linux-kernel - [PATCH] srcu: Avoid local_irq_save() before acquiring spinlock

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Date:   Tue, 26 May 2020 15:41:34 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org
Subject: [PATCH] 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 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@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: rcu@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 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 0c71505f0e19c..9459bca58c380 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -764,14 +764,15 @@ 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.
@@ -851,9 +852,8 @@ 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));
-- 
2.27.0.rc0