1 files changed, 166 insertions, 15 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index bf2e6741ec12..63416bb2712a 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1548,6 +1548,146 @@ 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.
@@ -1753,12 +1893,19 @@ asmlinkage int vprintk_emit(int facility, int level,
         /* If called from the scheduler, we can not call up(). */
         if (!in_sched) {
                 /*
+                 * Disable preemption to avoid being preempted while holding
+                 * console_sem which would prevent anyone from printing to
+                 * console
+                 */
+                preempt_disable();
+                /*
                  * Try to acquire and then immediately release the console
                  * semaphore.  The release will print out buffers and wake up
                  * /dev/kmsg and syslog() users.
                  */
-                if (console_trylock())
+                if (console_trylock_spinning())
                         console_unlock();
+                preempt_enable();
         }
 
         return printed_len;
@@ -1859,6 +2006,8 @@ 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,
@@ -2084,20 +2233,7 @@ int console_trylock(void)
                 return 0;
         }
         console_locked = 1;
-        /*
-         * When PREEMPT_COUNT disabled we can't reliably detect if it's
-         * safe to schedule (e.g. calling printk while holding a spin_lock),
-         * because preempt_disable()/preempt_enable() are just barriers there
-         * and preempt_count() is always 0.
-         *
-         * RCU read sections have a separate preemption counter when
-         * PREEMPT_RCU enabled thus we must take extra care and check
-         * rcu_preempt_depth(), otherwise RCU read sections modify
-         * preempt_count().
-         */
-        console_may_schedule = !oops_in_progress &&
-                        preemptible() &&
-                        !rcu_preempt_depth();
+        console_may_schedule = 0;
         return 1;
 }
 EXPORT_SYMBOL(console_trylock);
@@ -2247,14 +2383,29 @@ skip:
                 console_seq++;
                 raw_spin_unlock(&logbuf_lock);
 
+                /*
+                 * While actively printing out messages, if another printk()
+                 * were to occur on another CPU, it may wait for this one to
+                 * finish. This task can not be preempted if there is a
+                 * waiter waiting to take over.
+                 */
+                console_lock_spinning_enable();
+
                 stop_critical_timings();        /* don't trace print latency */
                 call_console_drivers(ext_text, ext_len, text, len);
                 start_critical_timings();
+
+                if (console_lock_spinning_disable_and_check()) {
+                        printk_safe_exit_irqrestore(flags);
+                        return;
+                }
+
                 printk_safe_exit_irqrestore(flags);
 
                 if (do_cond_resched)
                         cond_resched();
         }
+
         console_locked = 0;
 
         /* Release the exclusive_console once it is used */