From: Ingo Molnar <mingo@kernel.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andy Lutomirski <luto@amacapital.net>,
Andrew Morton <akpm@linux-foundation.org>,
Denys Vlasenko <dvlasenk@redhat.com>,
Brian Gerst <brgerst@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code
Date: Mon, 15 Jun 2015 22:33:53 +0200 [thread overview]
Message-ID: <20150615203353.GB13273@gmail.com> (raw)
In-Reply-To: <20150615004030.GK3913@linux.vnet.ibm.com>
* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> On Sun, Jun 14, 2015 at 09:38:25PM +0200, Oleg Nesterov wrote:
> > On 06/14, Oleg Nesterov wrote:
> > >
> > > On 06/14, Ingo Molnar wrote:
> > > >
> > > > * Oleg Nesterov <oleg@redhat.com> wrote:
> > > >
> > > > > > + spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */
> > > > > ^^^^^^^^^^^^^^^^^^^^^^^
> > > > >
> > > > > Hmm, but it doesn't if PREEMPT_RCU? No, no, I do not pretend I understand how it
> > > > > actually works ;) But, say, rcu_check_callbacks() can be called from irq and
> > > > > since spin_lock() doesn't increment current->rcu_read_lock_nesting this can lead
> > > > > to rcu_preempt_qs()?
> > > >
> > > > No, RCU grace periods are still defined by 'heavy' context boundaries such as
> > > > context switches, entering idle or user-space mode.
> > > >
> > > > PREEMPT_RCU is like traditional RCU, except that blocking is allowed within the
> > > > RCU read critical section - that is why it uses a separate nesting counter
> > > > (current->rcu_read_lock_nesting), not the preempt count.
> > >
> > > Yes.
> > >
> > > > But if a piece of kernel code is non-preemptible, such as a spinlocked region or
> > > > an irqs-off region, then those are still natural RCU read lock regions, regardless
> > > > of the RCU model, and need no additional RCU locking.
> > >
> > > I do not think so. Yes I understand that rcu_preempt_qs() itself doesn't
> > > finish the gp, but if there are no other rcu-read-lock holders then it
> > > seems synchronize_rcu() on another CPU can return _before_ spin_unlock(),
> > > this CPU no longer needs rcu_preempt_note_context_switch().
> > >
> > > OK, I can be easily wrong, I do not really understand the implementation
> > > of PREEMPT_RCU. Perhaps preempt_disable() can actually act as rcu_read_lock()
> > > with the _current_ implementation. Still this doesn't look right even if
> > > happens to work, and Documentation/RCU/checklist.txt says:
> > >
> > > 11. Note that synchronize_rcu() -only- guarantees to wait until
> > > all currently executing rcu_read_lock()-protected RCU read-side
> > > critical sections complete. It does -not- necessarily guarantee
> > > that all currently running interrupts, NMIs, preempt_disable()
> > > code, or idle loops will complete. Therefore, if your
> > > read-side critical sections are protected by something other
> > > than rcu_read_lock(), do -not- use synchronize_rcu().
> >
> >
> > I've even checked this ;) I applied the stupid patch below and then
> >
> > $ taskset 2 perl -e 'syscall 157, 666, 5000' &
> > [1] 565
> >
> > $ taskset 1 perl -e 'syscall 157, 777'
> >
> > $
> > [1]+ Done taskset 2 perl -e 'syscall 157, 666, 5000'
> >
> > $ dmesg -c
> > SPIN start
> > SYNC start
> > SYNC done!
> > SPIN done!
>
> Please accept my apologies for my late entry to this thread.
> Youngest kid graduated from university this weekend, so my
> attention has been elsewhere.
Congratulations! :-)
> If you were to disable interrupts instead of preemption, I would expect
> that the preemptible-RCU grace period would be blocked -- though I am
> not particularly comfortable with people relying on disabled interrupts
> blocking a preemptible-RCU grace period.
>
> Here is what can happen if you try to block a preemptible-RCU grace
> period by disabling preemption, assuming that there are at least two
> online CPUs in the system:
>
> 1. CPU 0 does spin_lock(), which disables preemption.
>
> 2. CPU 1 starts a grace period.
