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In-Reply-To: <ce407424e98bf5f2b186df5d28dd5749a6cbfa45.camel@themaw.net>
From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 26 Jul 2023 15:49:35 +0200
Message-ID: <CAJfpegu2S04RTpuHMH9=ODVmLD1JdB64t=ieprx74teonCDHEg@mail.gmail.com>
Subject: Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read
To:     Ian Kent <raven@themaw.net>
Cc:     Anders Roxell <anders.roxell@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>, Tejun Heo <tj@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        elver@google.com, imran.f.khan@oracle.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, 20 Jul 2023 at 04:03, Ian Kent <raven@themaw.net> wrote:
>
> On Wed, 2023-07-19 at 12:23 +0800, Ian Kent wrote:
> > On 19/7/23 03:00, Anders Roxell wrote:
> > > On 2023-01-23 11:11, Ian Kent wrote:
> > > > On 29/12/22 21:07, Ian Kent wrote:
> > > > > On 29/12/22 17:20, Arnd Bergmann wrote:
> > > > > > On Fri, Dec 23, 2022, at 00:11, Ian Kent wrote:
> > > > > > > On 21/12/22 21:34, Anders Roxell wrote:
> > > > > > > > On 2022-10-31 12:30, Tejun Heo wrote:
> > > > > > > > > On Tue, Oct 18, 2022 at 10:32:42AM +0800, Ian Kent
> > > > > > > > > wrote:
> > > > > > > > > > The kernfs write lock is held when the kernfs node
> > > > > > > > > > inode attributes
> > > > > > > > > > are updated. Therefore, when either
> > > > > > > > > > kernfs_iop_getattr() or
> > > > > > > > > > kernfs_iop_permission() are called the kernfs node
> > > > > > > > > > inode attributes
> > > > > > > > > > won't change.
> > > > > > > > > >
> > > > > > > > > > Consequently concurrent kernfs_refresh_inode() calls
> > > > > > > > > > always copy the
> > > > > > > > > > same values from the kernfs node.
> > > > > > > > > >
> > > > > > > > > > So there's no need to take the inode i_lock to get
> > > > > > > > > > consistent values
> > > > > > > > > > for generic_fillattr() and generic_permission(), the
> > > > > > > > > > kernfs read lock
> > > > > > > > > > is sufficient.
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Ian Kent <raven@themaw.net>
> > > > > > > > > Acked-by: Tejun Heo <tj@kernel.org>
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > > Building an allmodconfig arm64 kernel on yesterdays next-
> > > > > > > > 20221220 and
> > > > > > > > booting that in qemu I see the following "BUG: KCSAN:
> > > > > > > > data-race in
> > > > > > > > set_nlink / set_nlink".
> > > > > > > I'll check if I missed any places where set_link() could be
> > > > > > > called where the link count could be different.
> > > > > > >
> > > > > > >
> > > > > > > If there aren't any the question will then be can writing
> > > > > > > the
> > > > > > > same value to this location in multiple concurrent threads
> > > > > > > corrupt it?
> > > > > > I think the race that is getting reported for set_nlink()
> > > > > > is about this bit getting called simulatenously on multiple
> > > > > > CPUs with only the read lock held for the inode:
> > > > > >
> > > > > >        /* Yes, some filesystems do change nlink from zero to
> > > > > > one */
> > > > > >        if (inode->i_nlink == 0)
> > > > > > atomic_long_dec(&inode->i_sb->s_remove_count);
> > > > > >        inode->__i_nlink = nlink;
> > > > > >
> > > > > > Since i_nlink and __i_nlink refer to the same memory
> > > > > > location,
> > > > > > the 'inode->i_nlink == 0' check can be true for all of them
> > > > > > before the nonzero nlink value gets set, and this results in
> > > > > > s_remove_count being decremented more than once.
> > > > >
> > > > > Thanks for the comment Arnd.
> > > >
> > > > Hello all,
> > > >
> > > >
> > > > I've been looking at this and after consulting Miklos and his
> > > > pointing
> > > >
> > > > out that it looks like a false positive the urgency dropped off a
> > > > bit. So
> > > >
> > > > apologies for taking so long to report back.
