linux-kernel - Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu

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Message-ID: <20200520132755.GM317569@hirez.programming.kicks-ass.net>
Date:   Wed, 20 May 2020 15:27:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        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: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data
 access

On Wed, May 20, 2020 at 02:06:08PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-05-20 12:24:07 [+0200], Peter Zijlstra wrote:

> > Would it perhaps make sense to stick the local_lock in struct srcu_data ?
> 
> In that case we would need something for pointer stability before the
> lock is acquired.

Maybe I need sleep; but I think this will work.

 &x->foo = x + foo-offset

 this_cpu_ptr(x) = x + cpu-offset

 &this_cpu_ptr(x)->foo = (x + cpu-offset) + foo-offset
                       = (x + foo-offset) + cpu-offset
		       = this_cpu_ptr(&x->foo)

---
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -13,6 +13,7 @@
 
 #include <linux/rcu_node_tree.h>
 #include <linux/completion.h>
+#include <linux/locallock.h>
 
 struct srcu_node;
 struct srcu_struct;
@@ -22,6 +23,8 @@ struct srcu_struct;
  * to rcu_node.
  */
 struct srcu_data {
+	struct local_lock llock;
+
 	/* Read-side state. */
 	unsigned long srcu_lock_count[2];	/* Locks per CPU. */
 	unsigned long srcu_unlock_count[2];	/* Unlocks per CPU. */
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -778,13 +778,13 @@ static bool srcu_might_be_idle(struct sr
 	unsigned long tlast;
 
 	/* If the local srcu_data structure has callbacks, not idle.  */
-	local_irq_save(flags);
+	local_lock_irqsave(&ssp->sda->llock, flags);
 	sdp = this_cpu_ptr(ssp->sda);
 	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
-		local_irq_restore(flags);
+		local_unlock_irqrestore(&ssp->sda->llock, flags);
 		return false; /* Callbacks already present, so not idle. */
 	}
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&ssp->sda->llock, flags);
 
 	/*
 	 * No local callbacks, so probabalistically probe global state.
@@ -864,7 +864,7 @@ static void __call_srcu(struct srcu_stru
 	}
 	rhp->func = func;
 	idx = srcu_read_lock(ssp);
-	local_irq_save(flags);
+	local_lock_irqsave(&ssp->sda->llock, flags);
 	sdp = this_cpu_ptr(ssp->sda);
 	spin_lock_rcu_node(sdp);
 	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
@@ -880,7 +880,8 @@ static void __call_srcu(struct srcu_stru
 		sdp->srcu_gp_seq_needed_exp = s;
 		needexp = true;
 	}
-	spin_unlock_irqrestore_rcu_node(sdp, flags);
+	spin_unlock_rcu_node(sdp);
+	local_unlock_irqrestore(&ssp->sda->llock, flags);
 	if (needgp)
 		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
 	else if (needexp)