Commit 458df9fd authored by Steven Rostedt's avatar Steven Rostedt Committed by Linus Torvalds
Browse files

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

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: default avatarSteven Rostedt <>
Signed-off-by: default avatarJan Kara <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 939f04be
......@@ -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 */
* 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;
level = -1;
in_sched = true;
......@@ -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,
/* 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
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())
......@@ -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;
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);
__this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
return r;
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment