Messages in this thread Patch in this message | | | Date | Tue, 27 Oct 2015 14:19:39 +0900 | From | Tejun Heo <> | Subject | Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless_dereference() |
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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@kernel.org> --- 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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