Date: Thu, 16 Sep 2010 23:24:29 -0400 (EDT) From: Benjamin Kaduk <kaduk@MIT.EDU> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Questions about mutex implementation in kern/kern_mutex.c Message-ID: <alpine.GSO.1.10.1009162317430.9337@multics.mit.edu> In-Reply-To: <201009161416.05759.jhb@freebsd.org> References: <20100915134415.GA23727@pm513-1.comsys.ntu-kpi.kiev.ua> <AANLkTimJV-oB_uTSbUTtbSrR5fXgWGk00dEV7L-Gobrf@mail.gmail.com> <20100916173307.GA1994@pm513-1.comsys.ntu-kpi.kiev.ua> <201009161416.05759.jhb@freebsd.org>
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On Thu, 16 Sep 2010, John Baldwin wrote: > On Thursday, September 16, 2010 1:33:07 pm Andrey Simonenko wrote: > >> The mtx_owned(9) macro uses this property, mtx_owned() does not use anything >> special to compare the value of m->mtx_lock (volatile) with current thread >> pointer, all other functions that update m->mtx_lock of unowned mutex use >> compare-and-set instruction. Also I cannot find anything special in >> generated Assembler code for volatile variables (except for ia64 where >> acquire loads and release stores are used). > > No, mtx_owned() is just not harmed by the races it loses. You can certainly > read a stale value of mtx_lock in mtx_owned() if some other thread owns the > lock or has just released the lock. However, we don't care, because in both > of those cases, mtx_owned() returns false. What does matter is that > mtx_owned() can only return true if we currently hold the mutex. This works > because 1) the same thread cannot call mtx_unlock() and mtx_owned() at the > same time, and 2) even CPUs that hold writes in store buffers will snoop their > store buffer for local reads on that CPU. That is, a given CPU will never > read a stale value of a memory word that is "older" than a write it has > performed to that word. Sorry for the naive question, but would you mind expounding a bit on what keeps the thread from migrating to a different CPU and getting a stale value there? (I can imagine a couple possible mechanisms, but don't know enough to know which one(s) are the real ones.) Thanks, Ben Kaduk
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