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    Subject[PATCH 5.15 13/41] fget: clarify and improve __fget_files() implementation
    Date
    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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/file.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++++--------------
    1 file changed, 56 insertions(+), 16 deletions(-)

    --- a/fs/file.c
    +++ b/fs/file.c
    @@ -841,28 +841,68 @@ void do_close_on_exec(struct files_struc
    spin_unlock(&files->file_lock);
    }

    -static struct file *__fget_files(struct files_struct *files, unsigned int fd,
    - fmode_t mask, unsigned int refs)
    +static inline struct file *__fget_files_rcu(struct files_struct *files,
    + unsigned int fd, fmode_t mask, unsigned int refs)
    {
    - struct file *file;
    + for (;;) {
    + struct file *file;
    + struct fdtable *fdt = rcu_dereference_raw(files->fdt);
    + struct file __rcu **fdentry;

    - rcu_read_lock();
    -loop:
    - file = files_lookup_fd_rcu(files, fd);
    - if (file) {
    - /* File object ref couldn't be taken.
    - * dup2() atomicity guarantee is the reason
    - * we loop to catch the new file (or NULL pointer)
    + if (unlikely(fd >= fdt->max_fds))
    + return NULL;
    +
    + fdentry = fdt->fd + array_index_nospec(fd, fdt->max_fds);
    + file = rcu_dereference_raw(*fdentry);
    + if (unlikely(!file))
    + return NULL;
    +
    + if (unlikely(file->f_mode & mask))
    + return NULL;
    +
    + /*
    + * Ok, we have a file pointer. However, because we do
    + * this all locklessly under RCU, we may be racing with
    + * that file being closed.
    + *
    + * Such a race can take two forms:
    + *
    + * (a) the file ref already went down to zero,
    + * and get_file_rcu_many() fails. Just try
    + * again:
    */
    - if (file->f_mode & mask)
    - file = NULL;
    - else if (!get_file_rcu_many(file, refs))
    - goto loop;
    - else if (files_lookup_fd_raw(files, fd) != file) {
    + if (unlikely(!get_file_rcu_many(file, refs)))
    + continue;
    +
    + /*
    + * (b) the file table entry has changed under us.
    + * Note that we don't need to re-check the 'fdt->fd'
    + * pointer having changed, because it always goes
    + * hand-in-hand with 'fdt'.
    + *
    + * If so, we need to put our refs and try again.
    + */
    + if (unlikely(rcu_dereference_raw(files->fdt) != fdt) ||
    + unlikely(rcu_dereference_raw(*fdentry) != file)) {
    fput_many(file, refs);
    - goto loop;
    + continue;
    }
    +
    + /*
    + * Ok, we have a ref to the file, and checked that it
    + * still exists.
    + */
    + return file;
    }
    +}
    +
    +static struct file *__fget_files(struct files_struct *files, unsigned int fd,
    + fmode_t mask, unsigned int refs)
    +{
    + struct file *file;
    +
    + rcu_read_lock();
    + file = __fget_files_rcu(files, fd, mask, refs);
    rcu_read_unlock();

    return file;

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