From: "Paul E. McKenney" <paulmck@linux•ibm.com>
To: Joel Fernandes <joel@joelfernandes•org>
Cc: linux-kernel@vger•kernel.org, Oleg Nesterov <oleg@redhat•com>,
Alexey Kuznetsov <kuznet@ms2•inr.ac.ru>,
Bjorn Helgaas <bhelgaas@google•com>,
Borislav Petkov <bp@alien8•de>,
c0d1n61at3@gmail•com, "David S. Miller" <davem@davemloft•net>,
edumazet@google•com,
Greg Kroah-Hartman <gregkh@linuxfoundation•org>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6•org>,
"H. Peter Anvin" <hpa@zytor•com>, Ingo Molnar <mingo@redhat•com>,
Jonathan Corbet <corbet@lwn•net>,
Josh Triplett <josh@joshtriplett•org>,
keescook@chromium•org, kernel-hardening@lists•openwall.com,
kernel-team@android•com, Lai Jiangshan <jiangshanlai@gmail•com>,
Len Brown <lenb@kernel•org>,
linux-acpi@vger•kernel.org, linux-doc@vger•kernel.org,
linux-pci@vger•kernel.org, linux-pm@vger•kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@efficios•com>,
neilb@suse•com, netdev@vger•kernel.org,
Pavel Machek <pavel@ucw•cz>,
peterz@infradead•org, "Rafael J. Wysocki" <rjw@rjwysocki•net>,
Rasmus Villemoes <rasmus.villemoes@prevas•dk>,
rcu@vger•kernel.org, Steven Rostedt <rostedt@goodmis•org>,
Tejun Heo <tj@kernel•org>, Thomas Gleixner <tglx@linutronix•de>,
will@kernel•org,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel•org>
Subject: Re: [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section
Date: Sun, 14 Jul 2019 11:50:27 -0700 [thread overview]
Message-ID: <20190714185027.GL26519@linux.ibm.com> (raw)
In-Reply-To: <20190714183820.GD34501@google.com>
On Sun, Jul 14, 2019 at 02:38:20PM -0400, Joel Fernandes wrote:
> On Sun, Jul 14, 2019 at 02:10:53PM -0400, Joel Fernandes wrote:
> > On Sat, Jul 13, 2019 at 02:28:12PM -0700, Paul E. McKenney wrote:
> [snip]
> > > > > > > > > >
> > > > > > > > > > Cc: Oleg Nesterov <oleg@redhat•com>
> > > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes•org>
> > > > > > > > > > ---
> > > > > > > > > > include/linux/rcu_sync.h | 4 +---
> > > > > > > > > > 1 file changed, 1 insertion(+), 3 deletions(-)
> > > > > > > > > >
> > > > > > > > > > diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
> > > > > > > > > > index 9b83865d24f9..0027d4c8087c 100644
> > > > > > > > > > --- a/include/linux/rcu_sync.h
> > > > > > > > > > +++ b/include/linux/rcu_sync.h
> > > > > > > > > > @@ -31,9 +31,7 @@ struct rcu_sync {
> > > > > > > > > > */
> > > > > > > > > > static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
> > > > > > > > > > {
> > > > > > > > > > - RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&
> > > > > > > > > > - !rcu_read_lock_bh_held() &&
> > > > > > > > > > - !rcu_read_lock_sched_held(),
> > > > > > > > > > + RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),
> > > > > > > > >
> > > > > > > > > I believe that replacing rcu_read_lock_sched_held() with preemptible()
> > > > > > > > > in a CONFIG_PREEMPT=n kernel will give you false-positive splats here.
> > > > > > > > > If you have not already done so, could you please give it a try?
> > > > > > > >
> > > > > > > > Hi Paul,
> > > > > > > > I don't think it will cause splats for !CONFIG_PREEMPT.
> > > > > > > >
> > > > > > > > Currently, rcu_read_lock_any_held() introduced in this patch returns true if
> > > > > > > > !preemptible(). This means that:
> > > > > > > >
> > > > > > > > The following expression above:
> > > > > > > > RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),...)
