On Wed, 2020-11-11 at 11:08 +0000, Luis Henriques wrote: > Jeff Layton <jlayton@xxxxxxxxxx> writes: > > > On Sat, 2019-12-14 at 10:46 +0800, Yan, Zheng wrote: > > > On Fri, Dec 13, 2019 at 1:32 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > I believe it's possible that we could end up with racing calls to > > > > __ceph_remove_cap for the same cap. If that happens, the cap->ci > > > > pointer will be zereoed out and we can hit a NULL pointer dereference. > > > > > > > > Once we acquire the s_cap_lock, check for the ci pointer being NULL, > > > > and just return without doing anything if it is. > > > > > > > > URL: https://tracker.ceph.com/issues/43272 > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > > > --- > > > > fs/ceph/caps.c | 21 ++++++++++++++++----- > > > > 1 file changed, 16 insertions(+), 5 deletions(-) > > > > > > > > This is the only scenario that made sense to me in light of Ilya's > > > > analysis on the tracker above. I could be off here though -- the locking > > > > around this code is horrifically complex, and I could be missing what > > > > should guard against this scenario. > > > > > > > > > > I think the simpler fix is, in trim_caps_cb, check if cap-ci is > > > non-null before calling __ceph_remove_cap(). this should work because > > > __ceph_remove_cap() is always called inside i_ceph_lock > > > > > > > Is that sufficient though? The stack trace in the bug shows it being > > called by ceph_trim_caps, but I think we could hit the same problem with > > other __ceph_remove_cap callers, if they happen to race in at the right > > time. > > Sorry for resurrecting this old thread, but we just got a report with this > issue on a kernel that includes commit d6e47819721a ("ceph: hold > i_ceph_lock when removing caps for freeing inode"). > > Looking at the code, I believe Zheng's suggestion should work as I don't > see any __ceph_remove_cap callers that don't hold the i_ceph_lock. So, > would something like the diff bellow be acceptable? > > Cheers, I'm still not convinced that's the correct fix. Why would trim_caps_cb be subject to this race when other __ceph_remove_cap callers are not? Maybe the right fix is to test for a NULL cap->ci in __ceph_remove_cap and just return early if it is? -- Jeff Layton <jlayton@xxxxxxxxxx>
