Messages in this thread | | | From | Uladzislau Rezki <> | Date | Mon, 20 Apr 2020 19:18:29 +0200 | Subject | Re: [PATCH rcu/dev -fixes 3/4] rcu/tree: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk() |
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On Mon, Apr 20, 2020 at 11:38:36AM -0400, Joel Fernandes (Google) wrote: > From: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > > There is no need to use xchg(), the access is serialized by krcp->lock. > The xchg() function adds some atomic barriers which remain hidden in > x86's disassembly but are visible on ARM for instance. > > Replace xchg() with a load + store. > > Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > kernel/rcu/tree.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index cd61649e1b001..f6eb3aee0935e 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3088,7 +3088,8 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, > /* Check if a new block is required. */ > if (!krcp->bhead || > krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) { > - bnode = xchg(&krcp->bcached, NULL); > + bnode = krcp->bcached; > + krcp->bcached = NULL; > if (!bnode) { > WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); > > But we populate the cache without holding the krcp->lock. That is why it is xchg here. See below:
<snip> if (cmpxchg(&krcp->bcached, NULL, bhead)) free_page((unsigned long) bhead); <snip>
we do not hold any krcp->lock here, we do not need it.
-- Vlad Rezki
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