linux-kernel - Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless

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Date:	Mon, 26 Oct 2015 19:02:27 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	Patrick Marlier <patrick.marlier@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use
 lockless_dereference()


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> > It's this new usage in fs/fs-writeback.c:
> > 
> > static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
> >                                   struct wb_writeback_work *base_work,
> >                                   bool skip_if_busy)
> > {
> >         struct bdi_writeback *last_wb = NULL;
> >         struct bdi_writeback *wb = list_entry_rcu(&bdi->wb_list,
> 
> I believe that the above should instead be:
> 
> 	struct bdi_writeback *wb = list_entry_rcu(bdi->wb_list.next,
> 
> After all, RCU read-side list primitives need to fetch pointers in order to 
> traverse those pointers in an RCU-safe manner.  The patch below clears this up 
> for me, does it also work for you?

Are you sure about that?

I considered this solution too, but the code goes like this:

static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
                                  struct wb_writeback_work *base_work,
                                  bool skip_if_busy)
{
        struct bdi_writeback *last_wb = NULL;
        struct bdi_writeback *wb = list_entry_rcu(&bdi->wb_list,
                                                struct bdi_writeback, bdi_node);

        might_sleep();
restart:
        rcu_read_lock();
        list_for_each_entry_continue_rcu(wb, &bdi->wb_list, bdi_node) {

and list_for_each_entry_continue_rcu() will start the iteration with the next 
entry. So if you initialize the head with .next, then we'll start with 
.next->next, i.e. we skip the first entry.

That seems to change behavior and break the logic.

Another solution I considered is to use bd->wb_list.next->prev, but that, beyond 
being ugly, causes actual extra runtime overhead - for something that seems 
academical.

Thanks,

	Ingo
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