Date: Thu, 12 Sep 2013 10:08:00 -0400 From: John Baldwin <jhb@freebsd.org> To: Davide Italiano <davide@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r254703 - in head: share/man/man9 sys/sys Message-ID: <201309121008.01115.jhb@freebsd.org> In-Reply-To: <CACYV=-FiOyVsjbnPA5qLXK2OvN_Y9AgWpqOyYzEuJ7a2o8dp1g@mail.gmail.com> References: <201308231412.r7NECdG7081565@svn.freebsd.org> <201308261502.13277.jhb@freebsd.org> <CACYV=-FiOyVsjbnPA5qLXK2OvN_Y9AgWpqOyYzEuJ7a2o8dp1g@mail.gmail.com>
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On Wednesday, September 11, 2013 2:10:11 pm Davide Italiano wrote: > [snip] > > > Well, I've thought about changing lc_lock/unlock to return a uintptr_t or > > void * instead of an int and then I could make rm_sleep work fine. However, > > that still doesn't solve the callout case. The callout case can't be fixed > > easily without explicitly allocating storage in the softclock thread itself. > > > > Also, I don't think you want a pointer in a lock_object. Imagine if two > > threads both locked and then slept on the same rm lock in succession while > > waiting for a wakeup. You would have two trackers to keep track of, but only > > one pointer in the lock_object. > > > > I'm not sure you need to revert your commit. It should pretty much panic > > instantly if someone tries to use it with a read lock instead of a write > > lock, even without INVARIANTS. > > > > -- > > John Baldwin > > Hi John, > I've finally gotten around to dedicate some time to this. > The following patch : > http://people.freebsd.org/~davide/review/callout_sharedrm.diff is a > proposed fix the problem for the callout_rm shared case, based on your > suggestions. > I've overloaded the lc_lock/lc_unlock functions so that they take an > additional void *arg argument which in the callout rm shared case is a > pointer to priotracker structure (I used a void * so that it could be > used for some other purposes in the future, if any). > I've also added a MPASS in the lc_lock()/lc_unlock() functions for all > the primitives but rmlock so that is ensured that arg passed is NULL. > I'm not completely sure this is required but I put that there as > safety belt. > The KPI is highly disturbed by this change, but unfortunately I wasn't > able to find out a way to workaround the problem without this > breakage. While we're breaking things, I guess we can fix the > rm_sleep() case as well instead of causing a double breakage. > My only doubt is what exactly we should return from lc_unlock/lc_lock, > as long as in the, e.g. rwlock case we need to return a boolean that > contains lock state (shared/exclusive) whilst in the rmlock case we > need to return both the lock state and a pointer to struct > rm_priotracker. My best guess is that of introducing a 'struct > lockstate' (sorry for the poor name choice, just to give you the idea) > which contains these two informations, but probably we can > alternatively use a single pointer and store the information about > lock state in the low order bits. > What's your opinion about? Hmm, I think I had envisioned something a bit simpler. Namely, I would change lc_lock/lc_unlock to return a uintptr_t instead of an int, and I would then change lc_unlock for an rm lock to return a pointer to the current thread's tracker as the 'how' and 0 for a write lock. Note that you have to use the existing tracker to make this work correctly for the sleep case where you unlock/lock. For the callout case I figured you would just hack it explicitly like this: if (flags & CALLOUT_SHAREDRM) rm_rlock(...) else lock->lc_lock(); However, if you make my suggested change to make the 'how' a uintptr_t that passes the tracker you can actually do this in the callout case: struct rm_priotracker tracker; uintptr_t how; how = 0; if (flags & CALLOUT_SHAREDLOCK) how = 1; else if (flags & CALLOUT_SHAREDRM) how = (uintptr_t)&tracker; ... class->lc_lock(lock, how); Now, it would be even nicer to make prevent footshooting perhaps by checking the lock class directly: how = 0; if (flags & CALLOUT_SHAREDLOCK) { if (class == &lock_class_rm || class == &lock_class_rm_sleepable) how = (uintptr_t)&tracker; else how = 1; } -- John Baldwin
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