From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Howells Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Date: Tue, 10 Oct 2017 10:35:46 +0100 Message-ID: <8079.1507628146@warthog.procyon.org.uk> References: <1507594969-8347-12-git-send-email-paulmck@linux.vnet.ibm.com> <20171010001951.GA6476@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT Return-path: In-Reply-To: <1507594969-8347-12-git-send-email-paulmck@linux.vnet.ibm.com> Content-ID: <8078.1507628146.1@warthog.procyon.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: "Paul E. McKenney" Cc: dhowells@redhat.com, linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, linux-arch@vger.kernel.org, peterz@infradead.org, will.deacon@arm.com, Jonathan Corbet , Alexander Kuleshov List-Id: linux-arch.vger.kernel.org Paul E. McKenney wrote: > static inline void *assoc_array_ptr_to_leaf(const struct assoc_array_ptr *x) > { > - return (void *)((unsigned long)x & ~ASSOC_ARRAY_PTR_TYPE_MASK); > + return (void *)((unsigned long)READ_ONCE(x) & /* Address dependency. */ > + ~ASSOC_ARRAY_PTR_TYPE_MASK); > } This is the wrong place to do this. assoc_array_ptr_to_leaf() is effectively no more than a special cast; it removes a metadata bit from a pointer. x is the value we're modifying, not *x, and x was read by the caller. > static inline > unsigned long __assoc_array_ptr_to_meta(const struct assoc_array_ptr *x) > { > - return (unsigned long)x & > + return (unsigned long)READ_ONCE(x) & /* Address dependency. */ > ~(ASSOC_ARRAY_PTR_SUBTYPE_MASK | ASSOC_ARRAY_PTR_TYPE_MASK); > } Ditto. > - ptr = READ_ONCE(node->slots[slot]); > + ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */ > has_meta |= (unsigned long)ptr; > if (ptr && assoc_array_ptr_is_leaf(ptr)) { > - /* We need a barrier between the read of the pointer > - * and dereferencing the pointer - but only if we are > - * actually going to dereference it. > - */ > - smp_read_barrier_depends(); > - For example, you can see the READ_ONCE() here; that is already done. Can you also not just delete the comment, but rephrase it to indicate that a barrier is necessary and it's done by READ_ONCE(), please? David