linux-kernel - Re: [PATCH 2/3] srcu: queue work without holding the lock

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Message-Id: <20170929011046.GW3521@linux.vnet.ibm.com>
Date:   Thu, 28 Sep 2017 18:10:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] srcu: queue work without holding the lock

On Thu, Sep 28, 2017 at 06:03:57PM +0200, Sebastian Andrzej Siewior wrote:
> On 2017-09-22 11:46:10 [-0700], Paul E. McKenney wrote:
> > On Fri, Sep 22, 2017 at 05:28:05PM +0200, Sebastian Andrzej Siewior wrote:
> > > On RT we can't invoke queue_delayed_work() within an atomic section
> > > (which is provided by raw_spin_lock_irqsave()).
> > > srcu_reschedule() invokes queue_delayed_work() outside of the
> > > raw_spin_lock_irq_rcu_node() section so this should be fine here, too.
> > > If the remaining callers of call_srcu() aren't atomic
> > > (spin_lock_irqsave() is fine) then this should work on RT, too.
> > 
> > Just to make sure I understand...   The problem is not the _irqsave,
> > but rather the raw_?
> 
> exactly. The _irqsave is translated into a sleeping lock on RT and does
> not matter. The raw_ ones stay as they are and queue_delayed_work() uses
> sleeping locks itself and this is where things fall apart.

OK, internally I could get rid of raw_ at the expense of some code bloat,
but in the call_srcu() case, the caller might well hold a raw_ lock.

Thoughts?

							Thanx, Paul