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* [tip: core/rcu] srcu: Avoid local_irq_save() before acquiring spinlock_t
@ 2020-07-31  9:22 tip-bot2 for Sebastian Andrzej Siewior
  0 siblings, 0 replies; only message in thread
From: tip-bot2 for Sebastian Andrzej Siewior @ 2020-07-31  9:22 UTC (permalink / raw)
  To: linux-tip-commits
  Cc: Lai Jiangshan, Paul E. McKenney, Josh Triplett, Steven Rostedt,
	Mathieu Desnoyers, rcu, Sebastian Andrzej Siewior, x86, LKML

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     bde50d8ff83e4ce9e576f7c5ba1edb48a3610a5b
Gitweb:        https://git.kernel.org/tip/bde50d8ff83e4ce9e576f7c5ba1edb48a3610a5b
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Tue, 26 May 2020 15:41:34 +02:00
Committer:     Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Mon, 29 Jun 2020 12:01:22 -07:00

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>
---
 kernel/rcu/srcutree.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 8ff71e5..c100acf 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -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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