Date: Thu, 17 Jul 2003 10:57:07 -0400 From: Mike Makonnen <mtm@identd.net> To: John Baldwin <jhb@FreeBSD.org> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/kern kern_umtx.c src/sys/sys proc.h Message-ID: <20030717145707.GA2205@kokeb.ambesa.net> In-Reply-To: <200307170911.43767.jhb@FreeBSD.org> References: <200307171106.h6HB6emd076862@repoman.freebsd.org> <200307170911.43767.jhb@FreeBSD.org>
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On Thu, Jul 17, 2003 at 09:12:30AM -0400, John Baldwin wrote: > > ... > > 2. Once a umtx was contested all future locks and unlocks > > were happening in the kernel, regardless of whether it > > was contested or not. To prevent this from happening, > > when a thread locks a umtx it checks the queue for that > > umtx and unsets the contested bit if there are no other > > threads waiting on it. Again, this is slightly more > > complicated than it needs to be because we can't hold > > a lock across casuptr(). So, the thread has to check > > the queue again after unseting the bit, and reset the > > contested bit if it finds that another thread has put > > itself on the queue in the mean time. > > ... > > The in-kernel mutexes handle this case during the unlock > routine. When unlocking a contested mutex, if there is only > one waiter, we wake up that waiter and set the lock cookie > to MTX_UNOWNED without the contested bit set. If the woken > thread is the only thread to try to acquire the lock, it > suceeds and the lock remains uncontested until another thread > blocks trying to lock it. This would mean that the next lock of > the lock would not need to bounce into the kernel if the lock > was not owned by anyone. Initially I was going to do it this way. But there were a couple of things that concerned me so I thought it would be safer to implement it the way I did. One of them is that it opens up a race between a thread that is trying to acquire the umtx for the first time, and the thread that the unlocker just woke up. I suppose we could introduce a UMTX_RESERVED or something (UMTX_UNOWNED is already used) that no one except a thread that just woke up could lock. Is that what you are suggesting? Cheers. -- Mike Makonnen | GPG-KEY: http://www.identd.net/~mtm/mtm.asc mtm@identd.net | D228 1A6F C64E 120A A1C9 A3AA DAE1 E2AF DBCC 68B9 mtm@FreeBSD.Org| FreeBSD - Unleash the Daemon!
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