console_unlock() allows to cond_resched() if its caller has
set `console_may_schedule' to 1, since
'commit 8d91f8b15361 ("printk: do cond_resched() between lines while
outputting to consoles")'.
The rules are:
-- console_lock() always sets `console_may_schedule' to 1
-- console_trylock() always sets `console_may_schedule' to 0
However, console_trylock() callers (among them is printk()) do
not always call printk() from atomic contexts, and some of them
can cond_resched() in console_unlock(), so console_trylock()
can set `console_may_schedule' to 1 for such processes.
For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock()
always sets `console_may_schedule' to 0.
It's possible to drop explicit preempt_disable()/preempt_enable()
in vprintk_emit(), because console_unlock() and console_trylock()
are now smart enough:
a) console_unlock() does not cond_resched() when it's unsafe
(console_trylock() takes care of that)
b) console_unlock() does can_use_console() check.
Signed-off-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com>
---
kernel/printk/printk.c | 19 ++++++++++---------
1 file changed, 10 insertions(+), 9 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 99925ce..097ca8b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1769,20 +1769,12 @@ asmlinkage int vprintk_emit(int facility, int level,
if (!in_sched) {
lockdep_off();
/*
- * Disable preemption to avoid being preempted while holding
- * console_sem which would prevent anyone from printing to
- * console
- */
- preempt_disable();
-
- /*
* Try to acquire and then immediately release the console
* semaphore. The release will print out buffers and wake up
* /dev/kmsg and syslog() users.
*/
if (console_trylock())
console_unlock();
- preempt_enable();
lockdep_on();
}
@@ -2115,7 +2107,16 @@ int console_trylock(void)
return 0;
}
console_locked = 1;
- console_may_schedule = 0;
+ /*
+ * On !PREEMPT_COUNT kernels we can't reliably detect if it's safe
+ * to schedule -- e.g. calling printk while holding a spin_lock,
+ * because preempt_disable()/preempt_enable() are just barriers and
+ * don't modify preempt_count() there. console_may_schedule is
+ * always 0 on !PREEMPT_COUNT kernels.
+ */
+ console_may_schedule = !oops_in_progress &&
+ preemptible() &&
+ !rcu_preempt_depth();
return 1;
}
EXPORT_SYMBOL(console_trylock);
--
2.7.0
