Date: Thu, 12 Sep 2013 14:00:29 -0400 From: Dheeraj Kandula <dkandula@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Alfred Perlstein <bright@mu.org>, Svatopluk Kraus <onwahe@gmail.com>, freebsd-arch@freebsd.org Subject: Re: Why do we need to acquire the current thread's lock before context switching? Message-ID: <CA%2BqNgxSmTk8S95%2BDL5BQ8UFZWorNV0YwP9hiikRbDOJrFJp-7A@mail.gmail.com> In-Reply-To: <201309120824.52916.jhb@freebsd.org> References: <CA%2BqNgxSVkSi88UC3gmfwigmP0UCO6dz%2B_Zxhf_=URK7p4c-Ghg@mail.gmail.com> <CAFHCsPXJkxvJrhfbZt5T=Bm=ZS8-%2BE9xL1cY7b6UENHJ74YR5Q@mail.gmail.com> <CA%2BqNgxT68eobU%2BG4AjKeU6wZb0xM_sktDdQ=jCcmYyzQR%2Basiw@mail.gmail.com> <201309120824.52916.jhb@freebsd.org>
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Thanks John for the detailed clarification. Wow that is lot of information. I will digest it and will email you any further questions that I may have. Dheeraj On Thu, Sep 12, 2013 at 8:24 AM, John Baldwin <jhb@freebsd.org> wrote: > On Thursday, September 12, 2013 7:16:20 am Dheeraj Kandula wrote: > > Thanks a lot Svatopluk for the clarification. Right after I replied to > > Alfred's mail, I realized that it can't be thread specific lock as it > > should also protect the scheduler variables. So if I understand it righ= t, > > even though it is a mutex, it can be unlocked by another thread which i= s > > usually not the case with regular mutexes as the thread that locks it > must > > unlock it unlike a binary semaphore. Isn't it? > > It's less complicated than that. :) It is a mutex, but to expand on what > Svatopluk said with an example: take a thread that is asleep on a sleep > queue. td_lock points to the relevant SC_LOCK() for the sleep queue chai= n > in that case, so any other thread that wants to examine that thread's > state ends up locking the sleep queue while it examines that thread. In > particular, the thread that is doing a wakeup() can resume all of the > sleeping threads for a wait channel by holding the one SC_LOCK() for that > wait channel since that will be td_lock for all those threads. > > In general mutexes are only unlocked by the thread that locks them, > and the td_lock of the old thread is unlocked during sched_switch(). > However, the old thread has to grab td_lock of the new thread during > sched_switch() and then hand it off to the new thread when it resumes. > This is why sched_throw() and sched_switch() in ULE directly assign > 'mtx_lock' of the run queue lock before calling cpu_throw() or > cpu_switch(). That gives the effect that the new thread resumes while > holding the lock pinted to by its td_lock. > > > Dheeraj > > > > > > On Thu, Sep 12, 2013 at 7:04 AM, Svatopluk Kraus <onwahe@gmail.com> > wrote: > > > > > Think about td_lock like something what is lent by current thread > owner. > > > If a thread is running, it's owned by scheduler and td_lock points > > > to scheduler lock. If a thread is sleeping, it's owned by sleeping > queue > > > and td_lock points to sleep queue lock. If a thread is contested, it'= s > > > owned by turnstile queue and td_lock points to turnstile queue lock. > And so > > > on. This way an owner can work with owned threads safely without gian= t > > > lock. The td_lock pointer is changed atomically, so it's safe. > > > > > > Svatopluk Kraus > > > > > > On Thu, Sep 12, 2013 at 12:48 PM, Dheeraj Kandula <dkandula@gmail.com > >wrote: > > > > > >> Thanks a lot Alfred for the clarification. So is the td_lock granula= r > i.e. > > >> one separate lock for each thread but also used for protecting the > > >> scheduler variables or is it just one lock used by all threads and t= he > > >> scheduler as well. I will anyway go through the code that you > suggested > > >> but > > >> just wanted to have a deeper understanding before I go about hunting > in > > >> the > > >> code. > > >> > > >> Dheeraj > > >> > > >> > > >> On Thu, Sep 12, 2013 at 3:10 AM, Alfred Perlstein <bright@mu.org> > wrote: > > >> > > >> > On 9/11/13 2:39 PM, Dheeraj Kandula wrote: > > >> > > > >> >> Hey All, > > >> >> > > >> >> When the current thread is being context switched with a newly > selected > > >> >> thread, why is the current thread's lock acquired before context > > >> switch =96 > > >> >> mi_switch() is invoked after thread_lock(td) is called. A thread > at any > > >> >> time runs only on one of the cores of a CPU. Hence when it is bei= ng > > >> >> context > > >> >> switched it is added either to the real time runq or the timeshar= e > > >> runq or > > >> >> the idle runq with the lock still held or it is added to the slee= p > > >> queue > > >> >> or > > >> >> the blocked queue. So this happens atomically even without the > lock. > > >> Isn't > > >> >> it? Am I missing something here? I don't see any contention for t= he > > >> thread > > >> >> in order to demand a lock for the thread which will basically > protect > > >> the > > >> >> contents of the thread structure for the thread. > > >> >> > > >> >> Dheeraj > > >> >> > > >> >> > > >> > The thread lock also happens to protect various scheduler variable= s: > > >> > > > >> > struct mtx *volatile td_lock; /* replaces sched lock = */ > > >> > > > >> > see sys/kern/sched_ule.c on how the thread lock td_lock is changed > > >> > depending on what the thread is doing. > > >> > > > >> > -- > > >> > Alfred Perlstein > > >> > > > >> > > > >> _______________________________________________ > > >> freebsd-arch@freebsd.org mailing list > > >> http://lists.freebsd.org/mailman/listinfo/freebsd-arch > > >> To unsubscribe, send any mail to " > freebsd-arch-unsubscribe@freebsd.org" > > >> > > > > > > > > _______________________________________________ > > freebsd-arch@freebsd.org mailing list > > http://lists.freebsd.org/mailman/listinfo/freebsd-arch > > To unsubscribe, send any mail to "freebsd-arch-unsubscribe@freebsd.org" > > > > -- > John Baldwin >
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