Date: Sun, 9 Apr 2017 19:51:45 -0600 From: Warner Losh <imp@bsdimp.com> To: Yubin Ruan <ablacktshirt@gmail.com> Cc: Ryan Stone <rysto32@gmail.com>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, Ed Schouten <ed@nuxi.nl> Subject: Re: Understanding the FreeBSD locking mechanism Message-ID: <CANCZdfoyjcSU%2BNHEVJF=bd8xz-Q-H1EupMPX%2BJk45r3DKZ9F9Q@mail.gmail.com> In-Reply-To: <3f93930c-7f10-4d0b-35f2-2b07d64081f0@gmail.com> References: <e99b6366-7d30-a889-b7db-4a3b3133ff5e@gmail.com> <CABh_MKkbVVi%2BgTkaBVDvVfRggS6pbHKJE_VbYBZpAaTCZ81b7Q@mail.gmail.com> <c72c0ee3-328d-3efc-e8a0-4d6c0d5c8cee@gmail.com> <CAFMmRNwWnaq-4vEDCByqdUzWfoiZeN0nM_M5rt8ST0P8xnUTsA@mail.gmail.com> <3f93930c-7f10-4d0b-35f2-2b07d64081f0@gmail.com>
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On Sun, Apr 9, 2017 at 7:28 PM, Yubin Ruan <ablacktshirt@gmail.com> wrote: > On 2017/4/10 0:24, Ryan Stone wrote: >> >> >> >> On Sun, Apr 9, 2017 at 6:13 AM, Yubin Ruan <ablacktshirt@gmail.com >> <mailto:ablacktshirt@gmail.com>> wrote: >> >> >> #######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 * >> >> >> In the FreeBSD locking style, a spinlock is only used in the case where >> one needs to synchronize with an interrupt handler. This is why spinlocks >> always disable local interrupts in FreeBSD. >> >> FreeBSD's lock for the first case is the MTX_DEF mutex, which is >> adaptively-spinning blocking mutex implementation. In short, the MTX_DEF >> mutex will spin waiting for the lock if the owner is running, but will >> block if the owner is deschedules. This prevents expensive trips through >> the scheduler for the common case where the mutex is only held for short >> periods, without wasting CPU cycles spinning in cases where the owner >> thread >> is descheduled and therefore will not be completing soon. > > > Great explanation! I read the man page at: > >> >> https://www.freebsd.org/cgi/man.cgi?query=mutex&sektion=9&apropos=0&manpath=FreeBSD+11.0-RELEASE+and+Ports > > and now clear about MTX_DEF and MTX_SPIN mutexs. But, still a few more > question, if you don't mind: > > Is it true that a thread holding a MTX_DEF mutex can be descheduled? > (shouldn't it disable interrupt like a MTX_SPIN mutex?) It is said on > the main page that MTX_DEF mutex is used by default in FreeBSD, so its > usecase must be very common. If a thread holding a MTX_DEF mutex can be > descheduled, which means that it did not disable interrupt, then we may > have lots of deadlock here, right? Yes, they can be descheduled. But that's not a problem. No other thread can acquire the MTX_DEF lock. If another thread tries, it will sleep and wait for the thread that holds the MTX_DEF lock to release it. Eventually, the thread will get time to run again, and then release the lock. Threads that just hold a MTX_DEF lock may also migrate from CPU to CPU too. Warner >> #######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?) >> >> >> FreeBSD's spin locks prevent priority inversion by preventing the holder >> thread from being descheduled. >> >> MTX_DEF locks implement priority inheritance. > > > Nice hints. Thanks! > > regards, > Yubin Ruan > > > _______________________________________________ > 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"
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