From: Tejun Heo <tj@xxxxxxxxxx>
Subject: printk: do cond_resched() between lines while outputting to consoles
@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.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Calvin Owens <calvinowens@xxxxxx>
Acked-by: Jan Kara <jack@xxxxxxxx>
Cc: Dave Jones <davej@xxxxxxxxxxxxxxxxx>
Cc: Kyle McMartin <kyle@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/console.h | 1 +
kernel/panic.c | 3 +--
kernel/printk/printk.c | 35 ++++++++++++++++++++++++++++++++++-
3 files changed, 36 insertions(+), 3 deletions(-)
diff -puN include/linux/console.h~printk-do-cond_resched-between-lines-while-outputting-to-consoles include/linux/console.h
--- a/include/linux/console.h~printk-do-cond_resched-between-lines-while-outputting-to-consoles
+++ a/include/linux/console.h
@@ -150,6 +150,7 @@ extern int console_trylock(void);
extern void console_unlock(void);
extern void console_conditional_schedule(void);
extern void console_unblank(void);
+extern void console_flush_on_panic(void);
extern struct tty_driver *console_device(int *);
extern void console_stop(struct console *);
extern void console_start(struct console *);
diff -puN kernel/panic.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles kernel/panic.c
--- a/kernel/panic.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles
+++ a/kernel/panic.c
@@ -180,8 +180,7 @@ void panic(const char *fmt, ...)
* panic() is not being callled from OOPS.
*/
debug_locks_off();
- console_trylock();
- console_unlock();
+ console_flush_on_panic();
if (!panic_blink)
panic_blink = no_blink;
diff -puN kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles
+++ a/kernel/printk/printk.c
@@ -2234,13 +2234,24 @@ void console_unlock(void)
static u64 seen_seq;
unsigned long flags;
bool wake_klogd = false;
- bool retry;
+ bool do_cond_resched, retry;
if (console_suspended) {
up_console_sem();
return;
}
+ /*
+ * Console drivers are called under logbuf_lock, so
+ * @console_may_schedule should be cleared before; however, we may
+ * end up dumping a lot of lines, for example, if called from
+ * console registration path, and should invoke cond_resched()
+ * between lines if allowable. Not doing so can cause a very long
+ * scheduling stall on a slow console leading to RCU stall and
+ * softlockup warnings which exacerbate the issue with more
+ * messages practically incapacitating the system.
+ */
+ do_cond_resched = console_may_schedule;
console_may_schedule = 0;
/* flush buffered message fragment immediately to console */
@@ -2312,6 +2323,9 @@ skip:
call_console_drivers(level, ext_text, ext_len, text, len);
start_critical_timings();
local_irq_restore(flags);
+
+ if (do_cond_resched)
+ cond_resched();
}
console_locked = 0;
@@ -2379,6 +2393,25 @@ void console_unblank(void)
console_unlock();
}
+/**
+ * console_flush_on_panic - flush console content on panic
+ *
+ * Immediately output all pending messages no matter what.
+ */
+void console_flush_on_panic(void)
+{
+ /*
+ * If someone else is holding the console lock, trylock will fail
+ * and may_schedule may be set. Ignore and proceed to unlock so
+ * that messages are flushed out. As this can be called from any
+ * context and we don't want to get preempted while flushing,
+ * ensure may_schedule is cleared.
+ */
+ console_trylock();
+ console_may_schedule = 0;
+ console_unlock();
+}
+
/*
* Return the console tty driver structure and its associated index
*/
_
--
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