Re: git: 40a42785dbba - main - fcntl(F_SETFL): only allow one thread to perform F_SETFL

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