On Sun, Nov 03, 2019 at 01:01:21PM +0800, Lai Jiangshan wrote: > > > 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. This one in -rcu, you mean? 5c5d9065e4eb ("rcu: Clear ->rcu_read_unlock_special only once") Some adjustment was needed due to my not applying the earlier patches that assumed nested interrupts. Please let me know if further adjustments are needed. Thanx, Paul > > 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 > > >