fs/super.c:1785:15: sparse: sparse: Using plain integer as NULL pointer

[kernel test robot <lkp@intel.com>]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   33cc938e65a98f1d29d0a18403dbbee050dcad9a
commit: 439bc39b3cf0014b1b75075812f7ef0f8baa9674 fs: move sb_init_dio_done_wq out of direct-io.c
date:   10 months ago
config: hexagon-randconfig-r121-20231117 (https://download.01.org/0day-ci/archive/20231205/202312050045.w3LqhNoX-lkp@intel.com/config)
compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project.git 4a5ac14ee968ff0ad5d2cc1ffa0299048db4c88a)
reproduce: (https://download.01.org/0day-ci/archive/20231205/202312050045.w3LqhNoX-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202312050045.w3LqhNoX-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
>> fs/super.c:1785:15: sparse: sparse: Using plain integer as NULL pointer

vim +1785 fs/super.c

  1767	
  1768	/*
  1769	 * Create workqueue for deferred direct IO completions. We allocate the
  1770	 * workqueue when it's first needed. This avoids creating workqueue for
  1771	 * filesystems that don't need it and also allows us to create the workqueue
  1772	 * late enough so the we can include s_id in the name of the workqueue.
  1773	 */
  1774	int sb_init_dio_done_wq(struct super_block *sb)
  1775	{
  1776		struct workqueue_struct *old;
  1777		struct workqueue_struct *wq = alloc_workqueue("dio/%s",
  1778							      WQ_MEM_RECLAIM, 0,
  1779							      sb->s_id);
  1780		if (!wq)
  1781			return -ENOMEM;
  1782		/*
  1783		 * This has to be atomic as more DIOs can race to create the workqueue
  1784		 */
> 1785		old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);

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