Date: Thu, 12 Sep 2013 13:57:13 -0400 From: Dheeraj Kandula <dkandula@gmail.com> To: Svatopluk Kraus <onwahe@gmail.com> Cc: Alfred Perlstein <bright@mu.org>, freebsd-arch@freebsd.org Subject: Re: Why do we need to acquire the current thread's lock before context switching? Message-ID: <CA%2BqNgxSeB0m2LK71OagjLb3TpmgoQFTNa-HHcoJ1r_Em9da1yA@mail.gmail.com> In-Reply-To: <CAFHCsPWgh3mF7qN_FFdzhjdKOovbn2kHm7cPXuY-33wHGSNSHg@mail.gmail.com> References: <CA%2BqNgxSVkSi88UC3gmfwigmP0UCO6dz%2B_Zxhf_=URK7p4c-Ghg@mail.gmail.com> <523168EE.4070508@mu.org> <CA%2BqNgxS7RHj2LpdyADhgyjSDYfZDJODgyjV4m1yT6o5DchHQ-w@mail.gmail.com> <CAFHCsPXJkxvJrhfbZt5T=Bm=ZS8-%2BE9xL1cY7b6UENHJ74YR5Q@mail.gmail.com> <CA%2BqNgxT68eobU%2BG4AjKeU6wZb0xM_sktDdQ=jCcmYyzQR%2Basiw@mail.gmail.com> <CAFHCsPWgh3mF7qN_FFdzhjdKOovbn2kHm7cPXuY-33wHGSNSHg@mail.gmail.com>
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Great. Now I understand it better. Thanks for the reply Svatopluk. Dheeraj On Thu, Sep 12, 2013 at 8:30 AM, Svatopluk Kraus <onwahe@gmail.com> wrote: > Yes, td_lock is a bit of magic and not used like ordinary mutex certainly= . > And more, some dirty (not standard and hidden) things happen to it at lea= st > during task switch. > > Svatopluk Kraus > > On Thu, Sep 12, 2013 at 1:16 PM, Dheeraj Kandula <dkandula@gmail.com>wrot= e: > >> 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 right= , >> even though it is a mutex, it can be unlocked by another thread which is >> usually not the case with regular mutexes as the thread that locks it mu= st >> unlock it unlike a binary semaphore. Isn't it? >> >> 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 queu= e >>> 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. An= d so >>> on. This way an owner can work with owned threads safely without giant >>> 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 granular >>>> 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 the >>>> scheduler as well. I will anyway go through the code that you suggeste= d >>>> but >>>> just wanted to have a deeper understanding before I go about hunting i= n >>>> 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 being >>>> >> context >>>> >> switched it is added either to the real time runq or the timeshare >>>> runq or >>>> >> the idle runq with the lock still held or it is added to the sleep >>>> 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 the >>>> 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 variables: >>>> > >>>> > 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= " >>>> >>> >>> >> >
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