[linux-next:master 14074/14219] include/linux/rcupdate.h:651:9: sparse: sparse: context imbalance in 'dump_tasks' - unexpected unlock

[linux-next:master 14074/14219] include/linux/rcupdate.h:651:9: sparse: sparse: context imbalance in 'dump_tasks' - unexpected unlock

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
head:   8b0eed9f6e36a5488967b0acc51444d658dd711b
commit: cc2ceda020c2d0ee9ea839fc0f4239a3f7055c95 [14074/14219] mm, oom: avoid printk() iteration under RCU
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.1-rc1-37-gd466a02-dirty
        git checkout cc2ceda020c2d0ee9ea839fc0f4239a3f7055c95
        make ARCH=x86_64 allmodconfig
        make C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__'

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>


sparse warnings: (new ones prefixed by >>)

   include/linux/rcupdate.h:651:9: sparse: sparse: context imbalance in 'find_lock_task_mm' - wrong count at exit
   include/linux/sched/mm.h:166:37: sparse: sparse: dereference of noderef expression
   mm/oom_kill.c:219:28: sparse: sparse: context imbalance in 'oom_badness' - unexpected unlock
>> include/linux/rcupdate.h:651:9: sparse: sparse: context imbalance in 'dump_tasks' - unexpected unlock
   include/linux/rcupdate.h:651:9: sparse: sparse: context imbalance in '__oom_kill_process' - unexpected unlock

vim +/dump_tasks +651 include/linux/rcupdate.h

^1da177e4c3f41 Linus Torvalds      2005-04-16  603  
^1da177e4c3f41 Linus Torvalds      2005-04-16  604  /*
^1da177e4c3f41 Linus Torvalds      2005-04-16  605   * So where is rcu_write_lock()?  It does not exist, as there is no
^1da177e4c3f41 Linus Torvalds      2005-04-16  606   * way for writers to lock out RCU readers.  This is a feature, not
^1da177e4c3f41 Linus Torvalds      2005-04-16  607   * a bug -- this property is what provides RCU's performance benefits.
^1da177e4c3f41 Linus Torvalds      2005-04-16  608   * Of course, writers must coordinate with each other.  The normal
^1da177e4c3f41 Linus Torvalds      2005-04-16  609   * spinlock primitives work well for this, but any other technique may be
^1da177e4c3f41 Linus Torvalds      2005-04-16  610   * used as well.  RCU does not care how the writers keep out of each
^1da177e4c3f41 Linus Torvalds      2005-04-16  611   * others' way, as long as they do so.
^1da177e4c3f41 Linus Torvalds      2005-04-16  612   */
3d76c082907e8f Paul E. McKenney    2009-09-28  613  
3d76c082907e8f Paul E. McKenney    2009-09-28  614  /**
ca5ecddfa8fcbd Paul E. McKenney    2010-04-28  615   * rcu_read_unlock() - marks the end of an RCU read-side critical section.
3d76c082907e8f Paul E. McKenney    2009-09-28  616   *
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  617   * In most situations, rcu_read_unlock() is immune from deadlock.
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  618   * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  619   * is responsible for deboosting, which it does via rt_mutex_unlock().
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  620   * Unfortunately, this function acquires the scheduler's runqueue and
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  621   * priority-inheritance spinlocks.  This means that deadlock could result
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  622   * if the caller of rcu_read_unlock() already holds one of these locks or
ec84b27f9b3b56 Anna-Maria Gleixner 2018-05-25  623   * any lock that is ever acquired while holding them.
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  624   *
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  625   * That said, RCU readers are never priority boosted unless they were
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  626   * preempted.  Therefore, one way to avoid deadlock is to make sure
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  627   * that preemption never happens within any RCU read-side critical
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  628   * section whose outermost rcu_read_unlock() is called with one of
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  629   * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  630   * a number of ways, for example, by invoking preempt_disable() before
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  631   * critical section's outermost rcu_read_lock().
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  632   *
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  633   * Given that the set of locks acquired by rt_mutex_unlock() might change
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  634   * at any time, a somewhat more future-proofed approach is to make sure
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  635   * that that preemption never happens within any RCU read-side critical
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  636   * section whose outermost rcu_read_unlock() is called with irqs disabled.
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  637   * This approach relies on the fact that rt_mutex_unlock() currently only
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  638   * acquires irq-disabled locks.
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  639   *
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  640   * The second of these two approaches is best in most situations,
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  641   * however, the first approach can also be useful, at least to those
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  642   * developers willing to keep abreast of the set of locks acquired by
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  643   * rt_mutex_unlock().
f27bc4873fa8b7 Paul E. McKenney    2014-05-04  644   *
3d76c082907e8f Paul E. McKenney    2009-09-28  645   * See rcu_read_lock() for more information.
3d76c082907e8f Paul E. McKenney    2009-09-28  646   */
bc33f24bdca8b6 Paul E. McKenney    2009-08-22  647  static inline void rcu_read_unlock(void)
bc33f24bdca8b6 Paul E. McKenney    2009-08-22  648  {
f78f5b90c4ffa5 Paul E. McKenney    2015-06-18  649  	RCU_LOCKDEP_WARN(!rcu_is_watching(),
bde23c6892878e Heiko Carstens      2012-02-01  650  			 "rcu_read_unlock() used illegally while idle");
bc33f24bdca8b6 Paul E. McKenney    2009-08-22 @651  	__release(RCU);
bc33f24bdca8b6 Paul E. McKenney    2009-08-22  652  	__rcu_read_unlock();
d24209bb689e2c Paul E. McKenney    2015-01-21  653  	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
bc33f24bdca8b6 Paul E. McKenney    2009-08-22  654  }
^1da177e4c3f41 Linus Torvalds      2005-04-16  655  

:::::: The code at line 651 was first introduced by commit
:::::: bc33f24bdca8b6e97376e3a182ab69e6cdefa989 rcu: Consolidate sparse and lockdep declarations in include/linux/rcupdate.h

:::::: TO: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
:::::: CC: Ingo Molnar <mingo@elte.hu>

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