linux-kernel - Re: rcu: Add might_sleep() check to synchronize

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Date:   Fri, 23 Mar 2018 21:21:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: rcu: Add might_sleep() check to synchronize_rcu()

On Fri, 23 Mar 2018 15:57:04 -0700
Joel Fernandes <joelaf@...gle.com> wrote:

> > diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> > index 194a7483bb93..857b494bee29 100644
> > --- a/net/l2tp/l2tp_core.c
> > +++ b/net/l2tp/l2tp_core.c
> > @@ -1677,6 +1677,8 @@ void __l2tp_session_unhash(struct l2tp_session *session)
> >  {
> >         struct l2tp_tunnel *tunnel = session->tunnel;
> >
> > +       might_sleep();
> > +
> >         /* Remove the session from core hashes */
> >         if (tunnel) {
> >                 /* Remove from the per-tunnel hash */  
> 
> Thanks Thomas and Steven, also shouldn't this code be calling
> synchronize_rcu_bh instead of synchronize_rcu, to complement the
> rcu_read_lock_bh? In which situations would you call one versus the
> other?

Probably, as the comment above rcu_read_lock_bh is:

 * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
 *
 * This is equivalent of rcu_read_lock(), but to be used when updates
 * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
 * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
 * softirq handler to be a quiescent state, a process in RCU read-side
 * critical section must be protected by disabling softirqs.

It appears that the reason to use rcu_read_lock_bh() is if you are
calling synchronize_rcu_bh(). Otherwise, one could just be using
straight rcu_read_lock().

-- Steve