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Date:   Mon, 16 Sep 2019 04:03:55 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     kernel test robot <lkp@intel.com>
Cc:     "zhengbin (A)" <zhengbin13@huawei.com>, jack@suse.cz,
        akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
        "zhangyi (F)" <yi.zhang@huawei.com>, renxudong1@huawei.com,
        Hou Tao <houtao1@huawei.com>,
        Linus Torvalds <torvalds@linux-foundation.org>, lkp@01.org
Subject: Re: 266a9a8b41: WARNING:possible_recursive_locking_detected
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On Mon, Sep 16, 2019 at 03:58:27AM +0100, Al Viro wrote:
> On Mon, Sep 16, 2019 at 10:04:34AM +0800, kernel test robot wrote:
> > FYI, we noticed the following commit (built with gcc-7):
> > 
> > commit: 266a9a8b41803281e192151ae99779a7d50fc391 ("[PATCH] Re: Possible FS race condition between iterate_dir and d_alloc_parallel")
> > url: https://github.com/0day-ci/linux/commits/Al-Viro/Re-Possible-FS-race-condition-between-iterate_dir-and-d_alloc_parallel/20190915-052109
> > 
> > 
> > in testcase: rcutorture
> > with following parameters:
> > 
> > 	runtime: 300s
> > 	test: default
> > 	torture_type: srcu
> > 
> > test-description: rcutorture is rcutorture kernel module load/unload test.
> > test-url: https://www.kernel.org/doc/Documentation/RCU/torture.txt
> > 
> > 
> > on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 4G
> > 
> > caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> 
> False positive; dget() on child while holding ->d_lock on parent is OK.
> We could turn that into explicit spin_lock_nested() on child + increment of
> ->d_count under that, but this is a pointless pessimization.  Not sure
> what's the best way to tell lockdep to STFU here, but in this case it
> really ought to - locking order is correct.

Perhaps lockref_get_nested(struct lockref *lockref, unsigned int subclass)?
With s/spin_lock/spin_lock_nested/ in the body...