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

Paul E. McKenney
2020-05-22T17:39:56Z

On Fri, May 22, 2020 at 05:12:55PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

Good!

> > 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: ?

It looks good to me, but I have not yet tested it. (Happy to let you
take the first crack at rcutorture in any case, scenarios SRCU-P and
SRCU-N.)

> 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.

It is fine. Beforehand, that check_init_srcu_struct() would have been
invoked very shortly thereafter from __call_srcu(), and there is no
instead harm invoking it a few microseconds earlier. ;-)

Thanx, Paul