printk: Hide console waiter logic into helpers

The commit ("printk: Add console owner and waiter logic to load balance
console writes") made vprintk_emit() and console_unlock() even more
complicated.

This patch extracts the new code into 3 helper functions. They should
help to keep it rather self-contained. It will be easier to use and
maintain.

This patch just shuffles the existing code. It does not change
the functionality.

Link: http://lkml.kernel.org/r/20180112160837.GD24497@linux.suse
Cc: akpm@linux-foundation.org
Cc: linux-mm@kvack.org
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: rostedt@home.goodmis.org
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 148 insertions, 97 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 040fb948924e..3a475f58b749 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,15 +86,8 @@ EXPORT_SYMBOL_GPL(console_drivers);
 static struct lockdep_map console_lock_dep_map = {
         .name = "console_lock"
 };
-static struct lockdep_map console_owner_dep_map = {
-        .name = "console_owner"
-};
 #endif
 
-static DEFINE_RAW_SPINLOCK(console_owner_lock);
-static struct task_struct *console_owner;
-static bool console_waiter;
-
 enum devkmsg_log_bits {
         __DEVKMSG_LOG_BIT_ON = 0,
         __DEVKMSG_LOG_BIT_OFF,
@@ -1551,6 +1544,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
 }
 
 /*
+ * Special console_lock variants that help to reduce the risk of soft-lockups.
+ * They allow to pass console_lock to another printk() call using a busy wait.
+ */
+
+#ifdef CONFIG_LOCKDEP
+static struct lockdep_map console_owner_dep_map = {
+        .name = "console_owner"
+};
+#endif
+
+static DEFINE_RAW_SPINLOCK(console_owner_lock);
+static struct task_struct *console_owner;
+static bool console_waiter;
+
+/**
+ * console_lock_spinning_enable - mark beginning of code where another
+ *      thread might safely busy wait
+ *
+ * This basically converts console_lock into a spinlock. This marks
+ * the section where the console_lock owner can not sleep, because
+ * there may be a waiter spinning (like a spinlock). Also it must be
+ * ready to hand over the lock at the end of the section.
+ */
+static void console_lock_spinning_enable(void)
+{
+        raw_spin_lock(&console_owner_lock);
+        console_owner = current;
+        raw_spin_unlock(&console_owner_lock);
+
+        /* The waiter may spin on us after setting console_owner */
+        spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+}
+
+/**
+ * console_lock_spinning_disable_and_check - mark end of code where another
+ *      thread was able to busy wait and check if there is a waiter
+ *
+ * This is called at the end of the section where spinning is allowed.
+ * It has two functions. First, it is a signal that it is no longer
+ * safe to start busy waiting for the lock. Second, it checks if
+ * there is a busy waiter and passes the lock rights to her.
+ *
+ * Important: Callers lose the lock if there was a busy waiter.
+ *      They must not touch items synchronized by console_lock
+ *      in this case.
+ *
+ * Return: 1 if the lock rights were passed, 0 otherwise.
+ */
+static int console_lock_spinning_disable_and_check(void)
+{
+        int waiter;
+
+        raw_spin_lock(&console_owner_lock);
+        waiter = READ_ONCE(console_waiter);
+        console_owner = NULL;
+        raw_spin_unlock(&console_owner_lock);
+
+        if (!waiter) {
+                spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+                return 0;
+        }
+
+        /* The waiter is now free to continue */
+        WRITE_ONCE(console_waiter, false);
+
+        spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+
+        /*
+         * Hand off console_lock to waiter. The waiter will perform
+         * the up(). After this, the waiter is the console_lock owner.
+         */
+        mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
+        return 1;
+}
+
+/**
+ * console_trylock_spinning - try to get console_lock by busy waiting
+ *
+ * This allows to busy wait for the console_lock when the current
+ * owner is running in specially marked sections. It means that
+ * the current owner is running and cannot reschedule until it
+ * is ready to lose the lock.
+ *
+ * Return: 1 if we got the lock, 0 othrewise
+ */
+static int console_trylock_spinning(void)
+{
+        struct task_struct *owner = NULL;
+        bool waiter;
+        bool spin = false;
+        unsigned long flags;
+
+        if (console_trylock())
+                return 1;
+
+        printk_safe_enter_irqsave(flags);
+
+        raw_spin_lock(&console_owner_lock);
+        owner = READ_ONCE(console_owner);
+        waiter = READ_ONCE(console_waiter);
+        if (!waiter && owner && owner != current) {
+                WRITE_ONCE(console_waiter, true);
+                spin = true;
+        }
+        raw_spin_unlock(&console_owner_lock);
+
+        /*
+         * If there is an active printk() writing to the
+         * consoles, instead of having it write our data too,
+         * see if we can offload that load from the active
+         * printer, and do some printing ourselves.
+         * Go into a spin only if there isn't already a waiter
+         * spinning, and there is an active printer, and
+         * that active printer isn't us (recursive printk?).
+         */
+        if (!spin) {
+                printk_safe_exit_irqrestore(flags);
+                return 0;
+        }
+
+        /* We spin waiting for the owner to release us */
+        spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+        /* Owner will clear console_waiter on hand off */
+        while (READ_ONCE(console_waiter))
+                cpu_relax();
+        spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+
+        printk_safe_exit_irqrestore(flags);
+        /*
+         * The owner passed the console lock to us.
+         * Since we did not spin on console lock, annotate
+         * this as a trylock. Otherwise lockdep will
+         * complain.
+         */
+        mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
+
+        return 1;
+}
+
+/*
  * Call the console drivers, asking them to write out
  * log_buf[start] to log_buf[end - 1].
  * The console_lock must be held.
@@ -1760,56 +1893,8 @@ asmlinkage int vprintk_emit(int facility, int level,
                  * semaphore.  The release will print out buffers and wake up
                  * /dev/kmsg and syslog() users.
                  */
-                if (console_trylock()) {
+                if (console_trylock_spinning())
                         console_unlock();
-                } else {
-                        struct task_struct *owner = NULL;
-                        bool waiter;
-                        bool spin = false;
-
-                        printk_safe_enter_irqsave(flags);
-
-                        raw_spin_lock(&console_owner_lock);
-                        owner = READ_ONCE(console_owner);
-                        waiter = READ_ONCE(console_waiter);
-                        if (!waiter && owner && owner != current) {
-                                WRITE_ONCE(console_waiter, true);
-                                spin = true;
-                        }
-                        raw_spin_unlock(&console_owner_lock);
-
-                        /*
-                         * If there is an active printk() writing to the
-                         * consoles, instead of having it write our data too,
-                         * see if we can offload that load from the active
-                         * printer, and do some printing ourselves.
-                         * Go into a spin only if there isn't already a waiter
-                         * spinning, and there is an active printer, and
-                         * that active printer isn't us (recursive printk?).
-                         */
-                        if (spin) {
-                                /* We spin waiting for the owner to release us */
-                                spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
-                                /* Owner will clear console_waiter on hand off */
-                                while (READ_ONCE(console_waiter))
-                                        cpu_relax();
-
-                                spin_release(&console_owner_dep_map, 1, _THIS_IP_);
-                                printk_safe_exit_irqrestore(flags);
-
-                                /*
-                                 * The owner passed the console lock to us.
-                                 * Since we did not spin on console lock, annotate
-                                 * this as a trylock. Otherwise lockdep will
-                                 * complain.
-                                 */
-                                mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
-                                console_unlock();
-                                printk_safe_enter_irqsave(flags);
-                        }
-                        printk_safe_exit_irqrestore(flags);
-
-                }
         }
 
