On (12/03/15 10:11), Sergey Senozhatsky wrote:
> On (12/02/15 15:57), akpm@xxxxxxxxxxxxxxxxxxxx wrote:
> [..]
> > @console_may_schedule tracks whether console_sem was acquired through lock
> > or trylock. If the former, we're inside a sleepable context and
> > console_conditional_schedule() performs cond_resched(). This allows
> > console drivers which use console_lock for synchronization to yield while
> > performing time-consuming operations such as scrolling.
> >
> > However, the actual console outputting is performed while holding irq-safe
> > logbuf_lock, so console_unlock() clears @console_may_schedule before
> > starting outputting lines. Also, only a few drivers call
> > console_conditional_schedule() to begin with. This means that when a lot
> > of lines need to be output by console_unlock(), for example on a console
> > registration, the task doing console_unlock() may not yield for a long
> > time on a non-preemptible kernel.
> >
> > If this happens with a slow console devices, for example a serial console,
> > the outputting task may occupy the cpu for a very long time. Long enough
> > to trigger softlockup and/or RCU stall warnings, which in turn pile more
> > messages, sometimes enough to trigger the next cycle of warnings
> > incapacitating the system.
> >
> > Fix it by making console_unlock() insert cond_resched() between lines if
> > @console_may_schedule.
>
> CPU2 still can cause lots of troubles. consider
>
> CPU0 CPU1 CPU2
> printk
> ... printk_deferred
> printk wake_up_klogd
> wake_up_klogd_work_func
> console_trylock
> console_unlock
>
> printk_deferred() may be issued by scheduler, for example.
IOW, may be we can start limiting the number of bytes printed in console_unlock()
from irq contexts. Which is quite ugly, yes. We basically don't know how much time
we spend in call_console_drivers(); some of the consoles can do 'internal' spin_lock
loops in ->write() handlers, etc. So something like this (below) probably will not
really help, but still it's not always OK to do `while (1)' loop in console_unlock()
for irqs.
-ss
(not even compile tested)
---
kernel/printk/printk.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9da39e7..221a230 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2235,6 +2235,7 @@ void console_unlock(void)
unsigned long flags;
bool wake_klogd = false;
bool do_cond_resched, retry;
+ int printed, irq_count = irq_count();
if (console_suspended) {
up_console_sem();
@@ -2257,6 +2258,7 @@ void console_unlock(void)
/* flush buffered message fragment immediately to console */
console_cont_flush(text, sizeof(text));
again:
+ printed = 0;
for (;;) {
struct printk_log *msg;
size_t ext_len = 0;
@@ -2326,6 +2328,8 @@ skip:
if (do_cond_resched)
cond_resched();
+ if (irq_count && printed > LOG_LINE_MAX)
+ break;
}
console_locked = 0;
@@ -2344,7 +2348,7 @@ skip:
* flush, no worries.
*/
raw_spin_lock(&logbuf_lock);
- retry = console_seq != log_next_seq;
+ retry = (console_seq != log_next_seq) && !!irq_count;
raw_spin_unlock_irqrestore(&logbuf_lock, flags);
if (retry && console_trylock())
--
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