On (01/13/17 14:15), Petr Mladek wrote:
> Some console drivers code calls console_conditional_schedule()
> that looks at @console_may_schedule. The value must be cleared
> when the drivers are called from console_unlock() with
> interrupts disabled. But rescheduling is fine when the same
> code is called, for example, from tty operations where the
> console semaphore is taken via console_lock().
>
> This is why @console_may_schedule is cleared before calling console
> drivers. The original value is stored to decide if we could sleep
> between lines.
>
> Now, @console_may_schedule is not cleared when we call
> console_trylock() and jump back to the "again" goto label.
> This has become a problem, since the commit 6b97a20d3a7909daa066
> ("printk: set may_schedule for some of console_trylock() callers").
so I think I'd prefer to revert that commit.
the reason I added the commit in question was to reduce the number of
printk() soft lockups that I observed back then. however, it obviously
didn't solve all of the printk() problems. now printk() is moving in a
completely different direction in term of lockups and deadlocks. there
will be no console_trylock() call in vprintk_emit() at all. we will
either do console_lock() from scheduleable printk_kthread or
console_trylock() from IRQ work. so 6b97a20d3a7909daa066 didn't buy us
a lot, and it still doesn't (+ it introduced a bug).
apart from that, Tetsuo wasn't really happy with the patch
http://www.spinics.net/lists/linux-mm/msg103099.html
so let's just return the old behavior back.
---
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7180088cbb23..ddfbd47398f8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2078,20 +2078,7 @@ int console_trylock(void)
return 0;
}
console_locked = 1;
- /*
- * When PREEMPT_COUNT disabled 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 there
- * and preempt_count() is always 0.
- *
- * RCU read sections have a separate preemption counter when
- * PREEMPT_RCU enabled thus we must take extra care and check
- * rcu_preempt_depth(), otherwise RCU read sections modify
- * preempt_count().
- */
- console_may_schedule = !oops_in_progress &&
- preemptible() &&
- !rcu_preempt_depth();
+ console_may_schedule = 0;
return 1;
}
EXPORT_SYMBOL(console_trylock);
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