rcu-tasks: Fix boot-time RCU tasks debug-only deadlock

rcu-tasks: Fix boot-time RCU tasks debug-only deadlock

In kernels built with CONFIG_PROVE_RCU=y (for example, lockdep kernels),
the following sequence of events can occur:

o	rcu_init_tasks_generic() is invoked just before init is spawned.
	It invokes rcu_spawn_tasks_kthread() and friends.

o	rcu_spawn_tasks_kthread() invokes rcu_spawn_tasks_kthread_generic(),
	which uses kthread_run() to create the needed kthread.

o	Control returns to rcu_init_tasks_generic(), which, because this
	is a CONFIG_PROVE_RCU=y kernel, invokes the version of the
	rcu_tasks_initiate_self_tests() function that actually does
	something, including invoking synchronize_rcu_tasks(), which
	in turn invokes synchronize_rcu_tasks_generic().

o	synchronize_rcu_tasks_generic() sees that the ->kthread_ptr is
	still NULL, because the newly spawned kthread has not yet
	started.

o	The new kthread starts, preempting synchronize_rcu_tasks_generic()
	just after its check.  This kthread invokes rcu_tasks_one_gp(),
	which acquires ->tasks_gp_mutex, and, seeing no work, blocks
	in rcuwait_wait_event().  Note that this step requires either
	a preemptible kernel or a fault-injection-style sleep at the
	beginning of mutex_lock().

o	synchronize_rcu_tasks_generic() resumes and invokes rcu_tasks_one_gp().

o	rcu_tasks_one_gp() attempts to acquire ->tasks_gp_mutex, which
	is still held by the newly spawned kthread's rcu_tasks_one_gp()
	function.  Deadlock.

Because the only reason for ->tasks_gp_mutex is to handle pre-kthread
synchronous grace periods, this commit avoids this deadlock by having
rcu_tasks_one_gp() momentarily release ->tasks_gp_mutex while invoking
rcuwait_wait_event().  This allows the call to rcu_tasks_one_gp() from
synchronize_rcu_tasks_generic() proceed.

Note that it is not necessary to release the mutex anywhere else in
rcu_tasks_one_gp() because rcuwait_wait_event() is the only function
that can block indefinitely.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Roy Hopkins <rhopkins@suse.de>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Roy Hopkins <rhopkins@suse.de>