From owner-freebsd-arch@FreeBSD.ORG Thu Sep 12 11:16:22 2013 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTP id EE222E0C for ; Thu, 12 Sep 2013 11:16:22 +0000 (UTC) (envelope-from dkandula@gmail.com) Received: from mail-we0-x22d.google.com (mail-we0-x22d.google.com [IPv6:2a00:1450:400c:c03::22d]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by mx1.freebsd.org (Postfix) with ESMTPS id 79E4329E1 for ; Thu, 12 Sep 2013 11:16:22 +0000 (UTC) Received: by mail-we0-f173.google.com with SMTP id w62so8122708wes.32 for ; Thu, 12 Sep 2013 04:16:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=mY+i6oKCk3zRjDb5yv3JQzgT326lc8BHRQnVx/xhDyY=; b=CtGu+qerNZwC2IuFOV6UuMmSQWV2pbjGQrBcTuD3tsV9lHvMFAKp0Qc66U3F2KD/5M JRFwRMADiQoaQQz/g9+WF6GjGrDcgAZeuzwqpbsg73Can2SDHWXXuDRgx5hjCK/sAlyS xHGJDN21I8aiigHi4rS6sPFUsJjdgA09uCmHZ79BxhwaJOwNy4vd6m3QlZrUFX97pxuh wynQspf5YPcVz4naxjUhvYZ786sIlcQNqTne3Zc4hfQOEf54ij+oXWX1RNBTxghDnVlw sGkA+1UF5lK358UMXc8ZAomTSrbFHyILnkz1++D41VQ23UrwZ8PfpDCyaNJaiso27B8w E/9w== MIME-Version: 1.0 X-Received: by 10.194.23.196 with SMTP id o4mr895225wjf.62.1378984580812; Thu, 12 Sep 2013 04:16:20 -0700 (PDT) Received: by 10.194.38.167 with HTTP; Thu, 12 Sep 2013 04:16:20 -0700 (PDT) In-Reply-To: References: <523168EE.4070508@mu.org> Date: Thu, 12 Sep 2013 07:16:20 -0400 Message-ID: Subject: Re: Why do we need to acquire the current thread's lock before context switching? From: Dheeraj Kandula To: Svatopluk Kraus Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable X-Content-Filtered-By: Mailman/MimeDel 2.1.14 Cc: Alfred Perlstein , freebsd-arch@freebsd.org X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 12 Sep 2013 11:16:23 -0000 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 must unlock it unlike a binary semaphore. Isn't it? Dheeraj On Thu, Sep 12, 2013 at 7:04 AM, Svatopluk Kraus 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 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 wro= te: > >> 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 >> 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 wrote: >> >> > On 9/11/13 2:39 PM, Dheeraj Kandula wrote: >> > >> >> Hey All, >> >> >> >> When the current thread is being context switched with a newly select= ed >> >> 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 a= ny >> >> 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" >> > >