> > > >
> > > >
> > > > Anyway it needs some description of conclusions reached so far.
> > > >
> > > >
> > > > I'm still looking around but in short, kernfs will set
> > > > directories to <# of
> > > >
> > > > directory entries> + 2 unconditionally for directories. I can't
> > > > yet find
> > > >
> > > > any other places where i_nlink is set or changed and if there are
> > > > none
> > > >
> > > > then i_nlink will never be set to zero so the race should not
> > > > occur.
> > > >
> > > >
> > > > Consequently my claim is this is a real false positive.
> > > >
> > > >
> > > > There are the file system operations that may be passed at mount
> > > > time
> > > >
> > > > but given the way kernfs sets i_nlink it pretty much dictates
> > > > those
> > > > operations
> > > >
> > > > (if there were any that modify it and there don't appear to be
> > > > any) leave it
> > > >
> > > > alone.
> > > >
> > > >
> > > > So it just doesn't make sense for users of kernfs to fiddle with
> > > > i_nlink ...
> > > On todays next tag, next-20230718 this KCSAN BUG poped up again.
> > > When I
> > > built an allmodconfig arm64 kernel and booted it in QEMU. Full log
> > > can
> > > be found http://ix.io/4AUd
> > >
> > > [ 1694.987789][  T137] BUG: KCSAN: data-race in inode_permission /
> > > kernfs_refresh_inode
> > > [ 1694.992912][  T137]
> > > [ 1694.994532][  T137] write to 0xffff00000bab6070 of 2 bytes by
> > > task 104 on cpu 0:
> > > [ 1694.999269][ T137] kernfs_refresh_inode
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:171)
> > > [ 1695.002707][ T137] kernfs_iop_permission
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:289)
> > > [ 1695.006148][ T137] inode_permission
> > > (/home/anders/src/kernel/next/fs/namei.c:461
> > > /home/anders/src/kernel/next/fs/namei.c:528)
> > > [ 1695.009420][ T137] link_path_walk
> > > (/home/anders/src/kernel/next/fs/namei.c:1720
> > > /home/anders/src/kernel/next/fs/namei.c:2267)
> > > [ 1695.012643][ T137] path_lookupat
> > > (/home/anders/src/kernel/next/fs/namei.c:2478 (discriminator 2))
> > > [ 1695.015781][ T137] filename_lookup
> > > (/home/anders/src/kernel/next/fs/namei.c:2508)
> > > [ 1695.019059][ T137] vfs_statx
> > > (/home/anders/src/kernel/next/fs/stat.c:238)
> > > [ 1695.022024][ T137] vfs_fstatat
> > > (/home/anders/src/kernel/next/fs/stat.c:276)
> > > [ 1695.025067][ T137] __do_sys_newfstatat
> > > (/home/anders/src/kernel/next/fs/stat.c:446)
> > > [ 1695.028497][ T137] __arm64_sys_newfstatat
> > > (/home/anders/src/kernel/next/fs/stat.c:440
> > > /home/anders/src/kernel/next/fs/stat.c:440)
> > > [ 1695.032080][ T137] el0_svc_common.constprop.0
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> > > [ 1695.035916][ T137] do_el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> > > [ 1695.038796][ T137] el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> > > [ 1695.041468][ T137] el0t_64_sync_handler
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> > > [ 1695.044889][ T137] el0t_64_sync
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> > > [ 1695.047904][  T137]
> > > [ 1695.049511][  T137] 1 lock held by systemd-udevd/104:
> > > [ 1695.052837][ T137] #0: ffff000006681e08 (&root-
> > > >kernfs_iattr_rwsem){++++}-{3:3}, at: kernfs_iop_permission
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:288)
> > > [ 1695.060241][  T137] irq event stamp: 82902
> > > [ 1695.063006][ T137] hardirqs last enabled at (82901):
> > > _raw_spin_unlock_irqrestore
> > > (/home/anders/src/kernel/next/arch/arm64/include/asm/alternative-
> > > macros.h:250
> > > /home/anders/src/kernel/next/arch/arm64/include/asm/irqflags.h:27
