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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Sun, 1 Mar 2020 13:07:02 +0100
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Suraj Jitindar Singh <surajjs@amazon.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] ext4: fix potential race between online resizing and
 write operations
Message-ID: <20200301120702.GA9762@pc636>
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> > > So in CONFIG_PREEMPT kernel we can identify if we are in atomic or not by
> > > using rcu_preempt_depth() and in_atomic(). When it comes to !CONFIG_PREEMPT
> > > then we skip it and consider as atomic. Something like:
> > > 
> > > <snip>
> > > static bool is_current_in_atomic()
> > 
> > Would be good to change this to is_current_in_rcu_reader() since
> > rcu_preempt_depth() does not imply atomicity.
> >
> can_current_synchronize_rcu()? If can we just call:
> 
> <snip>
>     synchronize_rcu() or synchronize_rcu_expedited();
>     kvfree();
> <snip>
> 
> > > {
> > > #ifdef CONFIG_PREEMPT_RCU
> > >     if (!rcu_preempt_depth() && !in_atomic())
> > >         return false;
> > 
> > I think use if (!rcu_preempt_depth() && preemptible()) here.
> > 
> > preemptible() checks for IRQ disabled section as well.
> > 
> Yes but in_atomic() does it as well, it also checks other atomic
> contexts like softirq handlers and NMI ones. So calling there
> synchronize_rcu() is not allowed.
> 
Ahh. Right you are. We also have to check if irqs are disabled
or not. preemptible() has to be added as well.

<snip>
can_current_synchronize_rcu()
{
    if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
        if (!rcu_preempt_depth() && !in_atomic() && preemptible()) {
            might_sleep();
            return true;
	}
    }

    return false;
}
<snip>

if we can synchronize:
    - we can directly inline kvfree() to current context;
    - we can attached the head using GFP_KERNEL | __GFP_RETRY_MAYFAIL.
    
Otherwise attached the rcu_head under atomic or as we are in RCU reader section.

Thoughts?

--
Vlad Rezki