From: Joel Fernandes <joel@joelfernandes.org> To: "Paul E. McKenney" <paulmck@linux.ibm.com> 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 14:38:20 -0400 Message-ID: <20190714183820.GD34501@google.com> (raw) In-Reply-To: <20190714181053.GB34501@google.com> 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! thanks, - Joel
next prev parent reply index 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 [this message] 2019-07-14 18:50 ` Paul E. McKenney 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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