LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Joel Fernandes <joel@joelfernandes.org>
To: Byungchul Park <byungchul.park@lge.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	jiangshanlai@gmail.com, dipankar@in.ibm.com,
	mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
	rostedt@goodmis.org, luto@kernel.org, kernel-team@lge.com
Subject: Re: [PATCH tip/core/rcu 06/19] rcu: Add warning to detect half-interrupts
Date: Fri, 15 Mar 2019 09:46:20 -0400
Message-ID: <20190315134620.GE3378@google.com> (raw)
In-Reply-To: <eea72fb5-c25d-746a-fa48-6d04c8d5d1af@lge.com>

On Fri, Mar 15, 2019 at 04:44:52PM +0900, Byungchul Park wrote:
> On 03/15/2019 04:31 PM, Byungchul Park wrote:
> > On Mon, Mar 11, 2019 at 09:39:39AM -0400, Joel Fernandes wrote:
> > > On Wed, Aug 29, 2018 at 03:20:34PM -0700, Paul E. McKenney wrote:
> > > > RCU's dyntick-idle code is written to tolerate half-interrupts, that it,
> > > > either an interrupt that invokes rcu_irq_enter() but never invokes the
> > > > corresponding rcu_irq_exit() on the one hand, or an interrupt that never
> > > > invokes rcu_irq_enter() but does invoke the "corresponding" rcu_irq_exit()
> > > > on the other.  These things really did happen at one time, as evidenced
> > > > by this ca-2011 LKML post:
> > > > 
> > > > http://lkml.kernel.org/r/20111014170019.GE2428@linux.vnet.ibm.com
> > > > 
> > > > The reason why RCU tolerates half-interrupts is that usermode helpers
> > > > used exceptions to invoke a system call from within the kernel such that
> > > > the system call did a normal return (not a return from exception) to
> > > > the calling context.  This caused rcu_irq_enter() to be invoked without
> > > > a matching rcu_irq_exit().  However, usermode helpers have since been
> > > > rewritten to make much more housebroken use of workqueues, kernel threads,
> > > > and do_execve(), and therefore should no longer produce half-interrupts.
> > > > No one knows of any other source of half-interrupts, but then again,
> > > > no one seems insane enough to go audit the entire kernel to verify that
> > > > half-interrupts really are a relic of the past.
> > > > 
> > > > This commit therefore adds a pair of WARN_ON_ONCE() calls that will
> > > > trigger in the presence of half interrupts, which the code will continue
> > > > to handle correctly.  If neither of these WARN_ON_ONCE() trigger by
> > > > mid-2021, then perhaps RCU can stop handling half-interrupts, which
> > > > would be a considerable simplification.
> > > Hi Paul and everyone,
> > > I was thinking some more about this patch and whether we can simplify this code
> > > much in 2021. Since 2021 is a bit far away, I thought working on it in again to
> > > keep it fresh in memory is a good idea ;-)
> > > 
> > > To me it seems we cannot easily combine the counters (dynticks_nesting and
> > > dynticks_nmi_nesting) even if we confirmed that there is no possibility of a
> > > half-interrupt scenario (assuming simplication means counter combining like
> > > Byungchul tried to do in https://goo.gl/X1U77X). The reason is because these
> > > 2 counters need to be tracked separately as they are used differently in the
> > > following function:
> > Hi Joel and Paul,
> > 
> > I always love the way to logically approach problems so I'm a fan of
> > all your works :) But I'm JUST curious about something here. Why can't
> > we combine them the way I tried even if we confirm no possibility of
> > half-interrupt? IMHO, the only thing we want to know through calling
> > rcu_is_cpu_rrupt_from_idle() is whether the interrupt comes from
> > RCU-idle or not - of course assuming the caller context always be an
> > well-defined interrupt context like e.g. the tick handler.
> > 
> > So the function can return true if the caller is within a RCU-idle
> > region except a well-known single interrupt nested.
> > 
> > Of course, now that we cannot confirm it yet, the crowbar is necessary.
> > But does it still have a problem even after confirming it? Why? What am
> > I missing? Could you explain why for me? :(
> 
> 
> Did you also want to consider the case the function is called from others
> than
> well-known interrupt contexts? If yes, then I agree with you, there doesn't
> seem to be the kind of code and it's not a good idea to let the function be
> called generally though.

We were discussing exactly this on the thread. I am going to be adding a
lockdep check to make sure the function isn't called from anywhere but an
interrupt context. Then once we can confirm that there are no more
half-interrupts in the future, we can apply your counter combining approach.

