Date: Thu, 12 Sep 2013 13:04:20 +0200 From: Svatopluk Kraus <onwahe@gmail.com> To: Dheeraj Kandula <dkandula@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: <CAFHCsPXJkxvJrhfbZt5T=Bm=ZS8-%2BE9xL1cY7b6UENHJ74YR5Q@mail.gmail.com> In-Reply-To: <CA%2BqNgxS7RHj2LpdyADhgyjSDYfZDJODgyjV4m1yT6o5DchHQ-w@mail.gmail.com> References: <CA%2BqNgxSVkSi88UC3gmfwigmP0UCO6dz%2B_Zxhf_=URK7p4c-Ghg@mail.gmail.com> <523168EE.4070508@mu.org> <CA%2BqNgxS7RHj2LpdyADhgyjSDYfZDJODgyjV4m1yT6o5DchHQ-w@mail.gmail.com>
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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 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 suggested b= ut > just wanted to have a deeper understanding before I go about hunting in t= he > 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 selecte= d > >> thread, why is the current thread's lock acquired before context switc= h > =96 > >> mi_switch() is invoked after thread_lock(td) is called. A thread at an= y > >> 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 run= q > or > >> the idle runq with the lock still held or it is added to the sleep que= ue > >> 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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