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Date: Fri, 20 Mar 2020 02:01:06 -0700 From: Davidlohr Bueso <dave@...olabs.net> To: Thomas Gleixner <tglx@...utronix.de> Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>, "Paul E . McKenney" <paulmck@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, Steven Rostedt <rostedt@...dmis.org>, Randy Dunlap <rdunlap@...radead.org>, Arnd Bergmann <arnd@...db.de>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Logan Gunthorpe <logang@...tatee.com>, Kurt Schwemmer <kurt.schwemmer@...rosemi.com>, Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org, Felipe Balbi <balbi@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-usb@...r.kernel.org, Kalle Valo <kvalo@...eaurora.org>, "David S. Miller" <davem@...emloft.net>, linux-wireless@...r.kernel.org, netdev@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>, Michael Ellerman <mpe@...erman.id.au>, linuxppc-dev@...ts.ozlabs.org Subject: Re: [patch V2 11/15] completion: Use simple wait queues On Wed, 18 Mar 2020, Thomas Gleixner wrote: >From: Thomas Gleixner <tglx@...utronix.de> > >completion uses a wait_queue_head_t to enqueue waiters. > >wait_queue_head_t contains a spinlock_t to protect the list of waiters >which excludes it from being used in truly atomic context on a PREEMPT_RT >enabled kernel. > >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 > > - wake_up() walks an unbounded number of list entries during the wake up > and may wake an unbounded number of waiters. > >For simplicity and performance reasons complete() should be usable on >PREEMPT_RT enabled kernels. > >completions do not use custom wakeup callbacks and are usually single >waiter, except for a few corner cases. > >Replace the wait queue in the completion with a simple wait queue (swait), >which uses a raw_spinlock_t for protecting the waiter list and therefore is >safe to use inside truly atomic regions on PREEMPT_RT. > >There is no semantical or functional change: > > - completions use the exclusive wait mode which is what swait provides > > - complete() wakes one exclusive waiter > > - complete_all() wakes all waiters while holding the lock which protects > the wait queue against newly incoming waiters. The conversion to swait > preserves this behaviour. > >complete_all() might cause unbound latencies with a large number of waiters >being woken at once, but most complete_all() usage sites are either in >testing or initialization code or have only a really small number of >concurrent waiters which for now does not cause a latency problem. Keep it >simple for now. > >The fixup of the warning check in the USB gadget driver is just a straight >forward conversion of the lockless waiter check from one waitqueue type to >the other. > >Signed-off-by: Thomas Gleixner <tglx@...utronix.de> >Cc: Arnd Bergmann <arnd@...db.de> Reviewed-by: Davidlohr Bueso <dbueso@...e.de>
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