Date: Sun, 23 Jun 2002 18:42:43 -0700 (PDT) From: Julian Elischer <julian@elischer.org> To: Jonathan Lemon <jlemon@flugsvamp.com> Cc: dillon@apollo.backplane.com, hackers@freebsd.org Subject: Re: Bug in wakeup() (stable and current) ? Message-ID: <Pine.BSF.4.21.0206231840270.44896-100000@InterJet.elischer.org> In-Reply-To: <200206232158.g5NLw9c49030@prism.flugsvamp.com>
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On Sun, 23 Jun 2002, Jonathan Lemon wrote: > In article <local.mail.freebsd-hackers/200206232032.g5NKWVZW063483@apollo.backplane.com> you write: > >:I'm pretty sure you only need to 'goto restart' if you call into > >:maybe_resched() as someone else may have manipulated the queues. > >: > >:The 'restart' label is only in there for restarting in case one of > >:the functions called may change the lists, if we restart _every_ > >:time we'll traverse the same procs where p->p_wchan != ident over > >:and over needlessly. > >: > >:-Alfred > > > > Look at the code carefully. It's *removing* the element from the list, > > the conditionally restarting rather then removing the element from the > > list and unconditionally restarting. The only reason it works at all > > is because sys/queue.h does not clear out the pointers in the node > > that was just removed. The code is just plain wrong, though, because > > the queue mechanisms make no such (documented) guarentee. > > Looks like the original damage happened in r1.21, where the temporary > variable (used to hold the next item on the list) was replaced by a > dereference through the pointer of the item that was just removed. > > The code works simply because it relies TAILQ_REMOVE() not changing > the tqe_next pointer. I suppose that this should either be documented, > or the loop changed back to use a temp variable: > > for (td = TAILQ_FIRST(qp); td != NULL; td = tdq) { > tdq = TAILQ_NEXT(td, td_slpq); > ... > } I just added debug code in the TAILQ code that sets the forward pointor to -1. Since Matt had this it's possible that this is what hit him? I do this to stop people accessingthings that they shouldn't be counting on.. > > -- > Jonathan > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-hackers" in the body of the message > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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