Re: [PATCH V10] cgroup/rstat: Avoid flushing if there is an ongoing root flush

[kernel test robot]

Hi Jesper,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tj-cgroup/for-next]
[also build test WARNING on axboe-block/for-next akpm-mm/mm-everything linus/master v6.11-rc7 next-20240909]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Jesper-Dangaard-Brouer/cgroup-rstat-Avoid-flushing-if-there-is-an-ongoing-root-flush/20240905-034221
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next
patch link:    https://lore.kernel.org/r/172547884995.206112.808619042206173396.stgit%40firesoul
patch subject: [PATCH V10] cgroup/rstat: Avoid flushing if there is an ongoing root flush
config: hexagon-randconfig-r121-20240910 (https://download.01.org/0day-ci/archive/20240910/202409101356.VXPFCu6l-lkp@xxxxxxxxx/config)
compiler: clang version 14.0.6 (https://github.com/llvm/llvm-project f28c006a5895fc0e329fe15fead81e37457cb1d1)
reproduce: (https://download.01.org/0day-ci/archive/20240910/202409101356.VXPFCu6l-lkp@xxxxxxxxx/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@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202409101356.VXPFCu6l-lkp@xxxxxxxxx/

sparse warnings: (new ones prefixed by >>)
>> kernel/cgroup/rstat.c:339:23: sparse: sparse: Using plain integer as NULL pointer
   kernel/cgroup/rstat.c:87:18: sparse: sparse: context imbalance in 'cgroup_rstat_updated' - different lock contexts for basic block
   kernel/cgroup/rstat.c:75:9: sparse: sparse: context imbalance in 'cgroup_rstat_updated_list' - wrong count at exit
   kernel/cgroup/rstat.c:320:13: sparse: sparse: context imbalance in 'cgroup_rstat_trylock_flusher' - wrong count at exit
   kernel/cgroup/rstat.c:300:34: sparse: sparse: context imbalance in 'cgroup_rstat_unlock_flusher' - unexpected unlock
   kernel/cgroup/rstat.c:373:9: sparse: sparse: context imbalance in 'cgroup_rstat_flush_locked' - different lock contexts for basic block
   kernel/cgroup/rstat.c:447:9: sparse: sparse: context imbalance in 'cgroup_rstat_flush_hold' - wrong count at exit
   kernel/cgroup/rstat.c:457:6: sparse: sparse: context imbalance in 'cgroup_rstat_flush_release' - wrong count at exit

vim +339 kernel/cgroup/rstat.c

   308	
   309	/**
   310	 * cgroup_rstat_trylock_flusher - Trylock that checks for on ongoing flusher
   311	 * @cgrp: target cgroup
   312	 *
   313	 * Function return value follow trylock semantics. Returning true when lock is
   314	 * obtained. Returning false when not locked and it detected flushing can be
   315	 * skipped as another ongoing flusher is taking care of the flush.
   316	 *
   317	 * For callers that depend on flush completing before returning a strict option
   318	 * is provided.
   319	 */
   320	static bool cgroup_rstat_trylock_flusher(struct cgroup *cgrp, bool strict)
   321	{
   322		struct cgroup *ongoing;
   323	
   324		if (strict)
   325			goto lock;
   326	
   327		/*
   328		 * Check if ongoing flusher is already taking care of this.  Descendant
   329		 * check is necessary due to cgroup v1 supporting multiple root's.
   330		 */
   331		ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
   332		if (ongoing && cgroup_is_descendant(cgrp, ongoing))
   333			return false;
   334	
   335		/* Grab right to be ongoing flusher */
   336		if (!ongoing && cgroup_is_root(cgrp)) {
   337			struct cgroup *old;
   338	
 > 339			old = cmpxchg(&cgrp_rstat_ongoing_flusher, NULL, cgrp);
   340			if (old) {
   341				/* Lost race for being ongoing flusher */
   342				if (cgroup_is_descendant(cgrp, old))
   343					return false;
   344			}
   345			/* Due to lock yield combined with strict mode record ID */
   346			WRITE_ONCE(cgrp_rstat_ongoing_flusher_ID, current);
   347		}
   348	lock:
   349		__cgroup_rstat_lock(cgrp, -1);
   350	
   351		return true;
   352	}
   353	

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