From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id 2F9996B00C4 for ; Fri, 20 Nov 2009 10:17:47 -0500 (EST) Subject: Re: lockdep complaints in slab allocator From: Peter Zijlstra In-Reply-To: <20091120144855.GB22527@linux.vnet.ibm.com> References: <20091118181202.GA12180@linux.vnet.ibm.com> <84144f020911192249l6c7fa495t1a05294c8f5b6ac8@mail.gmail.com> <1258709153.11284.429.camel@laptop> <84144f020911200238w3d3ecb38k92ca595beee31de5@mail.gmail.com> <1258714328.11284.522.camel@laptop> <4B067816.6070304@cs.helsinki.fi> <20091120144855.GB22527@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Date: Fri, 20 Nov 2009 16:17:40 +0100 Message-ID: <1258730260.4104.240.camel@laptop> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: paulmck@linux.vnet.ibm.com Cc: Pekka Enberg , linux-mm@kvack.org, cl@linux-foundation.org, mpm@selenic.com, LKML , Nick Piggin List-ID: On Fri, 2009-11-20 at 06:48 -0800, Paul E. McKenney wrote: > On Fri, Nov 20, 2009 at 01:05:58PM +0200, Pekka Enberg wrote: > > Peter Zijlstra kirjoitti: > >> On Fri, 2009-11-20 at 12:38 +0200, Pekka Enberg wrote: > >>> > >>> On Fri, Nov 20, 2009 at 11:25 AM, Peter Zijlstra > >>> wrote: > >>>> 2) propagate the nesting information and user spin_lock_nested(), given > >>>> that slab is already a rat's nest, this won't make it any less obvious. > >>> spin_lock_nested() doesn't really help us here because there's a > >>> _real_ possibility of a recursive spin lock here, right? > >> Well, I was working under the assumption that your analysis of it being > >> a false positive was right ;-) > >> I briefly tried to verify that, but got lost and gave up, at which point > >> I started looking for ways to annotate. > > > > Uh, ok, so apparently I was right after all. There's a comment in > > free_block() above the slab_destroy() call that refers to the comment above > > alloc_slabmgmt() function definition which explains it all. > > > > Long story short: ->slab_cachep never points to the same kmalloc cache > > we're allocating or freeing from. Where do we need to put the > > spin_lock_nested() annotation? Would it be enough to just use it in > > cache_free_alien() for alien->lock or do we need it in cache_flusharray() > > as well? > > Hmmm... If the nc->lock spinlocks are always from different slabs > (as alloc_slabmgmt()'s block comment claims), why not just give each > array_cache structure's lock its own struct lock_class_key? They > are zero size unless you have lockdep enabled. Because more classes: - takes more (static/limited) lockdep resources - make more chains, weakening lock dependency tracking because it can no longer use the state observed in one branch on state observed in another branch. Suppose you have 3 locks and 2 classes, lock 1 and 2 part of class A and lock 3 of class B Then if we observe 1 -> 3, and 3 -> 2, we'd see A->B and B->A, and go yell. Now if we split class A into two classes and these locks get into separate classes we loose that cycle. Now in this case we want to break a cycle, so the above will be correct, but all resulting chains will be equivalent for 99% (with the one exception of this funny recursion case) wasting lots of resources and state matching opportunity. Therefore it would be much better to use the _nested annotation if possible. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org