From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: Possible btrfs deadlock coused by commit 660d3f6c Date: Mon, 16 Jan 2012 20:45:29 +0000 Message-ID: <20120116204528.GP23916@ZenIV.linux.org.uk> References: <20120116085322.GB2105@redhat.com> <20120116142540.GB8265@dhcp231-144.rdu.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Stanislaw Gruszka , Patrick , linux-btrfs@vger.kernel.org To: Josef Bacik Return-path: In-Reply-To: <20120116142540.GB8265@dhcp231-144.rdu.redhat.com> List-ID: On Mon, Jan 16, 2012 at 09:26:00AM -0500, Josef Bacik wrote: > On Mon, Jan 16, 2012 at 09:53:23AM +0100, Stanislaw Gruszka wrote: > > Hi Josef, > > > > commit 660d3f6cde552323578b85fc5a09a6742f1fe804 > > Author: Josef Bacik > > Date: Fri Dec 9 11:18:51 2011 -0500 > > > > Btrfs: fix how we do delalloc reservations and how we free reservations on error > > > > introduced possible deadlock. According to comment before > > btrfs_page_mkwrite(), that function is not allowed to take > > node->i_mutex . > > > > The problem was detected by Patrick (full lockdep info is provided): > > http://bugzilla.kernel.org/attachment.cgi?id=72072 > > during investigating of some other issue (kernel.org bug 42576) > > > > It's not a deadlock since you can't mmap a directory, but I posted a fix for > this to linux-btrfs last week, afaik it's headed to -rc1. Thanks, Sigh... You know, I'm getting really annoyed with the "can't mmap a directory" bogosity resurfacing every couple of months or so. No, you can't mmap a directory. You also can't bugger a hedgehog, which is about as relevant. What you *can* do is write(2) on a regular file. Which will happily grab ->i_mutex (in case of btrfs, that's done in the beginning of btrfs_file_aio_write()) and eventually get the user pages in (in case of btrfs, iov_iter_fault_in_readable() from __btrfs_buffered_write() from btrfs_file_aio_write()). IOW, you do get pagefaults with ->i_mutex held. Which means ->mmap_sem grabbed while holding ->i_mutex. No directories involved...