From: "Paul E. McKenney" <paulmck@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
Joel Fernandes <joel@joelfernandes.org>,
Steven Rostedt <rostedt@goodmis.org>,
Randy Dunlap <rdunlap@infradead.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Logan Gunthorpe <logang@deltatee.com>,
Kurt Schwemmer <kurt.schwemmer@microsemi.com>,
Bjorn Helgaas <bhelgaas@google.com>,
linux-pci@vger.kernel.org, Felipe Balbi <balbi@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-usb@vger.kernel.org, Kalle Valo <kvalo@codeaurora.org>,
"David S. Miller" <davem@davemloft.net>,
linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
Oleg Nesterov <oleg@redhat.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Michael Ellerman <mpe@ellerman.id.au>,
Arnd Bergmann <arnd@arndb.de>,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [patch V2 08/15] Documentation: Add lock ordering and nesting documentation
Date: Fri, 20 Mar 2020 14:02:43 -0700 [thread overview]
Message-ID: <20200320210243.GT3199@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <87mu8apzxr.fsf@nanos.tec.linutronix.de>
On Fri, Mar 20, 2020 at 08:51:44PM +0100, Thomas Gleixner wrote:
> "Paul E. McKenney" <paulmck@kernel.org> writes:
> >
> > - The soft interrupt related suffix (_bh()) still disables softirq
> > handlers. However, unlike non-PREEMPT_RT kernels (which disable
> > preemption to get this effect), PREEMPT_RT kernels use a per-CPU
> > lock to exclude softirq handlers.
>
> I've made that:
>
> - The soft interrupt related suffix (_bh()) still disables softirq
> handlers.
>
> Non-PREEMPT_RT kernels disable preemption to get this effect.
>
> PREEMPT_RT kernels use a per-CPU lock for serialization. The lock
> disables softirq handlers and prevents reentrancy by a preempting
> task.
That works! At the end, I would instead say "prevents reentrancy
due to task preemption", but what you have works.
> On non-RT this is implicit through preemption disable, but it's non
> obvious for RT as preemption stays enabled.
>
> > PREEMPT_RT kernels preserve all other spinlock_t semantics:
> >
> > - Tasks holding a spinlock_t do not migrate. Non-PREEMPT_RT kernels
> > avoid migration by disabling preemption. PREEMPT_RT kernels instead
> > disable migration, which ensures that pointers to per-CPU variables
> > remain valid even if the task is preempted.
> >
> > - Task state is preserved across spinlock acquisition, ensuring that the
> > task-state rules apply to all kernel configurations. In non-PREEMPT_RT
> > kernels leave task state untouched. However, PREEMPT_RT must change
> > task state if the task blocks during acquisition. Therefore, the
> > corresponding lock wakeup restores the task state. Note that regular
> > (not lock related) wakeups do not restore task state.
>
> - Task state is preserved across spinlock acquisition, ensuring that the
> task-state rules apply to all kernel configurations. Non-PREEMPT_RT
> kernels leave task state untouched. However, PREEMPT_RT must change
> task state if the task blocks during acquisition. Therefore, it
> saves the current task state before blocking and the corresponding
> lock wakeup restores it. A regular not lock related wakeup sets the
> task state to RUNNING. If this happens while the task is blocked on
> a spinlock then the saved task state is changed so that correct
> state is restored on lock wakeup.
>
> Hmm?
I of course cannot resist editing the last two sentences:
... Other types of wakeups unconditionally set task state to RUNNING.
If this happens while a task is blocked while acquiring a spinlock,
then the task state is restored to its pre-acquisition value at
lock-wakeup time.
> > But this code failes on PREEMPT_RT kernels because the memory allocator
> > is fully preemptible and therefore cannot be invoked from truly atomic
> > contexts. However, it is perfectly fine to invoke the memory allocator
> > while holding a normal non-raw spinlocks because they do not disable
> > preemption::
> >
> >> + spin_lock(&lock);
> >> + p = kmalloc(sizeof(*p), GFP_ATOMIC);
> >> +
> >> +Most places which use GFP_ATOMIC allocations are safe on PREEMPT_RT as the
> >> +execution is forced into thread context and the lock substitution is
> >> +ensuring preemptibility.
> >
> > Interestingly enough, most uses of GFP_ATOMIC allocations are
> > actually safe on PREEMPT_RT because the the lock substitution ensures
> > preemptibility. Only those GFP_ATOMIC allocations that are invoke
> > while holding a raw spinlock or with preemption otherwise disabled need
> > adjustment to work correctly on PREEMPT_RT.
> >
> > [ I am not as confident of the above as I would like to be... ]
>
> I'd leave that whole paragraph out. This documents the rules and from
> the above code examples it's pretty clear what works and what not :)
Works for me! ;-)
> > And meeting time, will continue later!
>
> Enjoy!
Not bad, actually, as meetings go.
