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>,
Arnd Bergmann <arnd@arndb.de>,
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>,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [patch V2 11/15] completion: Use simple wait queues
Date: Thu, 19 Mar 2020 01:44:26 +0100 [thread overview]
Message-ID: <87a74ddvh1.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200319003351.GA211584@google.com>
Joel,
Joel Fernandes <joel@joelfernandes.org> writes:
> On Wed, Mar 18, 2020 at 09:43:13PM +0100, Thomas Gleixner wrote:
>> The spinlock in the wait queue head cannot be replaced by a raw_spinlock
>> because:
>>
>> - wait queues can have custom wakeup callbacks, which acquire other
>> spinlock_t locks and have potentially long execution times
>
> Cool, makes sense.
>
>> - wake_up() walks an unbounded number of list entries during the wake up
>> and may wake an unbounded number of waiters.
>
> Just to clarify here, wake_up() will really wake up just 1 waiter if all the
> waiters on the queue are exclusive right? So in such scenario at least, the
> "unbounded number of waiters" would not be an issue if everything waiting was
> exclusive and waitqueue with wake_up() was used. Please correct me if I'm
> wrong about that though.
Correct.
> So the main reasons to avoid waitqueue in favor of swait (as you mentioned)
> would be the sleep-while-atomic issue in truly atomic context on RT, and the
> fact that callbacks can take a long time.
Yes.
Thanks,
tglx
next prev parent reply other threads:[~2020-03-19 0:45 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
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 [this message]
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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