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Mon, 13 Jan 2020 15:58:25 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 7FC6A304121; Mon, 13 Jan 2020 16:56:48 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id DECF420427392; Mon, 13 Jan 2020 16:58:23 +0100 (CET) Date: Mon, 13 Jan 2020 16:58:23 +0100 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org, Bart Van Assche Subject: Re: [PATCH v2 4/6] locking/lockdep: Reuse freed chain_hlocks entries Message-ID: <20200113155823.GY2844@hirez.programming.kicks-ass.net> References: <20191216151517.7060-1-longman@redhat.com> <20191216151517.7060-5-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191216151517.7060-5-longman@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 16, 2019 at 10:15:15AM -0500, Waiman Long wrote: > +/* > + * Return offset of a chain block of the right size or -1 if not found. > + */ > +static inline int alloc_chain_hlocks_from_buckets(int size) > +{ > + int prev, curr, next; > + > + if (!nr_free_chain_hlocks) > + return -1; > + > + if (size <= MAX_CHAIN_BUCKETS) { > + curr = chain_block_buckets[size - 1]; > + if (curr < 0) > + return -1; > + > + chain_block_buckets[size - 1] = next_chain_block(curr); > + nr_free_chain_hlocks -= size; > + return curr; > + } > + > + /* > + * Look for a free chain block of the given size > + * > + * It is rare to have a lock chain with depth > MAX_CHAIN_BUCKETS. > + * It is also more expensive as we may iterate the whole list > + * without finding one. > + */ > + prev = -1; > + curr = chain_block_buckets[0]; > + while (curr >= 0) { > + next = next_chain_block(curr); > + if (chain_block_size(curr) == size) { > + set_chain_block(prev, 0, next); > + nr_free_chain_hlocks -= size; > + nr_large_chain_blocks--; > + return curr; > + } > + prev = curr; > + curr = next; > + } > + return -1; > +} > +/* > + * The graph lock must be held before calling this function. > + * > + * Return: an offset to chain_hlocks if successful, or > + * -1 with graph lock released > + */ > +static int alloc_chain_hlocks(int size) > +{ > + int curr; > + > + if (size < 2) > + size = 2; > + > + curr = alloc_chain_hlocks_from_buckets(size); > + if (curr >= 0) > + return curr; > + > + BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks)); > + BUILD_BUG_ON((1UL << 6) <= ARRAY_SIZE(current->held_locks)); > + BUILD_BUG_ON((1UL << 8*sizeof(chain_hlocks[0])) <= > + ARRAY_SIZE(lock_classes)); > + > + /* > + * Allocate directly from chain_hlocks. > + */ > + if (likely(nr_chain_hlocks + size <= MAX_LOCKDEP_CHAIN_HLOCKS)) { > + curr = nr_chain_hlocks; > + nr_chain_hlocks += size; > + return curr; > + } > + if (!debug_locks_off_graph_unlock()) > + return -1; > + > + print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!"); > + dump_stack(); > + return -1; > +} *groan*.... That's _two_ allocators :/ And it can trivially fail, even if there's plenty space available. Consider nr_chain_hlocks is exhaused, and @size is empty, but size+1 still has blocks. I'm guessing you didn't make it a single allocator because you didn't want to implement block splitting? why?