Commit bde50d8f authored May 26, 2020 by Sebastian Andrzej Siewior Committed by Paul E. McKenney Jun 29, 2020

srcu: Avoid local_irq_save() before acquiring spinlock_t



SRCU disables interrupts to get a stable per-CPU pointer and then
acquires the spinlock which is in the per-CPU data structure. The
release uses spin_unlock_irqrestore(). While this is correct on a non-RT
kernel, this conflicts with the RT semantics because the spinlock is
converted to a 'sleeping' spinlock. Sleeping locks can obviously not be
acquired with interrupts disabled.

Acquire the per-CPU pointer `ssp->sda' without disabling preemption and
then acquire the spinlock_t of the per-CPU data structure. The lock will
ensure that the data is consistent.

The added call to check_init_srcu_struct() is now needed because a
statically defined srcu_struct may remain uninitialized until this
point and the newly introduced locking operation requires an initialized
spinlock_t.

This change was tested for four hours with 8*SRCU-N and 8*SRCU-P without
causing any warnings.

Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: rcu@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

parent 7fef6cff

kernel/rcu/srcutree.c

+7 −7

Original line number	Diff line number	Diff line
		@@ -777,14 +777,15 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
		unsigned long t;
		unsigned long tlast;

		check_init_srcu_struct(ssp);
		/* If the local srcu_data structure has callbacks, not idle. */
		local_irq_save(flags);
		sdp = this_cpu_ptr(ssp->sda);
		sdp = raw_cpu_ptr(ssp->sda);
		spin_lock_irqsave_rcu_node(sdp, flags);
		if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
		local_irq_restore(flags);
		spin_unlock_irqrestore_rcu_node(sdp, flags);
		return false; /* Callbacks already present, so not idle. */
		}
		local_irq_restore(flags);
		spin_unlock_irqrestore_rcu_node(sdp, flags);

		/*
		* No local callbacks, so probabalistically probe global state.
		@@ -864,9 +865,8 @@ static void __call_srcu(struct srcu_struct ssp, struct rcu_head rhp,
		}
		rhp->func = func;
		idx = srcu_read_lock(ssp);
		local_irq_save(flags);
		sdp = this_cpu_ptr(ssp->sda);
		spin_lock_rcu_node(sdp);
		sdp = raw_cpu_ptr(ssp->sda);
		spin_lock_irqsave_rcu_node(sdp, flags);
		rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
		rcu_segcblist_advance(&sdp->srcu_cblist,
		rcu_seq_current(&ssp->srcu_gp_seq));

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