Date: Tue, 23 Sep 2025 00:05:15 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Mateusz Guzik <mjguzik@gmail.com>, 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: <aNG6Cwgb3nPgw6J0@kib.kiev.ua> In-Reply-To: <a66cd016-2198-42e1-85d7-ae46ee10deb3@FreeBSD.org> References: <202509191419.58JEJsvj031867@gitrepo.freebsd.org> <92831372-745d-4612-b38f-aeb235dd8cca@FreeBSD.org> <CAGudoHGZs3iOLmbRBwhanNHtDRmd5BE%2Buorq8onCAbCkFw39iw@mail.gmail.com> <ac66d750-3daa-47c6-8b97-51bb69f9a88a@FreeBSD.org> <CAGudoHEMeDzTUBrFo1Gzzq-NhGOkOS0U_-JcfZS_ukKRZYt9XQ@mail.gmail.com> <a66cd016-2198-42e1-85d7-ae46ee10deb3@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Mon, Sep 22, 2025 at 06:42:34PM +0100, John Baldwin wrote: > On 9/22/25 13:40, Mateusz Guzik wrote: > > On Mon, Sep 22, 2025 at 7:39 PM John Baldwin <jhb@freebsd.org> wrote: > > > > > > 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. > > > > > > > Per my previous e-mail the offset lock is already contested. I am curious how they are? If several threads do read(2)/write(2) kind of io on the same fd, they get what they deserve. > > > > Using a pool over a lock embedded into the struct would hinder performance. > > > > I explained why I don't consider embedding sx into struct file to be a problem. Are you saying that struct file falls into 128-byte zone, and has enough space for sx?
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?aNG6Cwgb3nPgw6J0>