Date: Mon, 22 Sep 2025 18:39:02 +0100 From: John Baldwin <jhb@FreeBSD.org> To: Mateusz Guzik <mjguzik@gmail.com> Cc: Konstantin Belousov <kib@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 40a42785dbba - main - fcntl(F_SETFL): only allow one thread to perform F_SETFL Message-ID: <ac66d750-3daa-47c6-8b97-51bb69f9a88a@FreeBSD.org> In-Reply-To: <CAGudoHGZs3iOLmbRBwhanNHtDRmd5BE%2Buorq8onCAbCkFw39iw@mail.gmail.com> References: <202509191419.58JEJsvj031867@gitrepo.freebsd.org> <92831372-745d-4612-b38f-aeb235dd8cca@FreeBSD.org> <CAGudoHGZs3iOLmbRBwhanNHtDRmd5BE%2Buorq8onCAbCkFw39iw@mail.gmail.com>
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On 9/22/25 04:54, Mateusz Guzik wrote: > On Mon, Sep 22, 2025 at 10:41 AM John Baldwin <jhb@freebsd.org> wrote: >> >> On 9/19/25 10:19, Konstantin Belousov wrote: >>> The branch main has been updated by kib: >>> >>> URL: https://cgit.FreeBSD.org/src/commit/?id=40a42785dbba93cc5196178fc49d340c1a89cabe >>> >>> commit 40a42785dbba93cc5196178fc49d340c1a89cabe >>> Author: Konstantin Belousov <kib@FreeBSD.org> >>> AuthorDate: 2025-09-11 10:05:04 +0000 >>> Commit: Konstantin Belousov <kib@FreeBSD.org> >>> CommitDate: 2025-09-19 14:19:13 +0000 >>> >>> fcntl(F_SETFL): only allow one thread to perform F_SETFL >>> >>> Use f_vflags file locking for this. >>> Allowing more than one thread handling F_SETFL might cause de-sync >>> between real driver state and flags. >>> >>> Reviewed by: markj >>> Tested by: pho >>> Sponsored by: The FreeBSD Foundation >>> MFC after: 2 weeks >>> Differential revision: https://reviews.freebsd.org/D52487 >> >> Thanks for fixing this. I still slightly worry that "home-grown" locks >> aren't visible to WITNESS and it's checking. >> > > Another problem with these is that they don't do adaptive spinning. > > In particular for file offset, it *is* putting threads off cpu in real > workloads when it plausibly could be avoided. > > I think the real thing to do here is to drop the hand-rolled machinery > and use an sx lock. > > Currently struct file is 80 bytes which is a very nasty size from > caching standpoint. > > Locks are 32 bytes in size, which is another problem, but ultimately > one can be added here without growing the struct past 128 bytes. > > The only issue here is that files are marked as NOFREE, so this memory > can *never* be reclaimed. > > One could be tempted to use smr here, but the cost of smr_enter is > prohibitive. There is a lazy variant which does not do atomics, which > perhaps could work, but that 0 users in the tree and was probably > never tested. > > With 32-bit archs going away I don't think it's a big deal though. > > For interested, on Linux the struct is 256 bytes. I had suggested in an earlier review adding an sx-pool similar to our existing mtxpool and using that. That would avoid bloating the structure with a dedicated lock. -- John Baldwin
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