Based on the comments on the rrupt_from_idle() function, it wasn't clear to
me if the function was intended to be called from any context. That's why I
thought the counter combing approach would not work, but you are right - it
should work.

I will CC you on the lockdep check patch. Also hope you are subscribed to
the new shiny rcu@vger.kernel.org list as well :-)

thanks,

 - Joel


  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-29 22:20 [PATCH tip/core/rcu 0/19] RCU flavor-consolidation changes for v4.20/v5.0 Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 01/19] rcu: Refactor rcu_{nmi,irq}_{enter,exit}() Paul E. McKenney
2018-08-30 18:10   ` Steven Rostedt
2018-08-30 23:02     ` Paul E. McKenney
2018-08-31  2:25     ` Byungchul Park
2018-08-29 22:20 ` [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt quiescent states when disabled Paul E. McKenney
2018-10-29 11:24   ` Ran Rozenstein
2018-10-29 14:27     ` Paul E. McKenney
2018-10-30  3:44       ` Joel Fernandes
2018-10-30 12:58         ` Paul E. McKenney
2018-10-30 22:21           ` Joel Fernandes
2018-10-31 18:22             ` Paul E. McKenney
2018-11-02 19:43               ` Paul E. McKenney
2018-11-26 13:55                 ` Ran Rozenstein
2018-11-26 19:00                   ` Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 03/19] rcutorture: Test extended "rcu" read-side critical sections Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 04/19] rcu: Allow processing deferred QSes for exiting RCU-preempt readers Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 05/19] rcu: Remove now-unused ->b.exp_need_qs field from the rcu_special union Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 06/19] rcu: Add warning to detect half-interrupts Paul E. McKenney
2019-03-11 13:39   ` Joel Fernandes
2019-03-11 22:29     ` Paul E. McKenney
2019-03-12 15:05       ` Joel Fernandes
2019-03-12 15:20         ` Paul E. McKenney
2019-03-13 15:09           ` Joel Fernandes
2019-03-13 15:27             ` Steven Rostedt
2019-03-13 15:51               ` Paul E. McKenney
2019-03-13 16:51                 ` Steven Rostedt
2019-03-13 18:07                   ` Paul E. McKenney
2019-03-14 12:31                     ` Joel Fernandes
2019-03-14 13:36                       ` Steven Rostedt
2019-03-14 13:37                         ` Steven Rostedt
2019-03-14 21:27                           ` Joel Fernandes
2019-03-15  7:31     ` Byungchul Park
2019-03-15  7:44       ` Byungchul Park
2019-03-15 13:46         ` Joel Fernandes [this message]
2018-08-29 22:20 ` [PATCH tip/core/rcu 07/19] rcu: Apply RCU-bh QSes to RCU-sched and RCU-preempt when safe Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 08/19] rcu: Report expedited grace periods at context-switch time Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 09/19] rcu: Define RCU-bh update API in terms of RCU Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 10/19] rcu: Update comments and help text for no more RCU-bh updaters Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 11/19] rcu: Drop "wake" parameter from rcu_report_exp_rdp() Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 12/19] rcu: Fix typo in rcu_get_gp_kthreads_prio() header comment Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 13/19] rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 14/19] rcu: Express Tiny RCU updates in terms of RCU rather than RCU-sched Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 15/19] rcu: Remove RCU_STATE_INITIALIZER() Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 16/19] rcu: Eliminate rcu_state structure's ->call field Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 17/19] rcu: Remove rcu_state structure's ->rda field Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 18/19] rcu: Remove rcu_state_p pointer to default rcu_state structure Paul E. McKenney
2018-08-29 22:20 ` [PATCH tip/core/rcu 19/19] rcu: Remove rcu_data_p pointer to default rcu_data structure Paul E. McKenney
2018-08-29 22:22 ` [PATCH tip/core/rcu 0/19] RCU flavor-consolidation changes for v4.20/v5.0 Paul E. McKenney

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190315134620.GE3378@google.com \
    --to=joel@joelfernandes.org \
    --cc=byungchul.park@lge.com \
    --cc=dipankar@in.ibm.com \
    --cc=jiangshanlai@gmail.com \
    --cc=josh@joshtriplett.org \
    --cc=kernel-team@lge.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=mathieu.desnoyers@efficios.com \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=rcu@vger.kernel.org \
    --cc=rostedt@goodmis.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git