From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [GIT PULL] Ceph fixes for 5.1-rc7 Date: Fri, 26 Apr 2019 17:50:55 +0100 Message-ID: <20190426165055.GY2217@ZenIV.linux.org.uk> References: <20190425174739.27604-1-idryomov@gmail.com> <342ef35feb1110197108068d10e518742823a210.camel@kernel.org> <20190425200941.GW2217@ZenIV.linux.org.uk> <86674e79e9f24e81feda75bc3c0dd4215604ffa5.camel@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <86674e79e9f24e81feda75bc3c0dd4215604ffa5.camel@kernel.org> Sender: linux-kernel-owner@vger.kernel.org To: Jeff Layton Cc: Linus Torvalds , Ilya Dryomov , ceph-devel@vger.kernel.org, Linux List Kernel Mailing List-Id: ceph-devel.vger.kernel.org On Fri, Apr 26, 2019 at 12:25:03PM -0400, Jeff Layton wrote: > It turns out though that using name_snapshot from ceph is a bit more > tricky. In some cases, we have to call ceph_mdsc_build_path to build up > a full path string. We can't easily populate a name_snapshot from there > because struct external_name is only defined in fs/dcache.c. Explain, please. For ceph_mdsc_build_path() you don't need name snapshots at all and existing code is, AFAICS, just fine, except for pointless pr_err() there. I _probably_ would take allocation out of the loop (e.g. make it __getname(), called unconditionally) and turned it into the d_path.c-style read_seqbegin_or_lock()/need_seqretry()/done_seqretry() loop, so that the first pass would go under rcu_read_lock(), while the second (if needed) would just hold rename_lock exclusive (without bumping the refcount). But that's a matter of (theoretical) livelock avoidance, not the locking correctness for ->d_name accesses. Oh, and *base = ceph_ino(d_inode(temp)); *plen = len; probably belongs in critical section - _that_ might be a correctness issue, since temp is not held by anything once you are out of there. > I could add some routines to do this, but it feels a lot like I'm > abusing internal dcache interfaces. I'll keep thinking about it though. > > While we're on the subject though: > > struct external_name { > union { > atomic_t count; > struct rcu_head head; > } u; > unsigned char name[]; > }; > > Is it really ok to union the count and rcu_head there? > > I haven't trawled through all of the code yet, but what prevents someone > from trying to access the count inside an RCU critical section, after > call_rcu has been called on it? The fact that no lockless accesses to ->count are ever done?