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The purpose of exit_rcu() is to handle cases where buggy code causes a task to exit within an RCU read-side critical section. It currently does that in the case where said RCU read-side critical section was preempted at least once, but fails to handle cases where preemption did not occur. This case needs to be handled because otherwise the final context switch away from the exiting task will incorrectly behave as if task exit were instead a preemption of an RCU read-side critical section, and will therefore queue the exiting task. The exiting task will have exited, and thus won't ever execute rcu_read_unlock(), which means that it will remain queued forever, blocking all subsequent grace periods, and eventually resulting in OOM. Although this is arguably better than letting grace periods proceed and having a later rcu_read_unlock() access the now-freed task structure that once belonged to the exiting tasks, it would obviously be better to correctly handle this case. This commit therefore sets ->rcu_read_lock_nesting to 1 in that case, so that the subsequence call to __rcu_read_unlock() causes the exiting task to exit its dangling RCU read-side critical section. Note that deferred quiescent states need not be considered. The reason is that removing the task from the ->blkd_tasks[] list in the call to rcu_preempt_deferred_qs() handles the per-task component of any deferred quiescent state, and all other components of any deferred quiescent state are associated with the CPU, which isn't going anywhere until some later CPU-hotplug operation, which will report any remaining deferred quiescent states from within the rcu_report_dead() function. Note also that negative values of ->rcu_read_lock_nesting need not be considered. First, these won't show up in exit_rcu() unless there is a serious bug in RCU, and second, setting ->rcu_read_lock_nesting sets the state so that the RCU read-side critical section will be exited normally. Again, this code has no effect unless there has been some prior bug that prevents a task from leaving an RCU read-side critical section before exiting. Furthermore, there have been no reports of the bug fixed by this commit appearing in production. This commit is therefore absolutely -not- recommended for backporting to -stable. Reported-by: ABHISHEK DUBEY <dabhishek@iisc.ac.in> Reported-by: BHARATH Y MOURYA <bharathm@iisc.ac.in> Reported-by: Aravinda Prasad <aravinda@iisc.ac.in> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Tested-by: ABHISHEK DUBEY <dabhishek@iisc.ac.in> |
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| .. | ||
| Kconfig | ||
| Kconfig.debug | ||
| Makefile | ||
| rcu_segcblist.c | ||
| rcu_segcblist.h | ||
| rcu.h | ||
| rcuperf.c | ||
| rcutorture.c | ||
| srcutiny.c | ||
| srcutree.c | ||
| sync.c | ||
| tiny.c | ||
| tree_exp.h | ||
| tree_plugin.h | ||
| tree.c | ||
| tree.h | ||
| update.c | ||