Messages in this thread | | | Subject | Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs() | From | Lai Jiangshan <> | Date | Sun, 3 Nov 2019 13:01:21 +0800 |
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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@linux.alibaba.com> >> --- >> 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 >>
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