> > > /home/anders/src/kernel/next/arch/arm64/include/asm/irqflags.h:140
> > > /home/anders/src/kernel/next/include/linux/spinlock_api_smp.h:151
> > > /home/anders/src/kernel/next/kernel/locking/spinlock.c:194)
> > > [ 1695.069673][ T137] hardirqs last disabled at (82902):
> > > el1_interrupt
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:472
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:488)
> > > [ 1695.075474][ T137] softirqs last enabled at (82792):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> > > [ 1695.082319][ T137] softirqs last disabled at (82790):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/include/linux/bottom_half.h:20
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> > > [ 1695.089049][  T137]
> > > [ 1695.090659][  T137] read to 0xffff00000bab6070 of 2 bytes by
> > > task 137 on cpu 0:
> > > [ 1695.095374][ T137] inode_permission
> > > (/home/anders/src/kernel/next/fs/namei.c:532)
> > > [ 1695.098655][ T137] link_path_walk
> > > (/home/anders/src/kernel/next/fs/namei.c:1720
> > > /home/anders/src/kernel/next/fs/namei.c:2267)
> > > [ 1695.101857][ T137] path_openat
> > > (/home/anders/src/kernel/next/fs/namei.c:3789 (discriminator 2))
> > > [ 1695.104885][ T137] do_filp_open
> > > (/home/anders/src/kernel/next/fs/namei.c:3820)
> > > [ 1695.108006][ T137] do_sys_openat2
> > > (/home/anders/src/kernel/next/fs/open.c:1418)
> > > [ 1695.111290][ T137] __arm64_sys_openat
> > > (/home/anders/src/kernel/next/fs/open.c:1433)
> > > [ 1695.114825][ T137] el0_svc_common.constprop.0
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> > > [ 1695.118662][ T137] do_el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> > > [ 1695.121555][ T137] el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> > > [ 1695.124207][ T137] el0t_64_sync_handler
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> > > [ 1695.127590][ T137] el0t_64_sync
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> > > [ 1695.130641][  T137]
> > > [ 1695.132241][  T137] no locks held by systemd-udevd/137.
> > > [ 1695.135618][  T137] irq event stamp: 3246
> > > [ 1695.138519][ T137] hardirqs last enabled at (3245):
> > > seqcount_lockdep_reader_access
> > > (/home/anders/src/kernel/next/include/linux/seqlock.h:105)
> > > [ 1695.145825][ T137] hardirqs last disabled at (3246):
> > > el1_interrupt
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:472
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:488)
> > > [ 1695.151942][ T137] softirqs last enabled at (3208):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> > > [ 1695.158950][ T137] softirqs last disabled at (3206):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/include/linux/bottom_half.h:20
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> > > [ 1695.166036][  T137]
> > > [ 1695.167621][  T137] Reported by Kernel Concurrency Sanitizer on:
> > > [ 1695.179990][  T137] Hardware name: linux,dummy-virt (DT)
> > > [ 1695.183687][  T137]
> > > ==================================================================
> >
> >
> > This one is different to the original.
> >
> >
> > I can't see where the problem is here, can someone help me out
> >
> > please.
> >
> >
> > I don't see any shared data values used by the call
> >
> > devcgroup_inode_permission(inode, mask) in
> > fs/namei.c:inode_permission()
> >
> > that might be a problem during the read except possibly inode-
> > >i_mode.
> >
> >
> > I'll check on that ... maybe something's been missed when
> > kernfs_rwsem
> >
> > was changed to a separate lock (kernfs_iattr_rwsem).
> >
> >
> > >
> > > [...]
