Date: Tue, 26 Aug 2014 15:12:18 -0400 From: John Baldwin <jhb@freebsd.org> To: Mateusz Guzik <mjguzik@gmail.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Mateusz Guzik <mjg@freebsd.org> Subject: Re: svn commit: r270648 - in head/sys: kern sys Message-ID: <201408261512.18569.jhb@freebsd.org> In-Reply-To: <20140826135522.GA10544@dft-labs.eu> References: <201408260817.s7Q8HMMT044455@svn.freebsd.org> <201408260945.39017.jhb@freebsd.org> <20140826135522.GA10544@dft-labs.eu>
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On Tuesday, August 26, 2014 9:55:22 am Mateusz Guzik wrote: > On Tue, Aug 26, 2014 at 09:45:38AM -0400, John Baldwin wrote: > > On Tuesday, August 26, 2014 4:17:22 am Mateusz Guzik wrote: > > > Author: mjg > > > Date: Tue Aug 26 08:17:22 2014 > > > New Revision: 270648 > > > URL: http://svnweb.freebsd.org/changeset/base/270648 > > > > > > Log: > > > Fix up races with f_seqcount handling. > > > > > > It was possible that the kernel would overwrite user-supplied hint. > > > > > > Abuse vnode lock for this purpose. > > > > > > In collaboration with: kib > > > MFC after: 1 week > > > > Do we care about this being correct enough to penalize reads by single- > > threading them? This is just an optimization, if you lose the race it doesn't > > actually break anything. > > > > We don't singlethread reads, vnode is locked exactly as it was > previously. > > What changes is that a rarely used (as compared to reads) function > playing with f_seqcount now takes exclusive lock. > > If anything, this is an optimization since it removes an unnecessary > load_acq. Ah, I misread the diff and had thought the fcntl() changes were in the read() path instead. -- John Baldwin
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