Re: [PATCH 2/2] mm: slince possible data races about pgdat->kswapd

[Kefeng Wang]

On 2022/8/24 16:24, David Hildenbrand wrote:
> On 24.08.22 09:19, Kefeng Wang wrote:
> > The pgdat->kswapd could be accessed concurrently by kswapd_run() and
> > kcompactd(), it don't be protected by any lock, which could leads to
> > data races, adding READ/WRITE_ONCE() to slince it.
> Okay, I think this patch here makes it clearer that we really just want
> proper synchronization instead of hacking around it.
>
> What speaks against protecting pgdat->kswapd this using some proper
> locking primitive?

So add a new lock into struct pglist_data to protect pgdat->kswapd，other

option, thanks.

> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > ---
> >   mm/compaction.c | 4 +++-
> >   mm/vmscan.c     | 8 ++++----
> >   2 files changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 640fa76228dd..aa1cfe47f046 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1983,7 +1983,9 @@ static inline bool is_via_compact_memory(int order)
> >   
> >   static bool kswapd_is_running(pg_data_t *pgdat)
> >   {
> > -	return pgdat->kswapd && task_is_running(pgdat->kswapd);
> > +	struct task_struct *t = READ_ONCE(pgdat->kswapd);
> > +
> > +	return t && task_is_running(t);
> >   }
> >   
> >   /*
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 08c6497f76c3..65b19ca8c8ee 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4644,7 +4644,7 @@ void kswapd_run(int nid)
> >   	pg_data_t *pgdat = NODE_DATA(nid);
> >   	struct task_struct *t;
> >   
> > -	if (pgdat->kswapd)
> > +	if (READ_ONCE(pgdat->kswapd))
> >   		return;
> >   
> >   	t = kthread_run(kswapd, pgdat, "kswapd%d", nid);
> > @@ -4653,7 +4653,7 @@ void kswapd_run(int nid)
> >   		BUG_ON(system_state < SYSTEM_RUNNING);
> >   		pr_err("Failed to start kswapd on node %d\n", nid);
> >   	} else {
> > -		pgdat->kswapd = t;
> > +		WRITE_ONCE(pgdat->kswapd, t);
> >   	}
> >   }
> >   
> > @@ -4663,11 +4663,11 @@ void kswapd_run(int nid)
> >    */
> >   void kswapd_stop(int nid)
> >   {
> > -	struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
> > +	struct task_struct *kswapd = READ_ONCE(NODE_DATA(nid)->kswapd);
> >   
> >   	if (kswapd) {
> >   		kthread_stop(kswapd);
> > -		NODE_DATA(nid)->kswapd = NULL;
> > +		WRITE_ONCE(NODE_DATA(nid)->kswapd, NULL);
> >   	}
> >   }
> >