> > >
> > > [ 1738.053819][  T104] BUG: KCSAN: data-race in set_nlink /
> > > set_nlink
> > > [ 1738.058223][  T104]
> > > [ 1738.059865][  T104] read to 0xffff00000bab6918 of 4 bytes by
> > > task 108 on cpu 0:
> > > [ 1738.064916][ T104] set_nlink
> > > (/home/anders/src/kernel/next/fs/inode.c:369)
> > > [ 1738.067845][ T104] kernfs_refresh_inode
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:180)
> > > [ 1738.071607][ T104] kernfs_iop_permission
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:289)
> > > [ 1738.075467][ T104] inode_permission
> > > (/home/anders/src/kernel/next/fs/namei.c:461
> > > /home/anders/src/kernel/next/fs/namei.c:528)
> > > [ 1738.078868][ T104] link_path_walk
> > > (/home/anders/src/kernel/next/fs/namei.c:1720
> > > /home/anders/src/kernel/next/fs/namei.c:2267)
> > > [ 1738.082270][ T104] path_lookupat
> > > (/home/anders/src/kernel/next/fs/namei.c:2478 (discriminator 2))
> > > [ 1738.085488][ T104] filename_lookup
> > > (/home/anders/src/kernel/next/fs/namei.c:2508)
> > > [ 1738.089101][ T104] user_path_at_empty
> > > (/home/anders/src/kernel/next/fs/namei.c:2907)
> > > [ 1738.092469][ T104] do_readlinkat
> > > (/home/anders/src/kernel/next/fs/stat.c:477)
> > > [ 1738.095970][ T104] __arm64_sys_readlinkat
> > > (/home/anders/src/kernel/next/fs/stat.c:504
> > > /home/anders/src/kernel/next/fs/stat.c:501
> > > /home/anders/src/kernel/next/fs/stat.c:501)
> > > [ 1738.099529][ T104] el0_svc_common.constprop.0
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> > > [ 1738.103696][ T104] do_el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> > > [ 1738.106560][ T104] el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> > > [ 1738.109613][ T104] el0t_64_sync_handler
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> > > [ 1738.113035][ T104] el0t_64_sync
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> > > [ 1738.116346][  T104]
> > > [ 1738.117924][  T104] 1 lock held by systemd-udevd/108:
> > > [ 1738.121580][ T104] #0: ffff000006681e08 (&root-
> > > >kernfs_iattr_rwsem){++++}-{3:3}, at: kernfs_iop_permission
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:288)
> > > [ 1738.129355][  T104] irq event stamp: 31000
> > > [ 1738.132088][ T104] hardirqs last enabled at (31000):
> > > seqcount_lockdep_reader_access
> > > (/home/anders/src/kernel/next/include/linux/seqlock.h:105)
> > > [ 1738.139417][ T104] hardirqs last disabled at (30999):
> > > seqcount_lockdep_reader_access
> > > (/home/anders/src/kernel/next/include/linux/seqlock.h:104)
> > > [ 1738.146781][ T104] softirqs last enabled at (30973):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> > > [ 1738.153891][ T104] softirqs last disabled at (30971):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/include/linux/bottom_half.h:20
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> > > [ 1738.161012][  T104]
> > > [ 1738.162663][  T104] write to 0xffff00000bab6918 of 4 bytes by
> > > task 104 on cpu 0:
> > > [ 1738.167730][ T104] set_nlink
> > > (/home/anders/src/kernel/next/fs/inode.c:372)
> > > [ 1738.170559][ T104] kernfs_refresh_inode
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:180)
> > > [ 1738.174355][ T104] kernfs_iop_permission
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:289)
> > > [ 1738.177829][ T104] inode_permission
> > > (/home/anders/src/kernel/next/fs/namei.c:461
> > > /home/anders/src/kernel/next/fs/namei.c:528)
> > > [ 1738.181403][ T104] link_path_walk
> > > (/home/anders/src/kernel/next/fs/namei.c:1720
> > > /home/anders/src/kernel/next/fs/namei.c:2267)
> > > [ 1738.184738][ T104] path_lookupat
> > > (/home/anders/src/kernel/next/fs/namei.c:2478 (discriminator 2))
> > > [ 1738.188268][ T104] filename_lookup
> > > (/home/anders/src/kernel/next/fs/namei.c:2508)
> > > [ 1738.191865][ T104] vfs_statx
