From mboxrd@z Thu Jan 1 00:00:00 1970 From: Greg KH Subject: Re: [PATCH v0] kernfs: Skip kernfs_put of parent from child node Date: Fri, 5 Apr 2019 14:10:20 +0200 Message-ID: <20190405121020.GA32479@kroah.com> References: <1554463267-30841-1-git-send-email-gkohli@codeaurora.org> <20190405113304.GA28420@kroah.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Gaurav Kohli Cc: tj@kernel.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Mukesh Ojha List-Id: linux-arm-msm@vger.kernel.org On Fri, Apr 05, 2019 at 05:13:00PM +0530, Gaurav Kohli wrote: > > On 4/5/2019 5:03 PM, Greg KH wrote: > > On Fri, Apr 05, 2019 at 04:51:07PM +0530, Gaurav Kohli wrote: > > > While adding kernfs node for child to the parent kernfs > > > node and when child node founds that parent kn count is > > > zero, then below comes like: > > > > > > WARNING: fs/kernfs/dir.c:494 kernfs_get+0x64/0x88 > > > > > > This indicates that parent is in kernfs_put path/ or already > > > freed, and if the child node keeps continue to > > > make new kernfs node, then there is chance of > > > below race for parent node: > > > > > > CPU0 CPU1 > > > //Parent node //child node > > > kernfs_put > > > atomic_dec_and_test(&kn->count) > > > //count is 0, so continue > > > kernfs_new_node(child) > > > kernfs_get(parent); > > > //increment parent count to 1 > > > //warning come as parent count is 0 > > > /* link in */ > > > kernfs_add_one(kn); > > > // this should fail as parent is > > > //in free path. > > > kernfs_put(child) > > > kmem_cache_free(parent) > > > kmem_cache_free(child) > > > kn = parent > > > atomic_dec_and_test(&kn->count)) > > > //this is 0 now, so release will > > > continue for parent. > > > kmem_cache_free(parent) > > > > > > To prevent this race, child simply has to decrement count of parent > > > kernfs node and keep continue the free path for itself. > > > > > > Signed-off-by: Gaurav Kohli > > > Signed-off-by: Mukesh Ojha > > > > > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c > > > index b84d635..d5a36e8 100644 > > > --- a/fs/kernfs/dir.c > > > +++ b/fs/kernfs/dir.c > > > @@ -515,7 +515,6 @@ void kernfs_put(struct kernfs_node *kn) > > > if (!kn || !atomic_dec_and_test(&kn->count)) > > > return; > > > root = kernfs_root(kn); > > > - repeat: > > > /* > > > * Moving/renaming is always done while holding reference. > > > * kn->parent won't change beneath us. > > > @@ -545,8 +544,8 @@ void kernfs_put(struct kernfs_node *kn) > > > kn = parent; > > > if (kn) { > > > - if (atomic_dec_and_test(&kn->count)) > > > - goto repeat; > > > + /* Parent may be on free path, so simply decrement the count */ > > That's the wrong indentation :( > > > > And how are you hitting this issue? What user of kernfs is causing > > this? > > Hi Greg, > > Thanks,  will fix comment indentation, seen during sys-executor running: > > We have only one instance , In logs below warning also came: > > WARNING: CPU: 4 kernel/msm-4.14/fs/kernfs/dir.c:494 kernfs_get+0x64/0x88 > > which indicated parent is in put path. > > [  160.125151] Disabling lock debugging due to kernel taint > [  160.130626] INFO: Allocated in __kernfs_new_node+0x8c/0x3c0 age=11 cpu=2 > pid=7098 > [  160.138314]     kmem_cache_alloc+0x358/0x388 > [  160.142445]     __kernfs_new_node+0x8c/0x3c0 > [  160.146590]     kernfs_new_node+0x80/0xc8 > [  160.150462]     kernfs_create_dir_ns+0x44/0xfc > [  160.154777]     sysfs_create_dir_ns+0xa8/0x130 > [  160.158416] CPU5: update max cpu_capacity 1024 > [  160.159085]     kobject_add_internal+0x278/0x650 > [  160.163567]     kobject_add_varg+0xe0/0x130 > [  160.167606]     kobject_add+0x15c/0x1d0 > [  160.168452] CPU5: update max cpu_capacity 780 > [  160.171287]     get_device_parent+0x2d0/0x34c > [  160.175510]     device_add+0x240/0xde0 > [  160.178371] CPU6: update max cpu_capacity 916 > [  160.179108]     input_register_device+0x5f4/0xa0c > [  160.183686]     uinput_ioctl_handler+0x1184/0x2198 > [  160.202436] INFO: Freed in kernfs_put+0x2c8/0x434 age=14 cpu=0 pid=7096 > [  160.209230]     kernfs_put+0x2c8/0x434 > [  160.212825]     kobject_del+0x50/0xcc > [  160.216332]     cleanup_glue_dir+0x124/0x16c > [  160.220456]     device_del+0x55c/0x5c8 > [  160.224047]     __input_unregister_device+0x274/0x2a8 > [  160.228974]     input_unregister_device+0x90/0xd0 > [  160.233553]     uinput_destroy_device+0x15c/0x1dc > [  160.238131]     uinput_release+0x44/0x5c > [  160.241898]     __fput+0x1f4/0x4e4 > [  160.245127]     ____fput+0x20/0x2c > > > during code review, I have found race between kernfs parent put call and > child get call. So this is a sysfs usage of this? Using input devices or cpu devices for the stress test? thanks, greg k-h