> > > > > > > >
> > > > > > > > Becomes:
> > > > > > > > RCU_LOCKDEP_WARN(preemptible(), ...)
> > > > > > > >
> > > > > > > > For, CONFIG_PREEMPT=n kernels, this means:
> > > > > > > > RCU_LOCKDEP_WARN(0, ...)
> > > > > > > >
> > > > > > > > Which would mean no splats. Or, did I miss the point?
> > > > > > >
> > > > > > > I suggest trying it out on a CONFIG_PREEMPT=n kernel.
> > > > > >
> > > > > > Sure, will do, sorry did not try it out yet because was busy with weekend
> > > > > > chores but will do soon, thanks!
> > > > >
> > > > > I am not faulting you for taking the weekend off, actually. ;-)
> > > >
> > > > ;-)
> > > >
> > > > I tried doing RCU_LOCKDEP_WARN(preemptible(), ...) in this code path and I
> > > > don't get any splats. I also disassembled the code and it seems to me
> > > > RCU_LOCKDEP_WARN() becomes a NOOP which also the above reasoning confirms.
> > >
> > > OK, very good. Could you do the same thing for the RCU_LOCKDEP_WARN()
> > > in synchronize_rcu()? Why or why not?
> > >
> >
> > Hi Paul,
> >
> > Yes synchronize_rcu() can also make use of this technique since it is
> > strictly illegal to call synchronize_rcu() within a reader section.
> >
> > I will add this to the set of my patches as well and send them all out next
> > week, along with the rcu-sync and bh clean ups we discussed.
>
> After sending this email, it occurs to me it wont work in synchronize_rcu()
> for !CONFIG_PREEMPT kernels. This is because in a !CONFIG_PREEMPT kernel,
> executing in kernel mode itself looks like being in an RCU reader. So we
> should leave that as is. However it will work fine for rcu_sync_is_idle (for
> CONFIG_PREEMPT=n kernels) as I mentioned earlier.
>
> Were trying to throw me a Quick-Quiz ? ;-) In that case, hope I passed!
You did pass. This time. ;-)
Thanx, Paul
next prev parent reply other threads:[~2019-07-14 18:53 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-12 17:00 [PATCH v2 0/9] Harden list_for_each_entry_rcu() and family Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 1/9] rcu/update: Remove useless check for debug_locks Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 2/9] rcu: Add support for consolidated-RCU reader checking Joel Fernandes (Google)
2019-07-16 18:22 ` Paul E. McKenney
2019-07-16 18:35 ` Joel Fernandes
2019-07-16 18:50 ` Paul E. McKenney
2019-07-12 17:00 ` [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section Joel Fernandes (Google)
2019-07-12 21:35 ` Joel Fernandes
2019-07-12 23:32 ` Paul E. McKenney
2019-07-13 3:01 ` Joel Fernandes
2019-07-13 3:10 ` Joel Fernandes
2019-07-13 8:21 ` Paul E. McKenney
2019-07-13 13:30 ` Joel Fernandes
2019-07-13 14:41 ` Paul E. McKenney
2019-07-13 15:36 ` Joel Fernandes
2019-07-13 15:50 ` Paul E. McKenney
2019-07-13 16:13 ` Joel Fernandes
2019-07-13 21:28 ` Paul E. McKenney
2019-07-14 18:10 ` Joel Fernandes
2019-07-14 18:38 ` Joel Fernandes
2019-07-14 18:50 ` Paul E. McKenney [this message]
2019-07-15 7:26 ` Oleg Nesterov
2019-07-16 18:26 ` Paul E. McKenney
2019-07-16 18:28 ` Paul E. McKenney
2019-07-12 17:00 ` [PATCH v2 4/9] ipv4: add lockdep condition to fix for_each_entry Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 5/9] driver/core: Convert to use built-in RCU list checking Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 6/9] workqueue: Convert for_each_wq to use built-in list check Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 7/9] x86/pci: Pass lockdep condition to pcm_mmcfg_list iterator Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 8/9] acpi: Use built-in RCU list checking for acpi_ioremaps list Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 9/9] doc: Update documentation about list_for_each_entry_rcu Joel Fernandes (Google)
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