> > > (/home/anders/src/kernel/next/fs/stat.c:238)
> > > [ 1738.196236][ T104] vfs_fstatat
> > > (/home/anders/src/kernel/next/fs/stat.c:276)
> > > [ 1738.200120][ T104] __do_sys_newfstatat
> > > (/home/anders/src/kernel/next/fs/stat.c:446)
> > > [ 1738.204095][ T104] __arm64_sys_newfstatat
> > > (/home/anders/src/kernel/next/fs/stat.c:440
> > > /home/anders/src/kernel/next/fs/stat.c:440)
> > > [ 1738.207676][ T104] el0_svc_common.constprop.0
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52
> > > /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> > > [ 1738.211820][ T104] do_el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> > > [ 1738.214815][ T104] el0_svc
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144
> > > /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> > > [ 1738.217709][ T104] el0t_64_sync_handler
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> > > [ 1738.221239][ T104] el0t_64_sync
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> > > [ 1738.224502][  T104]
> > > [ 1738.226090][  T104] 1 lock held by systemd-udevd/104:
> > > [ 1738.229747][ T104] #0: ffff000006681e08 (&root-
> > > >kernfs_iattr_rwsem){++++}-{3:3}, at: kernfs_iop_permission
> > > (/home/anders/src/kernel/next/fs/kernfs/inode.c:288)
> > > [ 1738.237504][  T104] irq event stamp: 108353
> > > [ 1738.240262][ T104] hardirqs last enabled at (108353):
> > > seqcount_lockdep_reader_access
> > > (/home/anders/src/kernel/next/include/linux/seqlock.h:105)
> > > [ 1738.247443][ T104] hardirqs last disabled at (108352):
> > > seqcount_lockdep_reader_access
> > > (/home/anders/src/kernel/next/include/linux/seqlock.h:104)
> > > [ 1738.254510][ T104] softirqs last enabled at (108326):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> > > [ 1738.262187][ T104] softirqs last disabled at (108324):
> > > fpsimd_restore_current_state
> > > (/home/anders/src/kernel/next/include/linux/bottom_half.h:20
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242
> > > /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> > > [ 1738.270239][  T104]
> > > [ 1738.272140][  T104] Reported by Kernel Concurrency Sanitizer on:
> > > [ 1738.285185][  T104] Hardware name: linux,dummy-virt (DT)
> > > [ 1738.288703][  T104]
> > > ==================================================================
> >
> > This looks just like the original except a different lock is being
> >
> > used now.
> >
> >
> > The link count can't be less than two if set_nlink() is called.
> >
> >
> > Maybe I am missing something but the directory count is changed only
> >
> > while holding the root->kernfs_iattr_rwsem so the value used by
> >
> > set_nlink() will not change during concurrent calls to
> > refresh_inode().
> >
> > Still looks like a false positive, I'll check the write operations
> > again just to be sure.
>
> I do see a problem with recent changes.
>
> I'll send this off to Greg after I've done some testing (primarily just
> compile and function).
>
> Here's a patch which describes what I found.
>
> Comments are, of course, welcome, ;)
>
> kernfs: fix missing kernfs_iattr_rwsem locking
>
> From: Ian Kent <raven@themaw.net>
>
> When the kernfs_iattr_rwsem was introduced a case was missed.
>
> The update of the kernfs directory node child count was also protected
> by the kernfs_rwsem and needs to be included in the change so that the
> child count (and so the inode n_link attribute) does not change while
> holding the rwsem for read.
>
> Fixes: 9caf696142 (kernfs: Introduce separate rwsem to protect inode
> attributes)
>
> Signed-off-by: Ian Kent <raven@themaw.net>
> Cc: Anders Roxell <anders.roxell@linaro.org>
> Cc: Imran Khan <imran.f.khan@oracle.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Eric Sandeen <sandeen@sandeen.net>

Looks good.

Acked-by: Miklos Szeredi <mszeredi@redhat.com>