         return printed_len;
@@ -1910,6 +1995,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
 static ssize_t msg_print_ext_body(char *buf, size_t size,
                                   char *dict, size_t dict_len,
                                   char *text, size_t text_len) { return 0; }
+static void console_lock_spinning_enable(void) { }
+static int console_lock_spinning_disable_and_check(void) { return 0; }
 static void call_console_drivers(const char *ext_text, size_t ext_len,
                                  const char *text, size_t len) {}
 static size_t msg_print_text(const struct printk_log *msg,
@@ -2196,7 +2283,6 @@ void console_unlock(void)
         static u64 seen_seq;
         unsigned long flags;
         bool wake_klogd = false;
-        bool waiter = false;
         bool do_cond_resched, retry;
 
         if (console_suspended) {
@@ -2291,31 +2377,16 @@ skip:
                  * finish. This task can not be preempted if there is a
                  * waiter waiting to take over.
                  */
-                raw_spin_lock(&console_owner_lock);
-                console_owner = current;
-                raw_spin_unlock(&console_owner_lock);
-
-                /* The waiter may spin on us after setting console_owner */
-                spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+                console_lock_spinning_enable();
 
                 stop_critical_timings();        /* don't trace print latency */
                 call_console_drivers(ext_text, ext_len, text, len);
                 start_critical_timings();
 
-                raw_spin_lock(&console_owner_lock);
-                waiter = READ_ONCE(console_waiter);
-                console_owner = NULL;
-                raw_spin_unlock(&console_owner_lock);
-
-                /*
-                 * If there is a waiter waiting for us, then pass the
-                 * rest of the work load over to that waiter.
-                 */
-                if (waiter)
-                        break;
-
-                /* There was no waiter, and nothing will spin on us here */
-                spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+                if (console_lock_spinning_disable_and_check()) {
+                        printk_safe_exit_irqrestore(flags);
+                        return;
+                }
 
                 printk_safe_exit_irqrestore(flags);
 
@@ -2323,26 +2394,6 @@ skip:
                         cond_resched();
         }
 
-        /*
-         * If there is an active waiter waiting on the console_lock.
-         * Pass off the printing to the waiter, and the waiter
-         * will continue printing on its CPU, and when all writing
-         * has finished, the last printer will wake up klogd.
-         */
-        if (waiter) {
-                WRITE_ONCE(console_waiter, false);
-                /* The waiter is now free to continue */
-                spin_release(&console_owner_dep_map, 1, _THIS_IP_);
-                /*
-                 * Hand off console_lock to waiter. The waiter will perform
-                 * the up(). After this, the waiter is the console_lock owner.
-                 */
-                mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
-                printk_safe_exit_irqrestore(flags);
-                /* Note, if waiter is set, logbuf_lock is not held */
-                return;
-        }
-
         console_locked = 0;
 
         /* Release the exclusive_console once it is used */