From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Yan, Zheng" Subject: Re: [PATCH 02/12] Btrfs: Kill allocate_wait in space_info Date: Mon, 19 Apr 2010 23:34:25 +0800 Message-ID: References: <4BCC3458.5030600@oracle.com> <20100419135713.GA2352@localhost.localdomain> <20100419144843.GC2352@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: linux-btrfs@vger.kernel.org To: Josef Bacik Return-path: In-Reply-To: <20100419144843.GC2352@localhost.localdomain> List-ID: On Mon, Apr 19, 2010 at 10:48 PM, Josef Bacik wrote: > On Mon, Apr 19, 2010 at 10:46:12PM +0800, Yan, Zheng wrote: >> On Mon, Apr 19, 2010 at 9:57 PM, Josef Bacik wrot= e: >> > The purpose of maybe_allocate_chunk was that there is no way to kn= ow if some >> > other CPU is currently trying to allocate a chunk for the given sp= ace info. =A0We >> > could have two cpu's come inot do_chunk_alloc at relatively the sa= me time and >> > end up allocating twice the amount of space, which is why I did th= e waitqueue >> > thing. =A0It seems like this is still a possibility with your patc= h. =A0Thanks, >> > >> This is impossible because the very first thing do_chunk_alloc does = is >> lock the chunk_mutex. >> > > Sure, that just means we don't get two things creating chunks at the = same time, > but not from creating them one right after another. =A0So CPU 0 and 1= come in to > the check free space stuff, realize they need to allocate a chunk, an= d race to > call do_chunk_alloc. =A0One of them wins, and the other blocks on the= chunk_mutex > lock. =A0When the first finishes the other one is able to continue an= d do what it > was originally going to do, and then you get two chunks when you real= ly only > wanted one. =A0Thanks, > there is a check in do_chunk_alloc, so the later one will do nothing if the first call adds enough space. Yan Zheng -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" = in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html