Messages in this thread | | | Date | Wed, 28 Sep 2011 14:44:38 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 1/7] rcu: Fix preempt-unsafe debug check of rcu extended quiescent state |
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On Wed, Sep 28, 2011 at 11:17:40AM +0800, Yong Zhang wrote: > On Tue, Sep 27, 2011 at 11:44:56PM +0200, Frederic Weisbecker wrote: > > On Tue, Sep 27, 2011 at 11:16:02AM -0400, Pavel Ivanov wrote: > > > > What matters is that we do that check by ensuring we are really > > > > checking the value of the cpu var in the CPU we are currently > > > > running and not some other random one that can change its dynticks > > > > value at the same time. > > > > > > Define the "CPU we are currently running on" in this context. Is it > > > CPU executing call to rcu_check_extended_qs() or is it CPU executing > > > return from rcu_check_extended_qs() ? These CPUs can be different both > > > before your patch and after that. And function can return extended_qs > > > state from either of these CPUs again both before and after the patch. > > > If the calling code wants these CPUs to be the same it has to disable > > > preemption before making the call. And if it does so then additional > > > preemption disabling inside the function is pointless. > > > > So, like Paul said rcu_read_lock() doesn't necessary imply to disable > > preemption. > > > > Hence by the time we call rcu_check_extended_qs() the current task > > can be migrated anytime before, while in the function (except > > a little part) or after. > > > > The CPU I was referring to when I talked about "CPU we are currently > > running on" is the CPU we are running between the call to get_cpu_var() > > and put_cpu_var(). This one can not be changed because get_cpu_var() > > disables preemption. > > > > So consider this piece of code: > > > > struct rcu_dynticks *rdtp = > > &get_cpu_var(rcu_dynticks); > > bool ext_qs = true; > > > > if (atomic_read(&rdtp->dynticks) & 0x1) > > ext_qs = false; > > > > put_cpu_var(rcu_dynticks); > > > > What I expect from preemption disabled is that when I read the local > > CPU variable rdtp->dyntick, I'm sure this is the CPU var of the local > > CPU and the rdtp->dyntick from another CPU. > > > > If I don't disable preemption, like it was without my patch: > > > > 0 struct rcu_dynticks *rdtp = > > 1 &__raw_get_cpu_var(rcu_dynticks); > > 2 > > 3 if (atomic_read(&rdtp->dynticks) & 0x1) > > 4 ext_qs = false; > > 5 > > 6 put_cpu_var(rcu_dynticks); > > > > I can fetch rdtp of CPU 1 in line 0. Then the task migrates on CPU 2. > > So on line 3 I'm reading rdtp->dynticks of CPU 1 from CPU 2 and this is > > racy because CPU 1 can change the value of rdtp->dynticks concurrently. > > > > Now indeed it's weird because we can migrate anytime outside that preempt > > disabled section. > > > > So let's explore the two cases where this function can be called: > > > > - From the idle task. For now this is the only place where we can > > run sections of code in RCU extended quiescent state. If any use > > of RCU is made on such section, it will hit our check. > > Here there is no head-scratching about the role of disabling preemption > > because the idle tasks can't be migrated. There is one per cpu so > > the rcu_dynticks variable we look at is always the same inside a > > given idle task. > > Yeah. > > > > > - From a normal task. We assume it can be migrated anytime. But > > normal tasks aren't supposed in RCU extended quiescent state. > > Still the check can be useful there and spot for example cases where > > we exit the idle task without calling rcu_exit_nohz(). > > > > Now from a normal task, when we call rcu_read_lock(), we assume > > we can read the value dynticks from any CPU, wherever we migrate > > to. So for example if we are running idle in CPU 1, then we exit > > idle without calling rcu_exit_nohz(), the next task running on this > > CPU is about to call rcu_read_lock(), but of on the last time before > > we do our check it migrates to CPU 2. It won't detect the issue in CPU 1 > > then. But it doesn't matter much, soon or later there are fair > > chances there will be a call to rcu_read_lock() on CPU 1 that > > will report the issue. > > So the main usage is to detect unbalanced rcu_enter_nohz()/rcu_exit_nohz(), > right?
In fact the primary and main goal is to detect rcu uses between the calls to rcu_enter_nohz() and rcu_exit_nohz(). We found several of these bugs in idle code.
But it might find other things like unbalanced rcu_enter_nohz()/rcu_exit_nohz().
Also rcu_enter_nohz() is used only in idle for now but that may find broader uses in the future. So we want this check everywhere.
> If so, I suggest this should be commented somewhere, like the commit log; > because I was focusing on the idle task before then think it's harmless > with/without this patch :)
You're right, I'll add a comment to explain that.
Thanks.
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