Thanx, Paul
next prev parent reply other threads:[~2020-03-20 21:02 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-18 20:43 [patch V2 00/15] Lock ordering documentation and annotation for lockdep Thomas Gleixner
2020-03-18 20:43 ` [patch V2 01/15] PCI/switchtec: Fix init_completion race condition with poll_wait() Thomas Gleixner
2020-03-18 21:25 ` Bjorn Helgaas
2020-03-18 20:43 ` [patch V2 02/15] pci/switchtec: Replace completion wait queue usage for poll Thomas Gleixner
2020-03-18 21:26 ` Bjorn Helgaas
2020-03-18 22:11 ` Logan Gunthorpe
2020-03-18 20:43 ` [patch V2 03/15] usb: gadget: Use completion interface instead of open coding it Thomas Gleixner
2020-03-19 8:41 ` Greg Kroah-Hartman
2020-03-18 20:43 ` [patch V2 04/15] orinoco_usb: Use the regular completion interfaces Thomas Gleixner
2020-03-19 8:40 ` Greg Kroah-Hartman
2020-03-18 20:43 ` [patch V2 05/15] acpi: Remove header dependency Thomas Gleixner
2020-03-18 20:43 ` [patch V2 06/15] rcuwait: Add @state argument to rcuwait_wait_event() Thomas Gleixner
2020-03-20 5:36 ` Davidlohr Bueso
2020-03-20 8:45 ` Sebastian Andrzej Siewior
2020-03-20 8:58 ` Davidlohr Bueso
2020-03-20 9:48 ` [PATCH 0/5] Remove mm.h from arch/*/include/asm/uaccess.h Sebastian Andrzej Siewior
2020-03-20 9:48 ` [PATCH 1/5] nds32: Remove mm.h from asm/uaccess.h Sebastian Andrzej Siewior
2020-03-20 9:48 ` [PATCH 2/5] csky: " Sebastian Andrzej Siewior
2020-03-21 11:24 ` Guo Ren
2020-03-21 12:08 ` Thomas Gleixner
2020-03-21 14:11 ` Guo Ren
2020-03-20 9:48 ` [PATCH 3/5] hexagon: " Sebastian Andrzej Siewior
2020-03-20 9:48 ` [PATCH 4/5] ia64: " Sebastian Andrzej Siewior
2020-03-20 9:48 ` [PATCH 5/5] microblaze: " Sebastian Andrzej Siewior
2020-03-18 20:43 ` [patch V2 07/15] powerpc/ps3: Convert half completion to rcuwait Thomas Gleixner
2020-03-19 9:00 ` Sebastian Andrzej Siewior
2020-03-19 9:18 ` Peter Zijlstra
2020-03-19 9:21 ` Davidlohr Bueso
2020-03-19 10:04 ` Christoph Hellwig
2020-03-19 10:26 ` Sebastian Andrzej Siewior
2020-03-20 0:01 ` Geoff Levand
2020-03-20 0:45 ` Michael Ellerman
2020-03-21 10:41 ` Thomas Gleixner
2020-03-18 20:43 ` [patch V2 08/15] Documentation: Add lock ordering and nesting documentation Thomas Gleixner
2020-03-18 22:31 ` Paul E. McKenney
2020-03-19 18:02 ` Thomas Gleixner
2020-03-20 16:01 ` Paul E. McKenney
2020-03-20 19:51 ` Thomas Gleixner
2020-03-20 21:02 ` Paul E. McKenney [this message]
2020-03-20 22:36 ` Thomas Gleixner
2020-03-21 2:29 ` Paul E. McKenney
2020-03-21 10:26 ` Thomas Gleixner
2020-03-21 17:23 ` Paul E. McKenney
2020-03-19 8:51 ` Davidlohr Bueso
2020-03-19 15:04 ` Jonathan Corbet
2020-03-19 18:04 ` Thomas Gleixner
2020-03-21 21:21 ` Joel Fernandes
2020-03-21 21:49 ` Thomas Gleixner
2020-03-22 1:36 ` Joel Fernandes
2020-03-18 20:43 ` [patch V2 09/15] timekeeping: Split jiffies seqlock Thomas Gleixner
2020-03-18 20:43 ` [patch V2 10/15] sched/swait: Prepare usage in completions Thomas Gleixner
2020-03-18 20:43 ` [patch V2 11/15] completion: Use simple wait queues Thomas Gleixner
2020-03-18 22:28 ` Logan Gunthorpe
2020-03-19 0:33 ` Joel Fernandes
2020-03-19 0:44 ` Thomas Gleixner
2020-03-19 8:42 ` Greg Kroah-Hartman
2020-03-19 17:12 ` Linus Torvalds
2020-03-19 23:25 ` Julian Calaby
2020-03-20 6:59 ` Christoph Hellwig
2020-03-20 9:01 ` Davidlohr Bueso
2020-03-20 8:50 ` [patch V2 00/15] Lock ordering documentation and annotation for lockdep Davidlohr Bueso
2020-03-20 8:55 ` [PATCH 16/15] rcuwait: Get rid of stale name comment Davidlohr Bueso
2020-03-20 8:55 ` [PATCH 17/15] rcuwait: Inform rcuwait_wake_up() users if a wakeup was attempted Davidlohr Bueso
2020-03-20 9:13 ` Sebastian Andrzej Siewior
2020-03-20 10:44 ` Peter Zijlstra
2020-03-20 8:55 ` [PATCH 18/15] kvm: Replace vcpu->swait with rcuwait Davidlohr Bueso
2020-03-20 11:20 ` Paolo Bonzini
2020-03-20 12:54 ` Peter Zijlstra
2020-03-22 16:33 ` Davidlohr Bueso
2020-03-22 22:32 ` Peter Zijlstra
2020-03-20 8:55 ` [PATCH 19/15] sched/swait: Reword some of the main description Davidlohr Bueso
2020-03-20 9:19 ` Sebastian Andrzej Siewior
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