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

Petr Mládek
2014-05-07T09:14:06Z

On Mon 2014-05-05 19:18:46, Steven Rostedt wrote:
> 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();

I wonder if we should call here console_trylock_for_printk() which checks
whether the console is really usable.

The check for usable console was introduced in the commit
76a8ad293912cd2f (Make printk work for really early debugging).
I think that this IRQ work could get called during early boot,
so the check would make sense here as well. Or have I missed something?

Best Regards,
Petr

> + 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;
> }