From: Thomas Gleixner <tglx@linutronix.de> To: Joel Fernandes <joel@joelfernandes.org> 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>, "Paul E . McKenney" <paulmck@kernel.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: Sat, 21 Mar 2020 22:49:04 +0100 [thread overview] Message-ID: <874kuhqsz3.fsf@nanos.tec.linutronix.de> (raw) In-Reply-To: <20200321212144.GA6475@google.com> Joel Fernandes <joel@joelfernandes.org> writes: >> +rwlock_t >> +======== >> + >> +rwlock_t is a multiple readers and single writer lock mechanism. >> + >> +On a non PREEMPT_RT enabled kernel rwlock_t is implemented as a spinning >> +lock and the suffix rules of spinlock_t apply accordingly. The >> +implementation is fair and prevents writer starvation. >> > > You mentioned writer starvation, but I think it would be good to also mention > that rwlock_t on a non-PREEMPT_RT kernel also does not have _reader_ > starvation problem, since it uses queued implementation. This fact is worth > mentioning here, since further below you explain that an rwlock in PREEMPT_RT > does have reader starvation problem. It's worth mentioning. But RT really has only write starvation not reader starvation. >> +rwlock_t and PREEMPT_RT >> +----------------------- >> + >> +On a PREEMPT_RT enabled kernel rwlock_t is mapped to a separate >> +implementation based on rt_mutex which changes the semantics: >> + >> + - Same changes as for spinlock_t >> + >> + - The implementation is not fair and can cause writer starvation under >> + certain circumstances. The reason for this is that a writer cannot grant >> + its priority to multiple readers. Readers which are blocked on a writer >> + fully support the priority inheritance protocol. > > Is it hard to give priority to multiple readers because the number of readers > to give priority to could be unbounded? Yes, and it's horribly complex and racy. We had an implemetation years ago which taught us not to try it again :) >> +PREEMPT_RT also offers a local_lock mechanism to substitute the >> +local_irq_disable/save() constructs in cases where a separation of the >> +interrupt disabling and the locking is really unavoidable. This should be >> +restricted to very rare cases. > > It would also be nice to mention where else local_lock() can be used, such as > protecting per-cpu variables without disabling preemption. Could we add a > section on protecting per-cpu data? (Happy to do that and send a patch if you > prefer). The local lock section will come soon when we post the local lock patches again. >> +rwsems have grown interfaces which allow non owner release for special >> +purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT >> +substitutes all locking primitives except semaphores with RT-mutex based >> +implementations to provide priority inheritance for all lock types except >> +the truly spinning ones. Priority inheritance on ownerless locks is >> +obviously impossible. >> + >> +For now the rwsem non-owner release excludes code which utilizes it from >> +being used on PREEMPT_RT enabled kernels. > > I could not parse the last sentence here, but I think you meant "For now, > PREEMPT_RT enabled kernels disable code that perform a non-owner release of > an rwsem". Correct me if I'm wrong. Right, that's what I wanted to say :) Care to send a delta patch? Thanks! tglx
WARNING: multiple messages have this Message-ID (diff)
From: Thomas Gleixner <tglx@linutronix.de> To: Joel Fernandes <joel@joelfernandes.org> Cc: Randy Dunlap <rdunlap@infradead.org>, Peter Zijlstra <peterz@infradead.org>, linux-pci@vger.kernel.org, Sebastian Andrzej Siewior <bigeasy@linutronix.de>, Oleg Nesterov <oleg@redhat.com>, Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>, Davidlohr Bueso <dave@stgolabs.net>, Arnd Bergmann <arnd@arndb.de>, Logan Gunthorpe <logang@deltatee.com>, "Paul E . McKenney" <paulmck@kernel.org>, linuxppc-dev@lists.ozlabs.org, Steven Rostedt <rostedt@goodmis.org>, Bjorn Helgaas <bhelgaas@google.com>, Kurt Schwemmer <kurt.schwemmer@microsemi.com>, Kalle Valo <kvalo@codeaurora.org>, Felipe Balbi <balbi@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, linux-usb@vger.kernel.org, linux-wireless@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>, netdev@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>, "David S. Miller" <davem@davemloft.net> Subject: Re: [patch V2 08/15] Documentation: Add lock ordering and nesting documentation Date: Sat, 21 Mar 2020 