Date: Sun, 9 Apr 2017 23:56:29 +0800 From: Yubin Ruan <ablacktshirt@gmail.com> To: vasanth sabavat <vasanth.raonaik@gmail.com> Cc: Ed Schouten <ed@nuxi.nl>, freebsd-hackers@freebsd.org Subject: Re: Understanding the FreeBSD locking mechanism Message-ID: <d26c0a68-579c-e39c-779c-b11689e745a6@gmail.com> In-Reply-To: <CAAuizBjVU4o9ofi8sAyg_kva-%2BognyxomQU46aeb5Q23Htn-SA@mail.gmail.com> References: <e99b6366-7d30-a889-b7db-4a3b3133ff5e@gmail.com> <CABh_MKkbVVi%2BgTkaBVDvVfRggS6pbHKJE_VbYBZpAaTCZ81b7Q@mail.gmail.com> <c72c0ee3-328d-3efc-e8a0-4d6c0d5c8cee@gmail.com> <CAAuizBjVU4o9ofi8sAyg_kva-%2BognyxomQU46aeb5Q23Htn-SA@mail.gmail.com>
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On 2017/4/9 21:28, vasanth sabavat wrote: > > On Sun, Apr 9, 2017 at 3:14 AM Yubin Ruan <ablacktshirt@gmail.com > <mailto:ablacktshirt@gmail.com>> wrote: > > On 2017/4/6 17:31, Ed Schouten wrote: > > Hi Yubin, > > > > 2017-04-06 11:16 GMT+02:00 Yubin Ruan <ablacktshirt@gmail.com > <mailto:ablacktshirt@gmail.com>>: > >> Does this function provides the ordinary "spinlock" > functionality? There > >> is no special "test-and-set" instruction, and neither any extra > locking > >> to protect internal data structure manipulation. Isn't this > subjected to > >> race condition? > > > > Locking a spinlock is done through macro mtx_lock_spin(), which > > expands to __mtx_lock_spin() in sys/sys/mutex.h. That macro first > > calls into the function you looked at, spinlock_enter(), to disable > > interrupts. It then calls into the _mtx_obtain_lock_fetch() to do the > > test-and-set operation you were looking for. > > Thanks for replying. I have read some of those codes. > > Just a few more questions, if you don't mind: > > (1) why are spinlocks forced to disable interrupt in FreeBSD? > > From the book "The design and implementation of the FreeBSD Operating > System", the authors say "spinning can result in deadlock if a thread > interrupted the thread that held a mutex and then tried to acquire the > mutex"...(section 4.3, Mutex Synchronization, paragraph 4) > > I don't get the point why a spinlock(or *spin mutex* in the FreeBSD > world) has to disable interrupt. Being interrupted does not necessarily > mean a deadlock. Assume that thread A holding a lock T gets interrupted > by another thread B(context switch here) and thread B try to acquire > the lock T. After finding out that lock T has already been acquired, > thread B will just spin until it gets preempted, after which thread A > gets waken up and run and release the lock T. > > > Assume single CPU, If thread B spins where will thread A get to run and > finish up its critical section and release the lock? The one CPU you > have is held by thread b for spinning. > > For spin locks on single core, it does not make sense to spin. We just > disable interrupts as we are currently the only ones running we just > need to make sure no others will get to preempt us. That's why spin > locks should be held for short duration. > > When you have multiple cores, ThreadA can spin on cpu1, while thread B > holding the lock on cpu2 can finish up and release it. We disable > interrupts only on cpu1 so we don't want to preempt threadA. The cost of > preemption is very high compared to short spin. Note: short spin. > > Look at adaptive spin locks. Can't the scheduler preempt thread B and put thread A to run? After all, we did not disable interrupt. regards, Yubin Ruan > > So, you see there is not > necessarily any deadlock even if thread A get interrupted. > > I can only remember two conditions where using spinlock without > disabling interrupts will cause deadlock: > > #######1, spinlock used in an interrupt handler > If a thread A holding a spinlock T get interrupted and the interrupt > handler responsible for this interrupt try to acquire T, then we have > deadlock, because A would never have a chance to run before the > interrupt handler return, and the interrupt handler, unfortunately, > will continue to spin ... so in this situation, one has to disable > interrupt before spinning. > > As far as I know, in Linux, they provide two kinds of spinlocks: > > spin_lock(..); /* spinlock that does not disable interrupts */ > spin_lock_irqsave(...); /* spinlock that disable local interrupt */ > > > #######2, priority inversion problem > If thread B with a higher priority get in and try to acquire the lock > that thread A currently holds, then thread B would spin, while at the > same time thread A has no chance to run because it has lower priority, > thus not being able to release the lock. > (I haven't investigate enough into the source code, so I don't know > how FreeBSD and Linux handle this priority inversion problem. Maybe > they use priority inheritance or random boosting?) > > thanks, > Yubin Ruan > _______________________________________________ > freebsd-hackers@freebsd.org <mailto:freebsd-hackers@freebsd.org> > mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to > "freebsd-hackers-unsubscribe@freebsd.org > <mailto:freebsd-hackers-unsubscribe@freebsd.org>" > > -- > Thanks, > Vasanth
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