From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: "Paul E. McKenney" <paulmck@kernel.org>
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>,
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: Fri, 22 May 2020 17:12:55 +0200 [thread overview]
Message-ID: <20200522151255.rtqnuk2cl3dpruou@linutronix.de> (raw)
In-Reply-To: <20200520184345.GU2869@paulmck-ThinkPad-P72>
On 2020-05-20 11:43:45 [-0700], Paul E. McKenney wrote:
>
> Yes, that CPU's rcu_segcblist structure does need mutual exclusion in
> this case. This is because rcu_segcblist_pend_cbs() looks not just
> at the ->tails[] pointer, but also at the pointer referenced by the
> ->tails[] pointer. This last pointer is in an rcu_head structure, and
> not just any rcu_head structure, but one that is ready to be invoked.
> So this callback could vanish into the freelist (or worse) at any time.
> But callback invocation runs on the CPU that enqueued the callbacks
> (as long as that CPU remains online, anyway), so disabling interrupts
> suffices in mainline.
>
> Now, we could have srcu_might_be_idle() instead acquire the sdp->lock
> to protect the structure.
Joel suggested that.
> What would be really nice is a primitive that acquires such a per-CPU
> lock and remains executing on that CPU, whether by the graces of
> preempt_disable(), local_irq_save(), migrate_disable(), or what have you.
It depends on what is required. migrate_disable() would limit you to
executing one CPU but would allow preemption. You would need a lock to
ensure exclusive access to the data structure. preempt_disable() /
local_irq_save() guarantee more than that.
Looking at the two call-sites there is no damage there is a CPU
migration after obtaining the per-CPU pointer. There could be a
CPU-migration before and after the pointer has been obtained so the code
before and after this function can not make any assumptions.
Would something like this work: ?
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
--- 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));
That check_init_srcu_struct() is needed, because otherwise:
| BUG: spinlock bad magic on CPU#2, swapper/0/1
| lock: 0xffff88803ed28ac0, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
| CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.7.0-rc6+ #81
| Call Trace:
| dump_stack+0x71/0xa0
| do_raw_spin_lock+0x6c/0xb0
| _raw_spin_lock_irqsave+0x33/0x40
| synchronize_srcu+0x24/0xc9
| wakeup_source_remove+0x4d/0x70
| wakeup_source_unregister.part.0+0x9/0x40
| device_wakeup_enable+0x99/0xc0
I'm not sure if there should be an explicit init of `wakeup_srcu' or if
an srcu function (like call_srcu()) is supposed to do it.
> Thanx, Paul
Sebastian
next prev parent reply other threads:[~2020-05-22 15:13 UTC|newest]
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 ` [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access 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
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 [this message]
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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