printk: remove separate printk_sched buffers and use printk buf instead

To prevent deadlocks with doing a printk inside the scheduler,
printk_sched() was created.  The issue is that printk has a console_sem
that it can grab and release.  The release does a wake up if there's a
task pending on the sem, and this wake up grabs the rq locks that is
held in the scheduler.  This leads to a possible deadlock if the wake up
uses the same rq as the one with the rq lock held already.

What printk_sched() does is to save the printk write in a per cpu buffer
and sets the PRINTK_PENDING_SCHED flag.  On a timer tick, if this flag is
set, the printk() is done against the buffer.

There's a couple of issues with this approach.

1) If two printk_sched()s are called before the tick, the second one
   will overwrite the first one.

2) The temporary buffer is 512 bytes and is per cpu.  This is a quite a
   bit of space wasted for something that is seldom used.

In order to remove this, the printk_sched() can use the printk buffer
instead, and delay the console_trylock()/console_unlock() to the queued
work.

Because printk_sched() would then be taking the logbuf_lock, the
logbuf_lock must not be held while doing anything that may call into the
scheduler functions, which includes wake ups.  Unfortunately, printk()
also has a console_sem that it uses, and on release, the up(&console_sem)
may do a wake up of any pending waiters.  This must be avoided while
holding the logbuf_lock.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 29 insertions, 18 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 4e22230f1f6c..247b0c1fadfc 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -68,6 +68,9 @@ int console_printk[4] = {
         DEFAULT_CONSOLE_LOGLEVEL,       /* default_console_loglevel */
 };
 
+/* Deferred messaged from sched code are marked by this special level */
+#define SCHED_MESSAGE_LOGLEVEL -2
+
 /*
  * Low level drivers may need that to know if they can schedule in
  * their unblank() callback or not. So let's export it.
@@ -229,7 +232,9 @@ struct printk_log {
 };
 
 /*
- * The logbuf_lock protects kmsg buffer, indices, counters.
+ * The logbuf_lock protects kmsg buffer, indices, counters.  This can be taken
+ * within the scheduler's rq lock. It must be released before calling
+ * console_unlock() or anything else that might wake up a process.
  */
 static DEFINE_RAW_SPINLOCK(logbuf_lock);
 
@@ -1577,14 +1582,19 @@ asmlinkage int vprintk_emit(int facility, int level,
         static int recursion_bug;
         static char textbuf[LOG_LINE_MAX];
         char *text = textbuf;
-        size_t text_len;
+        size_t text_len = 0;
         enum log_flags lflags = 0;
         unsigned long flags;
         int this_cpu;
         int printed_len = 0;
+        bool in_sched = false;
         /* cpu currently holding logbuf_lock in this function */
         static volatile unsigned int logbuf_cpu = UINT_MAX;
 
+        if (level == SCHED_MESSAGE_LOGLEVEL) {
+                level = -1;
+                in_sched = true;
+        }
 
         boot_delay_msec(level);
         printk_delay();
@@ -1631,7 +1641,12 @@ asmlinkage int vprintk_emit(int facility, int level,
          * The printf needs to come first; we need the syslog
          * prefix which might be passed-in as a parameter.
          */
-        text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
+        if (in_sched)
+                text_len = scnprintf(text, sizeof(textbuf),
+                                     KERN_WARNING "[sched_delayed] ");
+
+        text_len += vscnprintf(text + text_len,
+                               sizeof(textbuf) - text_len, fmt, args);
 
         /* mark and strip a trailing newline */
         if (text_len && text[text_len-1] == '\n') {
@@ -1713,6 +1728,10 @@ asmlinkage int vprintk_emit(int facility, int level,
         lockdep_on();
         local_irq_restore(flags);
 
+        /* If called from the scheduler, we can not call up(). */
+        if (in_sched)
+                return printed_len;
+
         /*
          * Disable preemption to avoid being preempted while holding
          * console_sem which would prevent anyone from printing to console
@@ -2532,21 +2551,19 @@ late_initcall(printk_late_init);
 /*
  * Delayed printk version, for scheduler-internal messages:
  */
-#define PRINTK_BUF_SIZE         512
-
 #define PRINTK_PENDING_WAKEUP   0x01
-#define PRINTK_PENDING_SCHED    0x02
+#define PRINTK_PENDING_OUTPUT   0x02
 
 static DEFINE_PER_CPU(int, printk_pending);
-static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
 
 static void wake_up_klogd_work_func(struct irq_work *irq_work)
 {
         int pending = __this_cpu_xchg(printk_pending, 0);
 
-        if (pending & PRINTK_PENDING_SCHED) {
-                char *buf = __get_cpu_var(printk_sched_buf);
-                pr_warn("[sched_delayed] %s", buf);
+        if (pending & PRINTK_PENDING_OUTPUT) {
+                /* If trylock fails, someone else is doing the printing */
+                if (console_trylock())
+                        console_unlock();
         }
 
         if (pending & PRINTK_PENDING_WAKEUP)
@@ -2570,21 +2587,15 @@ void wake_up_klogd(void)
 
 int printk_sched(const char *fmt, ...)
 {
-        unsigned long flags;
         va_list args;
-        char *buf;
         int r;
 
-        local_irq_save(flags);
-        buf = __get_cpu_var(printk_sched_buf);
-
         va_start(args, fmt);
-        r = vsnprintf(buf, PRINTK_BUF_SIZE, fmt, args);
+        r = vprintk_emit(0, SCHED_MESSAGE_LOGLEVEL, NULL, 0, fmt, args);
         va_end(args);
 
-        __this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
+        __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
         irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
-        local_irq_restore(flags);
 
         return r;
 }