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McKenney" To: Joel Fernandes Cc: Sebastian Andrzej Siewior , 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: <20190828155120.GQ26530@linux.ibm.com> Reply-To: paulmck@kernel.org References: <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> <20190828125426.GO26530@linux.ibm.com> <20190828131433.3gi4debho5zc7hgc@linutronix.de> <20190828135938.GA230957@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190828135938.GA230957@google.com> 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_07:,, 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-1908280160 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 09:59:38AM -0400, Joel Fernandes wrote: > On Wed, Aug 28, 2019 at 03:14:33PM +0200, Sebastian Andrzej Siewior wrote: > > On 2019-08-28 05:54:26 [-0700], Paul E. McKenney wrote: > > > 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: > > > > > 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? > > > > I would like to keep that check and that is why we have: > > > > | #if defined(CONFIG_PREEMPT_RT_FULL) > > | sleeping_l = t->sleeping_lock; > > | #endif > > | WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0 && !sleeping_l); > > > > in -RT and ->sleeping_lock is that counter that is incremented in > > spin_lock(). And the only reason why sleeping_lock_inc() was used in the > > patch was to disable _this_ warning. > > Makes sense, Sebastian. > > Paul, you meant "!" in front of the IS_ENABLED right in your code snippet right? > > The other issue with: > WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && !preempt && t->rcu_read_lock_nesting > 0); > > .. could be that, the warning will be disabled for -rt entirely, not just for > sleeping locks. And we probably do want to keep this warning for the cases in > -rt where we are blocking but it is not a sleeping lock. Right? Yes, my code was missing a "!", but I prefer Sebastian's and Scott's approach to mine anyway. ;-) Thanx, Paul