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From: Joel Fernandes <joel@joelfernandes.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	rcu@vger.kernel.org
Subject: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access
Date: Wed, 20 May 2020 13:42:59 -0400
Message-ID: <20200520174259.GA247557@google.com> (raw)
In-Reply-To: <20200520120608.mwros5jurmidxxfv@linutronix.de>

Hi Sebastian,

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:
> > On Tue, May 19, 2020 at 10:19:07PM +0200, Sebastian Andrzej Siewior wrote:
> > 
> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > index 0c71505f0e19c..8d2b5f75145d7 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -25,6 +25,7 @@
> > >  #include <linux/delay.h>
> > >  #include <linux/module.h>
> > >  #include <linux/srcu.h>
> > > +#include <linux/locallock.h>
> > >  
> > >  #include "rcu.h"
> > >  #include "rcu_segcblist.h"
> > > @@ -735,6 +736,7 @@ static void srcu_flip(struct srcu_struct *ssp)
> > >  	smp_mb(); /* D */  /* Pairs with C. */
> > >  }
> > >  
> > > +static DEFINE_LOCAL_LOCK(sda_lock);
> > >  /*
> > >   * If SRCU is likely idle, return true, otherwise return false.
> > >   *
> > > @@ -765,13 +767,13 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
> > >  	unsigned long tlast;
> > >  
> > >  	/* If the local srcu_data structure has callbacks, not idle.  */
> > > -	local_irq_save(flags);
> > > +	local_lock_irqsave(sda_lock, flags);
> > >  	sdp = this_cpu_ptr(ssp->sda);
> > >  	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
> > > -		local_irq_restore(flags);
> > > +		local_unlock_irqrestore(sda_lock, flags);
> > >  		return false; /* Callbacks already present, so not idle. */
> > >  	}
> > > -	local_irq_restore(flags);
> > > +	local_unlock_irqrestore(sda_lock, flags);
> > 
> > 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.

For pointer stability, can we just use get_local_ptr() and put_local_ptr()
instead of adding an extra lock? This keeps the pointer stable while keeping
the section preemptible on -rt. And we already have a lock in rcu_data, I
prefer not to add another lock if possible.

I wrote a diff below with get_local_ptr() (just build tested). Does this
solve your issue?

> I remember Paul looked at that patch a few years ago and he said that
> that disabling interrupts here is important and matches the other part
> instance where the interrupts are disabled. Looking at it now, it seems
> that there is just pointer stability but I can't tell if
> rcu_segcblist_pend_cbs() needs more than just this.

Which 'other part' are you referring to? Your patch removed local_irq_save()
from other places as well right?

thanks,

 - Joel

---8<-----------------------

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 8ff71e5d0fe8b..5f49919205317 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -778,13 +778,17 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
 	unsigned long tlast;
 
 	/* If the local srcu_data structure has callbacks, not idle.  */
-	local_irq_save(flags);
-	sdp = this_cpu_ptr(ssp->sda);
+	sdp = get_local_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);
+		put_local_ptr(sdp);
 		return false; /* Callbacks already present, so not idle. */
 	}
-	local_irq_restore(flags);
+
+	spin_unlock_irqrestore_rcu_node(sdp, flags);
+	put_local_ptr(sdp);
 
 	/*
 	 * No local callbacks, so probabalistically probe global state.
@@ -864,9 +868,8 @@ 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 = get_local_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));
@@ -886,6 +889,8 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 	else if (needexp)
 		srcu_funnel_exp_start(ssp, sdp->mynode, s);
 	srcu_read_unlock(ssp, idx);
+
+	put_local_ptr(sdp);
 }
 
 /**

  parent reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200519201912.1564477-1-bigeasy@linutronix.de>
2020-05-19 20:19 ` Sebastian Andrzej Siewior
2020-05-20 10:24   ` Peter Zijlstra
2020-05-20 12:06     ` Sebastian Andrzej Siewior
2020-05-20 13:27       ` Peter Zijlstra
2020-05-20 17:42       ` Joel Fernandes [this message]
2020-05-20 18:28         ` Sebastian Andrzej Siewior
2020-05-20 18:35           ` Peter Zijlstra
2020-05-20 18:44             ` Joel Fernandes
2020-05-20 18:50               ` Uladzislau Rezki
2020-05-20 18:59           ` Joel Fernandes
2020-05-20 18:43       ` Paul E. McKenney
2020-05-22 15:12         ` Sebastian Andrzej Siewior
2020-05-22 17:39           ` Paul E. McKenney
2020-05-23 15:08             ` Sebastian Andrzej Siewior
2020-05-23 16:59               ` Paul E. McKenney
2020-05-24 19:03               ` Sebastian Andrzej Siewior
2020-05-25  3:27                 ` Paul E. McKenney
2020-05-26 13:41                   ` [PATCH] srcu: Avoid local_irq_save() before acquiring spinlock_t Sebastian Andrzej Siewior
2020-05-26 16:16                     ` Paul E. McKenney
2020-05-26 16:31                       ` Sebastian Andrzej Siewior

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