Re: git: 40a42785dbba - main - fcntl(F_SETFL): only allow one thread to perform F_SETFL
Date: Mon, 22 Sep 2025 17:42:34 UTC
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. > > 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. Fair enough. Certainly simpler. -- John Baldwin