Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755408Ab3H1Uoo (ORCPT ); Wed, 28 Aug 2013 16:44:44 -0400 Received: from mail-pb0-f52.google.com ([209.85.160.52]:45198 "EHLO mail-pb0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751961Ab3H1Uom (ORCPT ); Wed, 28 Aug 2013 16:44:42 -0400 Date: Wed, 28 Aug 2013 13:44:54 -0700 From: Kent Overstreet To: Andrew Morton Cc: "Nicholas A. Bellinger" , target-devel , lf-virt , lkml , kvm-devel , "Michael S. Tsirkin" , Asias He , Jens Axboe , Tejun Heo , Ingo Molnar , Andi Kleen , Christoph Lameter , Oleg Nesterov , Christoph Lameter Subject: Re: [PATCH-v3 1/4] idr: Percpu ida Message-ID: <20130828204454.GC1357@kmo-pixel> References: <1376694549-20609-1-git-send-email-nab@linux-iscsi.org> <1376694549-20609-2-git-send-email-nab@linux-iscsi.org> <20130820143157.f91bf59d16352989b54e431e@linux-foundation.org> <20130828195317.GE8032@kmo-pixel> <20130828132332.6d5263ee9622235ae0fcc615@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130828132332.6d5263ee9622235ae0fcc615@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2186 Lines: 59 On Wed, Aug 28, 2013 at 01:23:32PM -0700, Andrew Morton wrote: > On Wed, 28 Aug 2013 12:53:17 -0700 Kent Overstreet wrote: > > > > > + while (1) { > > > > + spin_lock(&pool->lock); > > > > + > > > > + /* > > > > + * prepare_to_wait() must come before steal_tags(), in case > > > > + * percpu_ida_free() on another cpu flips a bit in > > > > + * cpus_have_tags > > > > + * > > > > + * global lock held and irqs disabled, don't need percpu lock > > > > + */ > > > > + prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE); > > > > + > > > > + if (!tags->nr_free) > > > > + alloc_global_tags(pool, tags); > > > > + if (!tags->nr_free) > > > > + steal_tags(pool, tags); > > > > + > > > > + if (tags->nr_free) { > > > > + tag = tags->freelist[--tags->nr_free]; > > > > + if (tags->nr_free) > > > > + set_bit(smp_processor_id(), > > > > + pool->cpus_have_tags); > > > > + } > > > > + > > > > + spin_unlock(&pool->lock); > > > > + local_irq_restore(flags); > > > > + > > > > + if (tag >= 0 || !(gfp & __GFP_WAIT)) > > > > + break; > > > > + > > > > + schedule(); > > > > + > > > > + local_irq_save(flags); > > > > + tags = this_cpu_ptr(pool->tag_cpu); > > > > + } > > > > > > What guarantees that this wait will terminate? > > > > It seems fairly clear to me from the break statement a couple lines up; > > if we were passed __GFP_WAIT we terminate iff we succesfully allocated a > > tag. If we weren't passed __GFP_WAIT we never actually sleep. > > OK ;) Let me rephrase. What guarantees that a tag will become available? > > If what we have here is an open-coded __GFP_NOFAIL then that is > potentially problematic. It's the same semantics as a mempool, really - it'll succeed when a tag gets freed. If we are sleeping then there isn't really anything else we can do, there isn't anything we're trying in the __GFP_WAIT case that we're not trying in the GFP_NOWAIT case. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/