sched/api: Introduce task_rcu_dereference() and try_get_task_struct() (150593bf) · Commits · 戴 / test

include/linux/sched.h

+3 −0

Original line number	Original line	Diff line number	Diff line
	@@ -2139,6 +2139,9 @@ static inline void put_task_struct(struct task_struct *t)
	__put_task_struct(t);		__put_task_struct(t);
	}		}

			struct task_struct task_rcu_dereference(struct task_struct *ptask);
			struct task_struct try_get_task_struct(struct task_struct *ptask);

	#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN		#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
	extern void task_cputime(struct task_struct *t,		extern void task_cputime(struct task_struct *t,
	cputime_t utime, cputime_t stime);		cputime_t utime, cputime_t stime);

kernel/exit.c

+76 −0

Original line number	Original line	Diff line number	Diff line
	@@ -210,6 +210,82 @@ repeat:
	goto repeat;		goto repeat;
	}		}

			/*
			* Note that if this function returns a valid task_struct pointer (!NULL)
			* task->usage must remain >0 for the duration of the RCU critical section.
			*/
			struct task_struct task_rcu_dereference(struct task_struct *ptask)
			{
			struct sighand_struct *sighand;
			struct task_struct *task;

			/*
			* We need to verify that release_task() was not called and thus
			* delayed_put_task_struct() can't run and drop the last reference
			* before rcu_read_unlock(). We check task->sighand != NULL,
			* but we can read the already freed and reused memory.
			*/
			retry:
			task = rcu_dereference(*ptask);
			if (!task)
			return NULL;

			probe_kernel_address(&task->sighand, sighand);

			/*
			* Pairs with atomic_dec_and_test() in put_task_struct(). If this task
			* was already freed we can not miss the preceding update of this
			* pointer.
			*/
			smp_rmb();
			if (unlikely(task != READ_ONCE(*ptask)))
			goto retry;

			/*
			* We've re-checked that "task == *ptask", now we have two different
			* cases:
			*
			* 1. This is actually the same task/task_struct. In this case
			* sighand != NULL tells us it is still alive.
			*
			* 2. This is another task which got the same memory for task_struct.
			* We can't know this of course, and we can not trust
			* sighand != NULL.
			*
			* In this case we actually return a random value, but this is
			* correct.
			*
			* If we return NULL - we can pretend that we actually noticed that
			* *ptask was updated when the previous task has exited. Or pretend
			* that probe_slab_address(&sighand) reads NULL.
			*
			* If we return the new task (because sighand is not NULL for any
			* reason) - this is fine too. This (new) task can't go away before
			* another gp pass.
			*
			* And note: We could even eliminate the false positive if re-read
			* task->sighand once again to avoid the falsely NULL. But this case
			* is very unlikely so we don't care.
			*/
			if (!sighand)
			return NULL;

			return task;
			}

			struct task_struct try_get_task_struct(struct task_struct *ptask)
			{
			struct task_struct *task;

			rcu_read_lock();
			task = task_rcu_dereference(ptask);
			if (task)
			get_task_struct(task);
			rcu_read_unlock();

			return task;
			}

	/*		/*
	* Determine if a process group is "orphaned", according to the POSIX		* Determine if a process group is "orphaned", according to the POSIX
	* definition in 2.2.2.52. Orphaned process groups are not to be affected		* definition in 2.2.2.52. Orphaned process groups are not to be affected

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