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McKenney" To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers , Steven Rostedt Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption Reply-To: paulmck@linux.ibm.com References: <20181014212955.95267-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181014212955.95267-1-joel@joelfernandes.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18101423-0060-0000-0000-000002C095D0 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009879; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000268; SDB=6.01102741; UDB=6.00570737; IPR=6.00882792; MB=3.00023757; MTD=3.00000008; XFM=3.00000015; UTC=2018-10-14 23:17:32 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18101423-0061-0000-0000-000046D8FB30 Message-Id: <20181014231731.GN2674@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-10-14_18:,, 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=8.0.1-1807170000 definitions=main-1810140221 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 14, 2018 at 02:29:55PM -0700, Joel Fernandes (Google) wrote: > The Requirements.html document says "Disabling Preemption Does Not Block > Grace Periods". However this is no longer true with the RCU > consolidation. Lets remove the obsolete (non-)requirement entirely. > > Signed-off-by: Joel Fernandes (Google) Good catch, queued, thank you! Thanx, Paul > --- > .../RCU/Design/Requirements/Requirements.html | 50 ------------------- > 1 file changed, 50 deletions(-) > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html > index 7efc1c1da7af..4fae55056c1d 100644 > --- a/Documentation/RCU/Design/Requirements/Requirements.html > +++ b/Documentation/RCU/Design/Requirements/Requirements.html > @@ -900,8 +900,6 @@ Except where otherwise noted, these non-guarantees were premeditated. > Grace Periods Don't Partition Read-Side Critical Sections >
  • > Read-Side Critical Sections Don't Partition Grace Periods > -
  • > - Disabling Preemption Does Not Block Grace Periods > > >

    Readers Impose Minimal Ordering

    > @@ -1259,54 +1257,6 @@ of RCU grace periods. >   > > > -

    > -Disabling Preemption Does Not Block Grace Periods

    > - > -

    > -There was a time when disabling preemption on any given CPU would block > -subsequent grace periods. > -However, this was an accident of implementation and is not a requirement. > -And in the current Linux-kernel implementation, disabling preemption > -on a given CPU in fact does not block grace periods, as Oleg Nesterov > -demonstrated. > - > -

    > -If you need a preempt-disable region to block grace periods, you need to add > -rcu_read_lock() and rcu_read_unlock(), for example > -as follows: > - > -

    > -
    > - 1 preempt_disable();
> - 2 rcu_read_lock();
> - 3 do_something();
> - 4 rcu_read_unlock();
> - 5 preempt_enable();
> - 6
> - 7 /* Spinlocks implicitly disable preemption. */
> - 8 spin_lock(&mylock);
> - 9 rcu_read_lock();
> -10 do_something();
> -11 rcu_read_unlock();
> -12 spin_unlock(&mylock);
> -
    > -
    > - > -

    > -In theory, you could enter the RCU read-side critical section first, > -but it is more efficient to keep the entire RCU read-side critical > -section contained in the preempt-disable region as shown above. > -Of course, RCU read-side critical sections that extend outside of > -preempt-disable regions will work correctly, but such critical sections > -can be preempted, which forces rcu_read_unlock() to do > -more work. > -And no, this is not an invitation to enclose all of your RCU > -read-side critical sections within preempt-disable regions, because > -doing so would degrade real-time response. > - > -

    > -This non-requirement appeared with preemptible RCU. > - >

    Parallelism Facts of Life

    > >

    > -- > 2.19.0.605.g01d371f741-goog >