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

Steven Rostedt
2014-05-06T23:46:32Z

On Tue, 6 May 2014 16:37:14 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> What I have in -next is very different from this version of the patch.
> What's happening?

Hmm, good question.

Let me review this one. The one I sent recently was one I pulled from
my "sent" folder. I thought it was the latest one, but perhaps I sent
another one after that.

I'll go investigate.

-- Steve

>
>
> From: Steven Rostedt <rostedt@goodmis.org>
> Subject: 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>
> ---
>
> kernel/printk/printk.c | 47 ++++++++++++++++++++++++---------------
> 1 file changed, 29 insertions(+), 18 deletions(-)
>
> diff -puN kernel/printk/printk.c~printk-remove-separate-printk_sched-buffers-and-use-printk-buf-instead kernel/printk/printk.c
> --- a/kernel/printk/printk.c~printk-remove-separate-printk_sched-buffers-and-use-printk-buf-instead
> +++ a/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
> 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
> * 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
> 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
> @@ -2531,21 +2550,19 @@ 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)
> @@ -2569,21 +2586,15 @@ 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;
> }
> _