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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Wed, 15 Jul 2020 21:02:43 +0200
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, "Paul E . McKenney" <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Matthew Wilcox <willy@infradead.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page
 allocator
Message-ID: <20200715190243.GA26735@pc636>
References: <20200715183537.4010-1-urezki@gmail.com>
 <20200715185628.7b4k3o5efp4gnbla@linutronix.de>
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On Wed, Jul 15, 2020 at 08:56:28PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-07-15 20:35:37 [+0200], Uladzislau Rezki (Sony) wrote:
> > @@ -3306,6 +3307,9 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> >  			if (IS_ENABLED(CONFIG_PREEMPT_RT))
> >  				return false;
> >  
> > +			preempt_disable();
> > +			krc_this_cpu_unlock(*krcp, *flags);
> 
> Now you enter memory allocator with disabled preemption. This isn't any
> better but we don't have a warning for this yet.
>
Please elaborate why it is not allowed? Consider the code:

<snip>
    spin_lock();
    __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
    spin_unlock();
<snip>

Also, please note we do it for regular kernel.

>
> What happened to the part where I asked for a spinlock_t?
> 
What do you mean?

--
Vlad Rezki