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Subject[PATCH] printk: clear console_may_schedule on panic flushing
Commit "printk: do cond_resched() between lines while outputting to
consoles" made console flushing perform cond_resched() after each line
if the context allows as determined by whether the console lock was
acquired with console_lock(). The condition is carried in
console_may_schedule.

During panic, console lock status is ignored and the messages are
forcifully flushed which is implemented by performing
console_trylock(); console_unlock(); sequence ignoring whether trylock
succeeds or fails. This means that the emergency flushing, after
trylock failure, may enter flushing path with console_may_schedule set
from the actual holder.

As a system may panic from any context, this can lead to
cond_resched() being invoked from a non-sleepable context triggering
an extra warning dump while panicking which is noisy and can be
confusing. Besides, even when panicking from a sleepable context, we
don't want to be yielding during emergency message dumping.

Currently, the emergency dumping is opencoded in panic(). This patch
replaces the open coded implementation with a new function,
console_flush_on_panic() and makes it explicitly clear
console_may_schedule before starting the emergency flushing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kernel test robot <ying.huang@linux.intel.com>
Link: http://lkml.kernel.org/g/878u3w95ke.fsf@yhuang-dev.intel.com
---
include/linux/console.h | 1 +
kernel/panic.c | 3 +--
kernel/printk/printk.c | 19 +++++++++++++++++++
3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index bd19434..ea731af 100644
--- a/include/linux/console.h
+++ b/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 --git a/kernel/panic.c b/kernel/panic.c
index b333380..d96469d 100644
--- a/kernel/panic.c
+++ b/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 --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d0b8697..7ebcfea 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2386,6 +2386,25 @@ 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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