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From:   Wander Lairson Costa <wander@redhat.com>
Date:   Mon, 8 May 2023 09:30:28 -0300
Message-ID: <CAAq0SUk5kSx0vr+OvxG3sm_XArnFm3GZWz0kec8nigZHLo-rZQ@mail.gmail.com>
Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
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        Namhyung Kim <namhyung@kernel.org>,
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        Adrian Hunter <adrian.hunter@intel.com>,
        Will Deacon <will@kernel.org>,
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On Thu, May 4, 2023 at 5:16=E2=80=AFPM Oleg Nesterov <oleg@redhat.com> wrot=
e:
>
> Hi Wander,
>
> I certainly missed something ;) plus I am already sleeping. but let me tr=
y to
> reply anyway.
>
> On 05/04, Wander Lairson Costa wrote:
> > On Thu, May 4, 2023 at 4:23=E2=80=AFPM Oleg Nesterov <oleg@redhat.com> =
wrote:
> > >
> > > On 05/04, Wander Lairson Costa wrote:
> > > >
> > > > On Thu, May 4, 2023 at 12:23=E2=80=AFPM Oleg Nesterov <oleg@redhat.=
com> wrote:
> > > > >
> > > > > Yes, but as Sebastian explained CONFIG_PROVE_RAW_LOCK_NESTING won=
't like it.
> > > > >
> > > > >         https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@linutronix=
.de/
> > > > >
> > > >
> > > > I think that was my confusion in that thread. My understanding is t=
hat
> > > > CONFIG_PROVE_RAW_LOCK_NESTING will check lock ordering but not
> > > > context.
> > >
> > > Sorry, I don't understand... perhaps I missed something. But iiuc
> > > the problem is simple.
> > >
> > > So, this code
> > >
> > >         raw_spin_lock(one);
> > >         spin_lock(two);
> > >
> > > is obviously wrong if CONFIG_PREEMPT_RT.
> > >
> > > Without PREEMPT_RT this code is fine because raw_spinlock_t and spinl=
ock_t
> > > are the same thing. Except they have different lockdep annotations if
> > > CONFIG_PROVE_RAW_LOCK_NESTING is true, LD_WAIT_SPIN and LD_WAIT_CONFI=
G.
> > >
> > > So if CONFIG_PROVE_RAW_LOCK_NESTING is set, lockdep will complain eve=
n
> > > on the !PREEMPT_RT kernel, iow it checks the nesting as if the code r=
uns
> > > on with PREEMPT_RT.
> > >
> > > Cough... not sure my explanation can help ;) It looks very confusing =
when
> > > I read it.
> > >
> >
> > Thanks for the explanation. That's my understanding too. The part I
> > don't get is why this would fail with a call_rcu() inside
> > put_task_struct().
>
> the problem is that call_rcu() won't be called if !IS_ENABLED(PREEMPT_RT)=
,
> ___put_task_struct() will be called.
>
> CONFIG_PROVE_RAW_LOCK_NESTING can't know this can't happen if PREEMPT_RT
> is set.
>
> IOW. To simplify, suppose we have
>
>         // can be called in atomic context, e.g. under
>         // raw_spin_lock() so it is wrong with PREEMPT_RT
>         void __put_task_struct(struct task_struct *tsk)
>         {
>                 spin_lock(some_lock);
>         }
>
> lets "fix" the code above, lets change __put_task_struct,
>
>         void __put_task_struct(struct task_struct *tsk)
>         {
>                 if (!IS_ENABLED(CONFIG_PREEMPT_RT))
>                         return;
>
>                 spin_lock(some_lock);
>         }
>
> Now, if CONFIG_PREEMPT_RT is true then __put_task_struct() is fine
> wrt lock nesting.
>
> But, if CONFIG_PREEMPT_RT is not set, then __put_task_struct() still
> does the same:
>
>         void __put_task_struct(struct task_struct *tsk)
>         {
>                 spin_lock(some_lock);
>         }
>
> and CONFIG_PROVE_RAW_LOCK_NESTING will complain. Because, once again,
> it checks the nesting as if CONFIG_PREEMPT_RT is true, and in this case
> __put_task_struct() if it is called under raw_spin_lock().
>

IIUC, this is a problem with the current implementation, isn't it?
Because the holder of raw_spin_lock is the put_task_struct() caller.

> Oleg.
>