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McKenney" To: Sebastian Andrzej Siewior Cc: Joel Fernandes , Scott Wood , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Juri Lelli , Clark Williams Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep Message-ID: <20190828125426.GO26530@linux.ibm.com> Reply-To: paulmck@kernel.org References: <20190821231906.4224-1-swood@redhat.com> <20190821231906.4224-3-swood@redhat.com> <20190823162024.47t7br6ecfclzgkw@linutronix.de> <433936e4c720e6b81f9b297fefaa592fd8a961ad.camel@redhat.com> <20190824031014.GB2731@google.com> <20190826152523.dcjbsgyyir4zjdol@linutronix.de> <20190826162945.GE28441@linux.ibm.com> <20190827092333.jp3darw7teyyw67g@linutronix.de> <20190827155306.GF26530@linux.ibm.com> <20190828092739.46mrffvzjv6v3de5@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190828092739.46mrffvzjv6v3de5@linutronix.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-28_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908280138 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 28, 2019 at 11:27:39AM +0200, Sebastian Andrzej Siewior wrote: > On 2019-08-27 08:53:06 [-0700], Paul E. McKenney wrote: > > > > On the other hand, within a PREEMPT=n kernel, the call to schedule() > > > > would split even an rcu_read_lock() critical section. Which is why I > > > > asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops > > > > in !PREEMPT_RT_BASE kernels. We would after all want the usual lockdep > > > > complaints in that case. > > > > > > sleeping_lock_inc() +dec() is only RT specific. It is part of RT's > > > spin_lock() implementation and used by RCU (rcu_note_context_switch()) > > > to not complain if invoked within a critical section. > > > > Then this is being called when we have something like this, correct? > > > > DEFINE_SPINLOCK(mylock); // As opposed to DEFINE_RAW_SPINLOCK(). > > > > ... > > > > rcu_read_lock(); > > do_something(); > > spin_lock(&mylock); // Can block in -rt, thus needs sleeping_lock_inc() > > ... > > rcu_read_unlock(); > > > > Without sleeping_lock_inc(), lockdep would complain about a voluntary > > schedule within an RCU read-side critical section. But in -rt, voluntary > > schedules due to sleeping on a "spinlock" are OK. > > > > Am I understanding this correctly? > > Everything perfect except that it is not lockdep complaining but the > WARN_ON_ONCE() in rcu_note_context_switch(). This one, right? WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0); Another approach would be to change that WARN_ON_ONCE(). This fix might be too extreme, as it would suppress other issues: WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && !preempt && t->rcu_read_lock_nesting > 0); But maybe what is happening under the covers is that preempt is being set when sleeping on a spinlock. Is that the case? Thanx, Paul