Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless_dereference()

[Tejun Heo]

Hello,

On Tue, Oct 27, 2015 at 12:37:16PM +0900, Linus Torvalds wrote:
> > I believe that the above should instead be:
> >
> >         struct bdi_writeback *wb = list_entry_rcu(bdi->wb_list.next,

I should have just used list_entry() here.  It's just offseting the
pointer to set up the initial iteration point.

...
> That said, I'm not sure why it doesn't just do the normal
> 
>     rcu_read_lock();
>     list_for_each_entry_rcu(wb, &bdi->wb_list, bdi_node) {
>         ....
>     }
>     rcu_read_unlock();
> 
> like the other places do. It looks like it wants that
> "list_for_each_entry_continue_rcu()" because it does that odd "pin
> entry and drop rcu lock and retake it and continue where you left
> off", but I'm not sure why the continue version would be so
> different.. It's going to do that "follow next entry" regardless, and
> the "goto restart" doesn't look like it actually adds anything. If
> following the next pointer is ok even after having released the RCU
> read lock, then I'm not seeing why the end of the loop couldn't just
> do
> 
>                 rcu_read_unlock();
>                 wb_wait_for_completion(bdi, &fallback_work_done);
>                 rcu_read_lock();
> 
> and just continue the loop (and the pinning of "wb" and releasing the
> "last_wb" thing in the *next* iteration should make it all work the
> same).
> 
> Adding Tejun to the cc, because this is his code and there's probably
> something subtle I'm missing. Tejun, can you take a look? It's
> bdi_split_work_to_wbs() in fs/fs-writeback.c.

Yeah, just releasing and regrabbing should work too as the iterator
doesn't depend on anything other than the current entry (e.g. as
opposed to imaginary list_for_each_entry_safe_rcu()).  It's slightly
icky to meddle with locking behind the iterator's back tho.  Either
way should be fine but how about something like the following?

Subject: writeback: don't use list_entry_rcu() for pointer offsetting in bdi_split_work_to_wbs()

bdi_split_work_to_wbs() uses list_for_each_entry_rcu_continue() to
walk @bdi->wb_list.  To set up the initial iteration condition, it
uses list_entry_rcu() to calculate the entry pointer corresponding to
the list head; however, this isn't an actual RCU dereference and using
list_entry_rcu() for it ended up breaking a proposed list_entry_rcu()
change because it was feeding an non-lvalue pointer into the macro.

Don't use the RCU variant for simple pointer offsetting.  Use
list_entry() instead.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 fs/fs-writeback.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 29e4599..7378169 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -779,8 +779,8 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 				  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);
+	struct bdi_writeback *wb = list_entry(&bdi->wb_list,
+					      struct bdi_writeback, bdi_node);
 
 	might_sleep();
 restart:
--
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