linux-kernel - Re: [PATCH kernel] srcu: Fix static initialization

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Message-ID: <20200916161224.GA30546@paulmck-ThinkPad-P72>
Date:   Wed, 16 Sep 2020 09:12:24 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     rcu@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel] srcu: Fix static initialization

On Fri, Sep 11, 2020 at 06:52:08AM -0700, Paul E. McKenney wrote:
> On Fri, Sep 11, 2020 at 03:09:41PM +1000, Alexey Kardashevskiy wrote:
> > On 11/09/2020 04:53, Paul E. McKenney wrote:
> > > On Wed, Sep 09, 2020 at 10:31:03PM +1000, Alexey Kardashevskiy wrote:

[ . . . ]

> > >> init_srcu_struct_nodes() assumes ssp->sda!=NULL but alloc_percpu() fails
> > >> here:
> > >>
> > >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/percpu.c#n1734
> > >> ===
> > >> 	} else if (mutex_lock_killable(&pcpu_alloc_mutex)) {
> > >> 			pcpu_memcg_post_alloc_hook(objcg, NULL, 0, size);
> > >> 			return NULL;
> > >> ===
> > >>
> > >> I am still up to reading that osr-rcuusage.pdf to provide better
> > >> analysis :) Thanks,
> > > 
> > > Ah, got it!  Does the following patch help?
> > > 
> > > There will likely also need to be changes to cleanup_srcu_struct(),
> > > but first let's see if I understand the problem.  ;-)
> > > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > index c13348e..6f7880a 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -177,11 +177,13 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> > >  	INIT_DELAYED_WORK(&ssp->work, process_srcu);
> > >  	if (!is_static)
> > >  		ssp->sda = alloc_percpu(struct srcu_data);
> > > +	if (!ssp->sda)
> > > +		return -ENOMEM;
> > >  	init_srcu_struct_nodes(ssp, is_static);
> > >  	ssp->srcu_gp_seq_needed_exp = 0;
> > >  	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
> > >  	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
> > 
> > The line above confuses me a bit. What you propose returns without
> > smp_store_release() called which should not matter I suppose.
> 
> The idea is that if init_srcu_struct() returns -ENOMEM, the structure
> has not been initialized and had better not be used.  If the calling code
> cannot handle that outcome, then the calling code needs to do something
> to insulate init_srcu_struct() from signals.  One thing that it could
> do would be to invoke init_srcu_struct() from a workqueue handler and
> wait for this handler to complete.
> 
> Please keep in mind that there is nothing init_srcu_struct() can do
> about this:  The srcu_struct is useless unless alloc_percpu() succeeds.
> 
> And yes, I do need to update the header comments to make this clear.
> 
> > Otherwise it should work, although I cannot verify right now as my box
> > went down and since it is across Pacific - it may take time to power
> > cycle it :) Thanks,
> 
> I know that feeling!  And here is hoping that the box is out of reach
> of the local hot spots.  ;-)

Just following up...  Did that patch help?

							Thanx, Paul