On Tue 05-02-13 20:05:48, Steven Rostedt wrote:
> [ I sent this in a reply to another thread, but wanted a bit more attention
> to it ]
>
> 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 instead use the printk
> buffer instead, and delay the console_trylock()/console_unlock() to the
> tick.
>
> 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.
>
> Luckily, there's not many places that do the unlock, or hold the
> logbuf_lock. By moving things around a little, the console_sem can be
> released without ever holding the logbuf_lock, and we can safely have
> printk_sched() use the printk buffer directly.
I like this! It looks better than what I did with workqueue. I'll
experiment how things behave if I use PRINTK_PENDING_OUTPUT instead of my
workqueue solution. Thanks for the patch. BTW the patch looks OK to me but
I don't feel like an expert in that code so take that with a grain of salt.
Honza
>
> Signed-off-by: Steven Rostedt <rost...@goodmis.org>
>
> diff --git a/kernel/printk.c b/kernel/printk.c
> index 357f714..ed630fa 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -219,6 +219,9 @@ struct log {
> /*
> * 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);
>
> @@ -1343,30 +1346,46 @@ static inline int can_use_console(unsigned int cpu)
> * 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);
> - raw_spin_unlock(&logbuf_lock);
> return retval;
> }
>
> @@ -1495,11 +1514,17 @@ 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;
> +
> + if (level == -2) {
> + level = -1;
> + in_sched = true;
> + }
>
> boot_delay_msec(level);
> printk_delay();
> @@ -1545,7 +1570,12 @@ 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') {
> @@ -1620,7 +1650,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> * 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();
> @@ -1962,18 +1992,20 @@ int is_console_locked(void)
> #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);
>
> void printk_tick(void)
> {
> if (__this_cpu_read(printk_pending)) {
> int pending = __this_cpu_xchg(printk_pending, 0);
> - if (pending & PRINTK_PENDING_SCHED) {
> - char *buf = __get_cpu_var(printk_sched_buf);
> - printk(KERN_WARNING "[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)
> wake_up_interruptible(&log_wait);
> @@ -2458,20 +2490,14 @@ late_initcall(printk_late_init);
>
> 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);
> - local_irq_restore(flags);
> + __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
>
> return r;
> }
>
>
--
Jan Kara <j...@suse.cz>
SUSE Labs, CR
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