On 2019/11/3 10:01 上午, Boqun Feng wrote:
Hi Jiangshan,
I haven't checked the correctness of this patch carefully, but..
On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote:
Don't need to set ->rcu_read_lock_nesting negative, irq-protected
rcu_preempt_deferred_qs_irqrestore() doesn't expect
->rcu_read_lock_nesting to be negative to work, it even
doesn't access to ->rcu_read_lock_nesting any more.
rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
eventually call swake_up() or its friends to wake up, say, the gp
kthread, and the wake up functions could go into the scheduler code
path which might have RCU read-side critical section in it, IOW,
accessing ->rcu_read_lock_nesting.
Sure, thank you for pointing it out.
I should rewrite the changelog in next round. Like this:
rcu: cleanup rcu_preempt_deferred_qs()
IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't
expect ->rcu_read_lock_nesting to be negative to work.
There might be RCU read-side critical section in it (from wakeup()
or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
will ensure that ->rcu_read_unlock_special is zero and these RCU
read-side critical sections will not call rcu_read_unlock_special().
Thanks
Lai
===
PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
not applied earlier, it will be protected by previous patch (patch1)
in this series
"rcu: use preempt_count to test whether scheduler locks is held"
when rcu_read_unlock_special() is called.
Again, haven't checked closely, but this argument in the commit log
seems untrue.
Regards,
Boqun
It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
may access to ->rcu_read_lock_nesting, but it is still safe
since rcu_read_unlock_special() can protect itself from NMI.
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree_plugin.h | 5 -----
1 file changed, 5 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index aba5896d67e3..2fab8be2061f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
static void rcu_preempt_deferred_qs(struct task_struct *t)
{
unsigned long flags;
- bool couldrecurse = t->rcu_read_lock_nesting >= 0;
if (!rcu_preempt_need_deferred_qs(t))
return;
- if (couldrecurse)
- t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
local_irq_save(flags);
rcu_preempt_deferred_qs_irqrestore(t, flags);
- if (couldrecurse)
- t->rcu_read_lock_nesting += RCU_NEST_BIAS;
}
/*
--
2.20.1
