ad7d45cce077..02a65d55434092581ae852d19abdc58fb57f9fb3 100644 (file)
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -127,6 +127,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
         * complain:
         */
        if (sysctl_hung_task_warnings) {
+               printk_prefer_direct_enter();
+
                if (sysctl_hung_task_warnings > 0)
                        sysctl_hung_task_warnings--;
                pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
@@ -142,6 +144,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 
                if (sysctl_hung_task_all_cpu_backtrace)
                        hung_task_show_all_bt = true;
+
+               printk_prefer_direct_exit();
        }
 
        touch_nmi_watchdog();
@@ -204,12 +208,17 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
        }
  unlock:
        rcu_read_unlock();
-       if (hung_task_show_lock)
+       if (hung_task_show_lock) {
+               printk_prefer_direct_enter();
                debug_show_all_locks();
+               printk_prefer_direct_exit();
+       }
 
        if (hung_task_show_all_bt) {
                hung_task_show_all_bt = false;
+               printk_prefer_direct_enter();
                trigger_all_cpu_backtrace();
+               printk_prefer_direct_exit();
        }
 
        if (hung_task_call_panic)

diff --git a/kernel/panic.c b/kernel/panic.c
index 55b50e052ec3a335900afb75b81d60ed564f8303..7d422597403f00d25f2a1ee7a952ab4265b3ead2 100644 (file)
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -560,6 +560,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 {
        disable_trace_on_warning();
 
+       printk_prefer_direct_enter();
+
        if (file)
                pr_warn("WARNING: CPU: %d PID: %d at %s:%d %pS\n",
                        raw_smp_processor_id(), current->pid, file, line,
@@ -597,6 +599,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 
        /* Just a warning, don't kill lockdep. */
        add_taint(taint, LOCKDEP_STILL_OK);
+
+       printk_prefer_direct_exit();
 }
 
 #ifndef __WARN_FLAGS

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index a06999d552785aae329916f6deaf3ffc33ae9886..ed7f738261cc7e7e8a5ff41c6a1fa04f13ec6a31 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -362,6 +362,34 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
 static DEFINE_MUTEX(syslog_lock);
 
 #ifdef CONFIG_PRINTK
+static atomic_t printk_prefer_direct = ATOMIC_INIT(0);
+
+/**
+ * printk_prefer_direct_enter - cause printk() calls to attempt direct
+ *                              printing to all enabled consoles
+ *
+ * Since it is not possible to call into the console printing code from any
+ * context, there is no guarantee that direct printing will occur.
+ *
+ * This globally effects all printk() callers.
+ *
+ * Context: Any context.
+ */
+void printk_prefer_direct_enter(void)
+{
+       atomic_inc(&printk_prefer_direct);
+}
+
+/**
+ * printk_prefer_direct_exit - restore printk() behavior
+ *
+ * Context: Any context.
+ */
+void printk_prefer_direct_exit(void)
+{
+       WARN_ON(atomic_dec_if_positive(&printk_prefer_direct) < 0);
+}
+
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* All 3 protected by @syslog_lock. */
 /* the next printk record to read by syslog(READ) or /proc/kmsg */

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 0c5d8516516af5780c266c87b4705b84739eeb16..d612707c2ed017efadf68af6891bbb021340cbec 100644 (file)
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -619,6 +619,7 @@ static void print_cpu_stall(unsigned long gps)
         * See Documentation/RCU/stallwarn.rst for info on how to debug
         * RCU CPU stall warnings.
         */
+       printk_prefer_direct_enter();
        trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
        pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
        raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
@@ -656,6 +657,7 @@ static void print_cpu_stall(unsigned long gps)
         */
        set_tsk_need_resched(current);
        set_preempt_need_resched();
+       printk_prefer_direct_exit();
 }
 
 static void check_cpu_stall(struct rcu_data *rdp)

