linux-kernel - Re: [PATCH percpu/for-4.7-fixes 1/2] percpu: fix synchronization between chunk->map_extend

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Message-ID: <20160526192154.GC23194@mtj.duckdns.org>
Date:	Thu, 26 May 2016 15:21:54 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Alexei Starovoitov <ast@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Linux-MM layout <linux-mm@...ck.org>,
	Marco Grassi <marco.gra@...il.com>
Subject: Re: [PATCH percpu/for-4.7-fixes 1/2] percpu: fix synchronization
 between chunk->map_extend_work and chunk destruction

Hello,

On Thu, May 26, 2016 at 11:19:06AM +0200, Vlastimil Babka wrote:
> >  	if (is_atomic) {
> >  		margin = 3;
> > 
> >  		if (chunk->map_alloc <
> > -		    chunk->map_used + PCPU_ATOMIC_MAP_MARGIN_LOW &&
> > -		    pcpu_async_enabled)
> > -			schedule_work(&chunk->map_extend_work);
> > +		    chunk->map_used + PCPU_ATOMIC_MAP_MARGIN_LOW) {
> > +			if (list_empty(&chunk->map_extend_list)) {

> So why this list_empty condition? Doesn't it deserve a comment then? And

Because doing list_add() twice corrupts the list.  I'm not sure that
deserves a comment.  We can do list_move() instead but that isn't
necessarily better.

> isn't using a list an overkill in that case?

That would require rebalance work to scan all chunks whenever it's
scheduled and if a lot of atomic allocations are taking place, it has
some possibility to become expensive with a lot of chunks.

Thanks.

-- 
tejun