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Date:   Wed, 21 Dec 2022 14:34:28 +0100
From:   Anders Roxell <anders.roxell@linaro.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     Ian Kent <raven@themaw.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        elver@google.com, arnd@arndb.de
Subject: Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read
Message-ID: <20221221133428.GE69385@mutt>
References: <166606025456.13363.3829702374064563472.stgit@donald.themaw.net>
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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".


==================================================================
[ 1540.388669][  T123] BUG: KCSAN: data-race in set_nlink / set_nlink
[ 1540.392779][  T123] 
[ 1540.394302][  T123] write to 0xffff00000adcc3e4 of 4 bytes by task 126 on cpu 0:
[ 1540.398828][ T123] set_nlink (/home/anders/src/kernel/next/fs/inode.c:371) 
[ 1540.401609][ T123] kernfs_refresh_inode (/home/anders/src/kernel/next/fs/kernfs/inode.c:181) 
[ 1540.404925][ T123] kernfs_iop_getattr (/home/anders/src/kernel/next/fs/kernfs/inode.c:194) 
[ 1540.408088][ T123] vfs_getattr_nosec (/home/anders/src/kernel/next/fs/stat.c:133) 
[ 1540.411334][ T123] vfs_statx (/home/anders/src/kernel/next/fs/stat.c:170) 
[ 1540.414224][ T123] vfs_fstatat (/home/anders/src/kernel/next/fs/stat.c:276) 
[ 1540.417166][ T123] __do_sys_newfstatat (/home/anders/src/kernel/next/fs/stat.c:446) 
[ 1540.420539][ T123] __arm64_sys_newfstatat (/home/anders/src/kernel/next/fs/stat.c:440 /home/anders/src/kernel/next/fs/stat.c:440) 
[ 1540.424003][ T123] 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:142) 
[ 1540.427648][ T123] do_el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:197) 
[ 1540.430378][ T123] 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:142 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:638) 
[ 1540.433053][ T123] el0t_64_sync_handler (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:656) 
[ 1540.436421][ T123] el0t_64_sync (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:584) 
[ 1540.439303][  T123] 
[ 1540.440828][  T123] 1 lock held by systemd-udevd/126:
[ 1540.444055][ T123] #0: ffff00000609b960 (&root->kernfs_rwsem){++++}-{3:3}, at: kernfs_iop_getattr (/home/anders/src/kernel/next/fs/kernfs/inode.c:193) 
[ 1540.450699][  T123] irq event stamp: 185034
[ 1540.453373][ T123] hardirqs last enabled at (185034): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/mm/page_alloc.c:5302) 
[ 1540.460087][ T123] hardirqs last disabled at (185033): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:103 (discriminator 4)) 
[ 1540.466686][ T123] softirqs last enabled at (185001): 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:1780) 
[ 1540.473310][ T123] softirqs last disabled at (184999): 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:1773) 
[ 1540.479872][  T123] 
[ 1540.481406][  T123] read to 0xffff00000adcc3e4 of 4 bytes by task 123 on cpu 0:
[ 1540.485893][ T123] set_nlink (/home/anders/src/kernel/next/fs/inode.c:368) 
[ 1540.488663][ T123] kernfs_refresh_inode (/home/anders/src/kernel/next/fs/kernfs/inode.c:181) 
[ 1540.491961][ T123] kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:290) 
[ 1540.495260][ T123] inode_permission (/home/anders/src/kernel/next/fs/namei.c:458 /home/anders/src/kernel/next/fs/namei.c:525) 
[ 1540.498450][ T123] link_path_walk (/home/anders/src/kernel/next/fs/namei.c:1715 /home/anders/src/kernel/next/fs/namei.c:2262) 
[ 1540.501552][ T123] path_lookupat (/home/anders/src/kernel/next/fs/namei.c:2473 (discriminator 2)) 
[ 1540.504592][ T123] filename_lookup (/home/anders/src/kernel/next/fs/namei.c:2503) 
[ 1540.507740][ T123] user_path_at_empty (/home/anders/src/kernel/next/fs/namei.c:2876) 
[ 1540.511010][ T123] do_readlinkat (/home/anders/src/kernel/next/fs/stat.c:477) 
[ 1540.514097][ T123] __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) 
[ 1540.517598][ T123] 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:142) 
[ 1540.521319][ T123] do_el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:197) 
[ 1540.524125][ T123] 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:142 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:638) 
[ 1540.526795][ T123] el0t_64_sync_handler (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:656) 
[ 1540.530224][ T123] el0t_64_sync (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:584) 
[ 1540.533176][  T123] 
[ 1540.534710][  T123] 1 lock held by systemd-udevd/123:
[ 1540.537977][ T123] #0: ffff00000609b960 (&root->kernfs_rwsem){++++}-{3:3}, at: kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:289) 
[ 1540.544892][  T123] irq event stamp: 216564
[ 1540.547603][ T123] hardirqs last enabled at (216564): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/mm/page_alloc.c:5302) 
[ 1540.554368][ T123] hardirqs last disabled at (216563): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:103 (discriminator 4)) 
[ 1540.561107][ T123] softirqs last enabled at (216533): 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:1780) 
[ 1540.567833][ T123] softirqs last disabled at (216531): 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:1773) 
[ 1540.574496][  T123] 
[ 1540.576050][  T123] Reported by Kernel Concurrency Sanitizer on:
[ 1540.587925][  T123] Hardware name: linux,dummy-virt (DT)
[ 1540.591282][  T123] ==================================================================


Reverting this patch I don't see the data race anymore.

Cheers,
Anders