>
> 3. CPU 0 takes a scheduling-clock interrupt. It raises softirq,
> and the RCU_SOFTIRQ handler notes that there is a new grace
> period and sets state so that a subsequent quiescent state on
> this CPU will be noted.
>
> 4. CPU 0 takes another scheduling-clock interrupt, which checks
> current->rcu_read_lock_nesting, and notes that there is no
> preemptible-RCU read-side critical section in progress. It
> again raises softirq, and the RCU_SOFTIRQ handler reports
> the quiescent state to core RCU.
>
> 5. Once each of the other CPUs report a quiescent state, the
> grace period can end, despite CPU 0 having preemption
> disabled the whole time.
>
> So Oleg's test is correct, disabling preemption is not sufficient
> to block a preemptible-RCU grace period.
I stand corrected!
> The usual suggestion would be to add rcu_read_lock() just after the lock is
> acquired and rcu_read_unlock() just before each release of that same lock.
Will fix it that way.
Thanks,
Ingo
next prev parent reply other threads:[~2015-06-15 20:34 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-13 9:49 [PATCH 00/12, v2] x86/mm: Implement lockless pgd_alloc()/pgd_free() Ingo Molnar
2015-06-13 9:49 ` [PATCH 01/12] x86/mm/pat: Don't free PGD entries on memory unmap Ingo Molnar
2015-06-13 9:49 ` [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code Ingo Molnar
2015-06-13 19:24 ` Oleg Nesterov
2015-06-14 7:36 ` Ingo Molnar
2015-06-14 19:24 ` Oleg Nesterov
2015-06-14 19:38 ` Oleg Nesterov
2015-06-15 0:40 ` Paul E. McKenney
2015-06-15 20:33 ` Ingo Molnar [this message]
2015-06-13 9:49 ` [PATCH 03/12] x86/mm/hotplug: Don't remove PGD entries in remove_pagetable() Ingo Molnar
2015-06-13 9:49 ` [PATCH 04/12] x86/mm/hotplug: Simplify sync_global_pgds() Ingo Molnar
2015-06-13 9:49 ` [PATCH 05/12] mm: Introduce arch_pgd_init_late() Ingo Molnar
2015-06-13 9:49 ` [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late() method Ingo Molnar
2015-06-13 9:49 ` [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code Ingo Molnar
2015-06-14 8:26 ` Ingo Molnar
2015-06-15 9:05 ` Ian Campbell
2015-06-15 10:30 ` David Vrabel
2015-06-15 20:35 ` Ingo Molnar
2015-06-16 14:15 ` David Vrabel
2015-06-16 14:19 ` Boris Ostrovsky
2015-06-16 14:27 ` David Vrabel
2015-06-13 9:49 ` [PATCH 08/12] x86/mm: Remove pgd_list use from vmalloc_sync_all() Ingo Molnar
2015-06-13 9:49 ` [PATCH 09/12] x86/mm/pat/32: Remove pgd_list use from the PAT code Ingo Molnar
2015-06-13 9:49 ` [PATCH 10/12] x86/mm: Make pgd_alloc()/pgd_free() lockless Ingo Molnar
2015-06-13 9:49 ` [PATCH 11/12] x86/mm: Remove pgd_list leftovers Ingo Molnar
2015-06-13 9:49 ` [PATCH 12/12] x86/mm: Simplify pgd_alloc() Ingo Molnar
2015-06-13 18:58 ` why do we need vmalloc_sync_all? Oleg Nesterov
2015-06-14 7:59 ` Ingo Molnar
2015-06-14 20:06 ` Oleg Nesterov
2015-06-15 2:47 ` Andi Kleen
2015-06-15 2:57 ` Andy Lutomirski
2015-06-15 20:28 ` Ingo Molnar
2015-06-15 20:48 ` Andy Lutomirski
-- strict thread matches above, loose matches on Subject: below --
2015-06-11 14:07 [RFC PATCH 00/12] x86/mm: Implement lockless pgd_alloc()/pgd_free() Ingo Molnar
2015-06-11 14:07 ` [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code Ingo Molnar
2015-06-12 22:22 ` Oleg Nesterov
2015-06-12 22:48 ` Waiman Long
2015-06-13 7:46 ` Ingo Molnar
2015-06-12 23:15 ` Oleg Nesterov
2015-06-13 7:48 ` Ingo Molnar
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