22:49:04 +0100 [thread overview] Message-ID: <874kuhqsz3.fsf@nanos.tec.linutronix.de> (raw) In-Reply-To: <20200321212144.GA6475@google.com> Joel Fernandes <joel@joelfernandes.org> writes: >> +rwlock_t >> +======== >> + >> +rwlock_t is a multiple readers and single writer lock mechanism. >> + >> +On a non PREEMPT_RT enabled kernel rwlock_t is implemented as a spinning >> +lock and the suffix rules of spinlock_t apply accordingly. The >> +implementation is fair and prevents writer starvation. >> > > You mentioned writer starvation, but I think it would be good to also mention > that rwlock_t on a non-PREEMPT_RT kernel also does not have _reader_ > starvation problem, since it uses queued implementation. This fact is worth > mentioning here, since further below you explain that an rwlock in PREEMPT_RT > does have reader starvation problem. It's worth mentioning. But RT really has only write starvation not reader starvation. >> +rwlock_t and PREEMPT_RT >> +----------------------- >> + >> +On a PREEMPT_RT enabled kernel rwlock_t is mapped to a separate >> +implementation based on rt_mutex which changes the semantics: >> + >> + - Same changes as for spinlock_t >> + >> + - The implementation is not fair and can cause writer starvation under >> + certain circumstances. The reason for this is that a writer cannot grant >> + its priority to multiple readers. Readers which are blocked on a writer >> + fully support the priority inheritance protocol. > > Is it hard to give priority to multiple readers because the number of readers > to give priority to could be unbounded? Yes, and it's horribly complex and racy. We had an implemetation years ago which taught us not to try it again :) >> +PREEMPT_RT also offers a local_lock mechanism to substitute the >> +local_irq_disable/save() constructs in cases where a separation of the >> +interrupt disabling and the locking is really unavoidable. This should be >> +restricted to very rare cases. > > It would also be nice to mention where else local_lock() can be used, such as > protecting per-cpu variables without disabling preemption. Could we add a > section on protecting per-cpu data? (Happy to do that and send a patch if you > prefer). The local lock section will come soon when we post the local lock patches again. >> +rwsems have grown interfaces which allow non owner release for special >> +purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT >> +substitutes all locking primitives except semaphores with RT-mutex based >> +implementations to provide priority inheritance for all lock types except >> +the truly spinning ones. Priority inheritance on ownerless locks is >> +obviously impossible. >> + >> +For now the rwsem non-owner release excludes code which utilizes it from >> +being used on PREEMPT_RT enabled kernels. > > I could not parse the last sentence here, but I think you meant "For now, > PREEMPT_RT enabled kernels disable code that perform a non-owner release of > an rwsem". Correct me if I'm wrong. Right, that's what I wanted to say :) Care to send a delta patch? Thanks! tglx
next prev parent reply other threads:[~2020-03-21 21:49 UTC|newest] Thread overview: 148+ 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 ` 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 20:43 ` Thomas Gleixner 2020-03-18 21:25 ` Bjorn Helgaas 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 20:43 ` Thomas Gleixner 2020-03-18 21:26 ` Bjorn Helgaas 2020-03-18 21:26 ` Bjorn Helgaas 2020-03-18 22:11 ` Logan Gunthorpe 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 2020-03-18 20:43 ` Thomas Gleixner 2020-03-19 8:41 ` Greg Kroah-Hartman 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 2020-03-18 20:43 ` Thomas Gleixner 2020-03-19 8:40 ` Greg Kroah-Hartman 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 ` Thomas Gleixner 2020-03-18 20:43 ` [patch V2 06/15] rcuwait: Add @state argument to rcuwait_wait_event() Thomas Gleixner 2020-03-18 20:43 ` Thomas Gleixner 2020-03-20 5:36 ` Davidlohr Bueso 2020-03-20 5:36 ` Davidlohr Bueso 2020-03-20 8:45 ` Sebastian Andrzej Siewior 2020-03-20 8:45 ` Sebastian Andrzej Siewior 2020-03-20 8:58 ` Davidlohr Bueso 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 ` 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 ` Sebastian Andrzej Siewior 2020-03-20 9:48 ` [PATCH 2/5] csky: " Sebastian Andrzej Siewior 2020-03-20 9:48 ` Sebastian Andrzej Siewior 2020-03-21 11:24 ` Guo Ren 