RE: [PATCH] fs/ceph/dir: fix `i_nlink` underrun during async unlink

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

On Tue, 2026-02-24 at 14:33 +0100, Max Kellermann wrote:
> There hasn't been a review of this patch and the bug is still present
> in 6.19 / 7.0rc.
> 
> On Fri, Sep 5, 2025 at 11:15 PM Max Kellermann <max.kellermann@ionos.com> wrote:
> > 
> > During async unlink, we drop the `i_nlink` counter before we receive
> > the completion (that will eventually update the `i_nlink`) because "we
> > assume that the unlink will succeed".  That is not a bad idea, but it
> > races against deletions by other clients (or against the completion of
> > our own unlink) and can lead to an underrun which emits a WARNING like
> > this one:
> > 
> >  WARNING: CPU: 85 PID: 25093 at fs/inode.c:407 drop_nlink+0x50/0x68
> >  Modules linked in:
> >  CPU: 85 UID: 3221252029 PID: 25093 Comm: php-cgi8.1 Not tainted 6.14.11-cm4all1-ampere #655
> >  Hardware name: Supermicro ARS-110M-NR/R12SPD-A, BIOS 1.1b 10/17/2023
> >  pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >  pc : drop_nlink+0x50/0x68
> >  lr : ceph_unlink+0x6c4/0x720
> >  sp : ffff80012173bc90
> >  x29: ffff80012173bc90 x28: ffff086d0a45aaf8 x27: ffff0871d0eb5680
> >  x26: ffff087f2a64a718 x25: 0000020000000180 x24: 0000000061c88647
> >  x23: 0000000000000002 x22: ffff07ff9236d800 x21: 0000000000001203
> >  x20: ffff07ff9237b000 x19: ffff088b8296afc0 x18: 00000000f3c93365
> >  x17: 0000000000070000 x16: ffff08faffcbdfe8 x15: ffff08faffcbdfec
> >  x14: 0000000000000000 x13: 45445f65645f3037 x12: 34385f6369706f74
> >  x11: 0000a2653104bb20 x10: ffffd85f26d73290 x9 : ffffd85f25664f94
> >  x8 : 00000000000000c0 x7 : 0000000000000000 x6 : 0000000000000002
> >  x5 : 0000000000000081 x4 : 0000000000000481 x3 : 0000000000000000
> >  x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff08727d3f91e8
> >  Call trace:
> >   drop_nlink+0x50/0x68 (P)
> >   vfs_unlink+0xb0/0x2e8
> >   do_unlinkat+0x204/0x288
> >   __arm64_sys_unlinkat+0x3c/0x80
> >   invoke_syscall.constprop.0+0x54/0xe8
> >   do_el0_svc+0xa4/0xc8
> >   el0_svc+0x18/0x58
> >   el0t_64_sync_handler+0x104/0x130
> >   el0t_64_sync+0x154/0x158
> > 
> > In ceph_unlink(), a call to ceph_mdsc_submit_request() submits the
> > CEPH_MDS_OP_UNLINK to the MDS, but does not wait for completion.
> > 
> > Meanwhile, between this call and the following drop_nlink() call, a
> > worker thread may process a CEPH_CAP_OP_IMPORT, CEPH_CAP_OP_GRANT or
> > just a CEPH_MSG_CLIENT_REPLY (the latter of which could be our own
> > completion).  These will lead to a set_nlink() call, updating the
> > `i_nlink` counter to the value received from the MDS.  If that new
> > `i_nlink` value happens to be zero, it is illegal to decrement it
> > further.  But that is exactly what ceph_unlink() will do then.
> > 
> > The WARNING can be reproduced this way:
> > 
> > 1. Force async unlink; only the async code path is affected.  Having
> >    no real clue about Ceph internals, I was unable to find out why the
> >    MDS wouldn't give me the "Fxr" capabilities, so I patched
> >    get_caps_for_async_unlink() to always succeed.
> > 
> >    (Note that the WARNING dump above was found on an unpatched kernel,
> >    without this kludge - this is not a theoretical bug.)
> > 
> > 2. Add a sleep call after ceph_mdsc_submit_request() so the unlink
> >    completion gets handled by a worker thread before drop_nlink() is
> >    called.  This guarantees that the `i_nlink` is already zero before
> >    drop_nlink() runs.
> > 
> > The solution is to skip the counter decrement when it is already zero,
> > but doing so without a lock is still racy (TOCTOU).  Since
> > ceph_fill_inode() and handle_cap_grant() both hold the
> > `ceph_inode_info.i_ceph_lock` spinlock while set_nlink() runs, this
> > seems like the proper lock to protect the `i_nlink` updates.
> > 
> > I found prior art in NFS and SMB (using `inode.i_lock`) and AFS (using
> > `afs_vnode.cb_lock`).  All three have the zero check as well.
> > 
> > Fixes: 2ccb45462aea ("ceph: perform asynchronous unlink if we have sufficient caps")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> > ---
> >  fs/ceph/dir.c | 15 ++++++++++++++-
> >  1 file changed, 14 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
> > index 8478e7e75df6..67f04e23f78a 100644
> > --- a/fs/ceph/dir.c
> > +++ b/fs/ceph/dir.c
> > @@ -1341,6 +1341,7 @@ static int ceph_unlink(struct inode *dir, struct dentry *dentry)
> >         struct ceph_client *cl = fsc->client;
> >         struct ceph_mds_client *mdsc = fsc->mdsc;
> >         struct inode *inode = d_inode(dentry);
> > +       struct ceph_inode_info *ci = ceph_inode(inode);
> >         struct ceph_mds_request *req;
> >         bool try_async = ceph_test_mount_opt(fsc, ASYNC_DIROPS);
> >         struct dentry *dn;
> > @@ -1427,7 +1428,19 @@ static int ceph_unlink(struct inode *dir, struct dentry *dentry)
> >                          * We have enough caps, so we assume that the unlink
> >                          * will succeed. Fix up the target inode and dcache.
> >                          */
> > -                       drop_nlink(inode);
> > +
> > +                       /*
> > +                        * Protect the i_nlink update with i_ceph_lock
> > +                        * to precent racing against ceph_fill_inode()
> > +                        * handling our completion on a worker thread
> > +                        * and don't decrement if i_nlink has already
> > +                        * been updated to zero by this completion.
> > +                        */
> > +                       spin_lock(&ci->i_ceph_lock);
> > +                       if (inode->i_nlink > 0)
> > +                               drop_nlink(inode);
> > +                       spin_unlock(&ci->i_ceph_lock);
> > +
> >                         d_delete(dentry);
> >                 } else {
> >                         spin_lock(&fsc->async_unlink_conflict_lock);
> > --
> > 2.47.2
> > 
> 

Makes sense.

Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>

Thanks,
Slava.