linux-kernel - Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator

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Message-ID: <20200715190243.GA26735@pc636>
Date:   Wed, 15 Jul 2020 21:02:43 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, "Paul E . McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page
 allocator

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