2020-03-21 11:24 ` Guo Ren 2020-03-21 12:08 ` Thomas Gleixner 2020-03-21 12:08 ` Thomas Gleixner 2020-03-21 14:11 ` Guo Ren 2020-03-21 14:11 ` Guo Ren 2020-03-20 9:48 ` [PATCH 3/5] hexagon: " Sebastian Andrzej Siewior 2020-03-20 9:48 ` Sebastian Andrzej Siewior 2020-03-20 9:48 ` [PATCH 4/5] ia64: " Sebastian Andrzej Siewior 2020-03-20 9:48 ` Sebastian Andrzej Siewior 2020-03-20 9:48 ` Sebastian Andrzej Siewior 2020-03-20 9:48 ` [PATCH 5/5] microblaze: " Sebastian Andrzej Siewior 2020-03-20 9:48 ` Sebastian Andrzej Siewior 2020-03-18 20:43 ` [patch V2 07/15] powerpc/ps3: Convert half completion to rcuwait Thomas Gleixner 2020-03-18 20:43 ` Thomas Gleixner 2020-03-19 9:00 ` Sebastian Andrzej Siewior 2020-03-19 9:00 ` Sebastian Andrzej Siewior 2020-03-19 9:18 ` Peter Zijlstra 2020-03-19 9:18 ` Peter Zijlstra 2020-03-19 9:21 ` Davidlohr Bueso 2020-03-19 9:21 ` Davidlohr Bueso 2020-03-19 10:04 ` Christoph Hellwig 2020-03-19 10:04 ` Christoph Hellwig 2020-03-19 10:26 ` Sebastian Andrzej Siewior 2020-03-19 10:26 ` Sebastian Andrzej Siewior 2020-03-20 0:01 ` Geoff Levand 2020-03-20 0:01 ` Geoff Levand 2020-03-20 0:45 ` Michael Ellerman 2020-03-20 0:45 ` Michael Ellerman 2020-03-21 10:41 ` Thomas Gleixner 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 20:43 ` Thomas Gleixner 2020-03-18 22:31 ` Paul E. McKenney 2020-03-18 22:31 ` Paul E. McKenney 2020-03-19 18:02 ` Thomas Gleixner 2020-03-19 18:02 ` Thomas Gleixner 2020-03-20 16:01 ` Paul E. McKenney 2020-03-20 16:01 ` Paul E. McKenney 2020-03-20 19:51 ` Thomas Gleixner 2020-03-20 19:51 ` Thomas Gleixner 2020-03-20 21:02 ` Paul E. McKenney 2020-03-20 21:02 ` Paul E. McKenney 2020-03-20 22:36 ` Thomas Gleixner 2020-03-20 22:36 ` Thomas Gleixner 2020-03-21 2:29 ` Paul E. McKenney 2020-03-21 2:29 ` Paul E. McKenney 2020-03-21 10:26 ` Thomas Gleixner 2020-03-21 10:26 ` Thomas Gleixner 2020-03-21 17:23 ` Paul E. McKenney 2020-03-21 17:23 ` Paul E. McKenney 2020-03-19 8:51 ` Davidlohr Bueso 2020-03-19 8:51 ` Davidlohr Bueso 2020-03-19 15:04 ` Jonathan Corbet 2020-03-19 15:04 ` Jonathan Corbet 2020-03-19 18:04 ` Thomas Gleixner 2020-03-19 18:04 ` Thomas Gleixner 2020-03-21 21:21 ` Joel Fernandes 2020-03-21 21:21 ` Joel Fernandes 2020-03-21 21:49 ` Thomas Gleixner [this message] 2020-03-21 21:49 ` Thomas Gleixner 2020-03-22 1:36 ` Joel Fernandes 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 ` Thomas Gleixner 2020-03-18 20:43 ` [patch V2 10/15] sched/swait: Prepare usage in completions Thomas Gleixner 2020-03-18 20:43 ` Thomas Gleixner 2020-03-18 20:43 ` [patch V2 11/15] completion: Use simple wait queues Thomas Gleixner 2020-03-18 20:43 ` Thomas Gleixner 2020-03-18 22:28 ` Logan Gunthorpe 2020-03-18 22:28 ` Logan Gunthorpe 2020-03-19 0:33 ` Joel Fernandes 2020-03-19 0:33 ` Joel Fernandes 2020-03-19 0:44 ` Thomas Gleixner 2020-03-19 0:44 ` Thomas Gleixner 2020-03-19 8:42 ` Greg Kroah-Hartman 2020-03-19 8:42 ` Greg Kroah-Hartman 2020-03-19 17:12 ` Linus Torvalds 2020-03-19 17:12 ` Linus Torvalds 2020-03-19 23:25 ` Julian Calaby 2020-03-19 23:25 ` Julian Calaby 2020-03-20 6:59 ` Christoph Hellwig 2020-03-20 6:59 ` Christoph Hellwig 2020-03-20 9:01 ` Davidlohr Bueso 2020-03-20 9:01 ` Davidlohr Bueso 2020-03-18 20:43 ` [patch V2 13/15] lockdep: Add hrtimer context tracing bits Thomas Gleixner 2020-03-18 20:43 ` [patch V2 14/15] lockdep: Annotate irq_work Thomas Gleixner 2020-03-18 20:43 ` [patch V2 15/15] lockdep: Add posixtimer context tracing bits Thomas Gleixner 2020-03-20 8:50 ` [patch V2 00/15] Lock ordering documentation and annotation for lockdep Davidlohr Bueso 2020-03-20 8:50 ` Davidlohr Bueso 2020-03-20 8:55 ` [PATCH 16/15] rcuwait: Get rid of stale name comment Davidlohr Bueso 2020-03-20 8:55 ` 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 8:55 ` Davidlohr Bueso 2020-03-20 9:13 ` Sebastian Andrzej Siewior 2020-03-20 9:13 ` Sebastian Andrzej Siewior 2020-03-20 10:44 ` Peter Zijlstra 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 8:55 ` Davidlohr Bueso 2020-03-20 11:20 ` Paolo Bonzini 2020-03-20 11:20 ` Paolo Bonzini 2020-03-20 12:54 ` Peter Zijlstra 2020-03-20 12:54 ` Peter Zijlstra 2020-03-22 16:33 ` Davidlohr Bueso 2020-03-22 16:33 ` Davidlohr Bueso 2020-03-22 22:32 ` Peter Zijlstra 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 8:55 ` Davidlohr Bueso 2020-03-20 9:19 ` Sebastian Andrzej Siewior 2020-03-20 9:19 ` Sebastian Andrzej Siewior
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