Re: [PATCH 5.4] fget: clarify and improve __fget_files() implementation

Greg KH
2022-02-28T11:09:56Z

On Sat, Feb 26, 2022 at 02:32:01PM +0800, Baokun Li wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
>
> commit e386dfc56f837da66d00a078e5314bc8382fab83 upstream.
>
> Commit 054aa8d439b9 ("fget: check that the fd still exists after getting
> a ref to it") fixed a race with getting a reference to a file just as it
> was being closed. It was a fairly minimal patch, and I didn't think
> re-checking the file pointer lookup would be a measurable overhead,
> since it was all right there and cached.
>
> But I was wrong, as pointed out by the kernel test robot.
>
> The 'poll2' case of the will-it-scale.per_thread_ops benchmark regressed
> quite noticeably. Admittedly it seems to be a very artificial test:
> doing "poll()" system calls on regular files in a very tight loop in
> multiple threads.
>
> That means that basically all the time is spent just looking up file
> descriptors without ever doing anything useful with them (not that doing
> 'poll()' on a regular file is useful to begin with). And as a result it
> shows the extra "re-check fd" cost as a sore thumb.
>
> Happily, the regression is fixable by just writing the code to loook up
> the fd to be better and clearer. There's still a cost to verify the
> file pointer, but now it's basically in the noise even for that
> benchmark that does nothing else - and the code is more understandable
> and has better comments too.
>
> [ Side note: this patch is also a classic case of one that looks very
> messy with the default greedy Myers diff - it's much more legible with
> either the patience of histogram diff algorithm ]
>
> Link: https://lore.kernel.org/lkml/20211210053743.GA36420@xsang-OptiPlex-9020/
> Link: https://lore.kernel.org/lkml/20211213083154.GA20853@linux.intel.com/
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Tested-by: Carel Si <beibei.si@intel.com>
> Cc: Jann Horn <jannh@google.com>
> Cc: Miklos Szeredi <mszeredi@redhat.com>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> ---
> fs/file.c | 73 +++++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 57 insertions(+), 16 deletions(-)

All now queued up, thanks for the backports!

greg k-h