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McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 18/20] doc: RCU scheduler spinlock rcu_read_unlock() restriction remains Date: Sun, 11 Nov 2018 11:56:43 -0800 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20181111195619.GA6958@linux.ibm.com> References: <20181111195619.GA6958@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18111119-0064-0000-0000-000003722F1A X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010028; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116023; UDB=6.00577118; IPR=6.00896123; MB=3.00024114; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-11 19:56:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111119-0065-0000-0000-00003B4D44E6 Message-Id: <20181111195645.9205-18-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-11_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 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=8.0.1-1807170000 definitions=main-1811110190 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Given RCU flavor consolidation, when rcu_read_unlock() is invoked with interrupts disabled, the reporting of the corresponding quiescent state is deferred until interrupts are re-enabled. There was therefore some hope that this would allow dropping the restriction against holding scheduler spinlocks across an rcu_read_unlock() without disabling interrupts across the entire corresponding RCU read-side critical section. Unfortunately, the need to quickly provide a quiescent state to expedited grace periods sometimes requires a call to raise_softirq() during rcu_read_unlock() execution. Because raise_softirq() can sometimes acquire the scheduler spinlocks, the restriction must remain in effect. This commit therefore updates the RCU requirements documentation accordingly. Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 44 ++++++++++++------- 1 file changed, 29 insertions(+), 15 deletions(-) diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index f74a2233865c..9fca73e03a98 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2475,23 +2475,37 @@ for context-switch-heavy CONFIG_NO_HZ_FULL=y workloads, but there is room for further improvement.

-In the past, it was forbidden to disable interrupts across an -rcu_read_unlock() unless that interrupt-disabled region -of code also included the matching rcu_read_lock(). -Violating this restriction could result in deadlocks involving the -scheduler's runqueue and priority-inheritance spinlocks. -This restriction was lifted when interrupt-disabled calls to -rcu_read_unlock() started deferring the reporting of -the resulting RCU-preempt quiescent state until the end of that +It is forbidden to hold any of scheduler's runqueue or priority-inheritance +spinlocks across an rcu_read_unlock() unless interrupts have been +disabled across the entire RCU read-side critical section, that is, +up to and including the matching rcu_read_lock(). +Violating this restriction can result in deadlocks involving these +scheduler spinlocks. +There was hope that this restriction might be lifted when interrupt-disabled +calls to rcu_read_unlock() started deferring the reporting of +the resulting RCU-preempt quiescent state until the end of the corresponding interrupts-disabled region. -This deferred reporting means that the scheduler's runqueue and -priority-inheritance locks cannot be held while reporting an RCU-preempt -quiescent state, which lifts the earlier restriction, at least from -a deadlock perspective. -Unfortunately, real-time systems using RCU priority boosting may +Unfortunately, timely reporting of the corresponding quiescent state +to expedited grace periods requires a call to raise_softirq(), +which can acquire these scheduler spinlocks. +In addition, real-time systems using RCU priority boosting need this restriction to remain in effect because deferred -quiescent-state reporting also defers deboosting, which in turn -degrades real-time latencies. +quiescent-state reporting would also defer deboosting, which in turn +would degrade real-time latencies. + +

+In theory, if a given RCU read-side critical section could be +guaranteed to be less than one second in duration, holding a scheduler +spinlock across that critical section's rcu_read_unlock() +would require only that preemption be disabled across the entire +RCU read-side critical section, not interrupts. +Unfortunately, given the possibility of vCPU preemption, long-running +interrupts, and so on, it is not possible in practice to guarantee +that a given RCU read-side critical section will complete in less than +one second. +Therefore, as noted above, if scheduler spinlocks are held across +a given call to rcu_read_unlock(), interrupts must be +disabled across the entire RCU read-side critical section.

Tracing and RCU

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