[PATCH] printk: Remove separate printk_sched buffers and use printk buf instead

Steven Rostedt
2014-05-05T23:18:52Z

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>
---
This version has been forward ported to the 3.15-rc releases.
---
kernel/printk/printk.c | 87 +++++++++++++++++++++++++++++++------------------
1 file changed, 56 insertions(+), 31 deletions(-)

Index: linux-trace.git/kernel/printk/printk.c
===================================================================
--- linux-trace.git.orig/kernel/printk/printk.c 2014-05-05 16:46:17.280815365 -0400
+++ linux-trace.git/kernel/printk/printk.c 2014-05-05 16:52:23.398378897 -0400
@@ -208,6 +208,9 @@
/*
* The logbuf_lock protects kmsg buffer, indices, counters. It is also
* used in interesting ways to provide interlocking in console_unlock();
+ * 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);

@@ -1338,27 +1341,43 @@
* interrupts disabled. It should return with 'lockbuf_lock'
* released but interrupts still disabled.
*/
-static int console_trylock_for_printk(unsigned int cpu)
+static int console_trylock_for_printk(unsigned int cpu, bool in_sched)
__releases(&logbuf_lock)
{
int retval = 0, wake = 0;

- if (console_trylock()) {
- retval = 1;
+ /* if called from the scheduler, we can not call up() */
+ if (in_sched)
+ goto out;

- /*
- * If we can't use the console, we need to release
- * the console semaphore by hand to avoid flushing
- * the buffer. We need to hold the console semaphore
- * in order to do this test safely.
- */
- if (!can_use_console(cpu)) {
- console_locked = 0;
- wake = 1;
- retval = 0;
- }
+ if (down_trylock(&console_sem))
+ goto out;
+
+ /*
+ * If we can't use the console, we need to release
+ * the console semaphore by hand to avoid flushing
+ * the buffer. We need to hold the console semaphore
+ * in order to do this test safely.
+ */
+ if (console_suspended || !can_use_console(cpu)) {
+ wake = 1;
+ goto out;
}
+
+ /* console is now locked */
+
+ console_locked = 1;
+ console_may_schedule = 0;
+ mutex_acquire(&console_lock_dep_map, 0, 1, _RET_IP_);
+
+ retval = 1;
+
+out:
logbuf_cpu = UINT_MAX;
+ /*
+ * The logbuf_lock must not be held when doing a wake up,
+ * which the up(&console_sem) can do.
+ */
raw_spin_unlock(&logbuf_lock);
if (wake)
up(&console_sem);
@@ -1490,11 +1509,17 @@
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;
+
+ if (level == -2) {
+ level = -1;
+ in_sched = true;
+ }

boot_delay_msec(level);
printk_delay();
@@ -1540,7 +1565,12 @@
* 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') {
@@ -1621,7 +1651,7 @@
* The console_trylock_for_printk() function will release 'logbuf_lock'
* regardless of whether it actually gets the console semaphore or not.
*/
- if (console_trylock_for_printk(this_cpu))
+ if (console_trylock_for_printk(this_cpu, in_sched))
console_unlock();

lockdep_on();
@@ -2440,18 +2470,20 @@
#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 (console_trylock())
+ console_unlock();
+ else
+ /* Try again later */
+ __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
}

if (pending & PRINTK_PENDING_WAKEUP)
@@ -2475,21 +2507,14 @@

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, -2, NULL, 0, fmt, args);
va_end(args);

- __this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
- irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
- local_irq_restore(flags);
+ __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);

return r;
}