diff --git a/kernel/reboot.c b/kernel/reboot.c
index 6bcc5d6a6572b7decf4b731d9f130374555bf7d5..4177645e74d6af001695d4443fc5d288e67a7ca3 100644 (file)
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -447,9 +447,11 @@ static int __orderly_reboot(void)
        ret = run_cmd(reboot_cmd);
 
        if (ret) {
+               printk_prefer_direct_enter();
                pr_warn("Failed to start orderly reboot: forcing the issue\n");
                emergency_sync();
                kernel_restart(NULL);
+               printk_prefer_direct_exit();
        }
 
        return ret;
@@ -462,6 +464,7 @@ static int __orderly_poweroff(bool force)
        ret = run_cmd(poweroff_cmd);
 
        if (ret && force) {
+               printk_prefer_direct_enter();
                pr_warn("Failed to start orderly shutdown: forcing the issue\n");
 
                /*
@@ -471,6 +474,7 @@ static int __orderly_poweroff(bool force)
                 */
                emergency_sync();
                kernel_power_off();
+               printk_prefer_direct_exit();
        }
 
        return ret;
@@ -528,6 +532,8 @@ EXPORT_SYMBOL_GPL(orderly_reboot);
  */
 static void hw_failure_emergency_poweroff_func(struct work_struct *work)
 {
+       printk_prefer_direct_enter();
+
        /*
         * We have reached here after the emergency shutdown waiting period has
         * expired. This means orderly_poweroff has not been able to shut off
@@ -544,6 +550,8 @@ static void hw_failure_emergency_poweroff_func(struct work_struct *work)
         */
        pr_emerg("Hardware protection shutdown failed. Trying emergency restart\n");
        emergency_restart();
+
+       printk_prefer_direct_exit();
 }
 
 static DECLARE_DELAYED_WORK(hw_failure_emergency_poweroff_work,
@@ -582,11 +590,13 @@ void hw_protection_shutdown(const char *reason, int ms_until_forced)
 {
        static atomic_t allow_proceed = ATOMIC_INIT(1);
 
+       printk_prefer_direct_enter();
+
        pr_emerg("HARDWARE PROTECTION shutdown (%s)\n", reason);
 
        /* Shutdown should be initiated only once. */
        if (!atomic_dec_and_test(&allow_proceed))
-               return;
+               goto out;
 
        /*
         * Queue a backup emergency shutdown in the event of
@@ -594,6 +604,8 @@ void hw_protection_shutdown(const char *reason, int ms_until_forced)
         */
        hw_failure_emergency_poweroff(ms_until_forced);
        orderly_poweroff(true);
+out:
+       printk_prefer_direct_exit();
 }
 EXPORT_SYMBOL_GPL(hw_protection_shutdown);
 

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 9166220457bcbbf12852a591d617b442da3b2c37..40024e03d42203833f7bfebfd9c754c26e77a255 100644 (file)
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -424,6 +424,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
                /* Start period for the next softlockup warning. */
                update_report_ts();
 
+               printk_prefer_direct_enter();
+
                pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
                        smp_processor_id(), duration,
                        current->comm, task_pid_nr(current));
@@ -442,6 +444,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
                add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
                if (softlockup_panic)
                        panic("softlockup: hung tasks");
+
+               printk_prefer_direct_exit();
        }
 
        return HRTIMER_RESTART;

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index 247bf0b1582ca1cf352f006aa1fd5f689f8f5859..701f35f0e2d4499325202c6c7b7b592e590fc330 100644 (file)
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -135,6 +135,8 @@ static void watchdog_overflow_callback(struct perf_event *event,
                if (__this_cpu_read(hard_watchdog_warn) == true)
                        return;
 
+               printk_prefer_direct_enter();
+
                pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n",
                         this_cpu);
                print_modules();
@@ -155,6 +157,8 @@ static void watchdog_overflow_callback(struct perf_event *event,
                if (hardlockup_panic)
                        nmi_panic(regs, "Hard LOCKUP");
 
+               printk_prefer_direct_exit();
+
                __this_cpu_write(hard_watchdog_warn, true);
                return;
        }