Date: Thu, 16 Aug 2007 19:04:06 -0400 From: John Baldwin <jhb@freebsd.org> To: Jeff Roberson <jroberson@chesapeake.net> Cc: freebsd-arch@freebsd.org Subject: Re: file locking. Message-ID: <200708161904.06299.jhb@freebsd.org> In-Reply-To: <20070816151932.R568@10.0.0.1> References: <20070815233852.X568@10.0.0.1> <200708161635.20935.jhb@freebsd.org> <20070816151932.R568@10.0.0.1>
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On Thursday 16 August 2007 06:31:13 pm Jeff Roberson wrote: > On Thu, 16 Aug 2007, John Baldwin wrote: > > > On Thursday 16 August 2007 04:18:51 pm Jeff Roberson wrote: > >> On Thu, 16 Aug 2007, John Baldwin wrote: > >> > >>> On Thursday 16 August 2007 03:01:18 am Jeff Roberson wrote: > >>>> I have been looking at file locking for 8.0 and have come up with a way > > to > >>>> completely eliminate the file lock, reduce the size of struct file by > >>>> ~4 pointers (22%), remove the global list of files and associated lock, > >>>> and restrict the scope of unp_gc while removing several race conditions. > >>> > >> > >> Thanks for the review. > >> > >>> I like finit(). I would maybe change socketpair() to pass so1 and so2 to > >>> finit() rather than setting f_data twice. > >> > >> I'm not sure what you mean? > > > > In socketpair() the new code does this: > > > > fp1->f_data = so1; > > ... > > fp2->f_data = so2; > > ... > > finit(fp1, ..., fp1->f_data, ...); > > finit(fp2, ..., fp2->f_data, ...); > > > > It might be cleaner to do this: > > > > ... > > ... > > finit(fp1, ..., so1, ...); > > finit(fp2, ..., so2, ...); > > I did not want to have one file pointing at another without an initialized > f_data field. However, I guess the underlying sockets are already setup > so this may not be important. The code did go to some effort to setup > f_data early before as well so I didn't want to change that. Until f_ops is set, f_data is irrelevant as badfileops ignores f_data. > > So, in the old code there's a really big comment about how it makes sure to > > only do closef() (via unp_discard()) once but does a sorflush() for each > > f_msgcount. Was that comment no longer true? > > The comment actually says: > > * > * It is incorrect to simply unp_discard each entry for f_msgcount > * times > > What we do is grab an extra ref to each struct file that is dead and then > explicitly sorflush() them. This closes all of the references held by > that socket, which would free any unreferenced non-unp descriptors. > However, we want to prevent the algorithm from recursing in on itself so > we hold the extra file ref for unp sockets that would be closed. Then > when we loop releasing this one last ref at the end the actually fo_close > will be called. > > This portion of the algorithm is not significantly different from before. > I just introduced an extra flag so I could remove the race from dropping > the lock inbetween operations and get an accurate count of how big the > array needs to be. Ok. > >> Do we have an official stance on libkvm? Now that we have sysctl for > >> run-time it's only useful for crashdump debugging. Really in most cases > >> it could be replaced with a reasonable set of gcc scripts. > > > > s/gcc/gdb/. At work we do mostly post-mortem analysis, so having working > > libkvm is still very important for us. xref the way I just fixed netstat to > > work again on coredumps recently. Breaking fstat on coredumps would probably > > be very annoying. libkvm can always use the same algo as the sysctl if > > necessary though. > > Yes, I'll do that. Cool, thanks! -- John Baldwin
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