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    SubjectRe: [PATCH v2 3/4]: sendfile: remove flags paramter of do_splice_direct()
    On Sun, May 30, 2010 at 12:20 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
    > [ Removed matthew@wil.cx since I'm getting errors in sending to that address ]
    >
    >
    > On Sat, 2010-05-29 at 09:59 +0800, Changli Gao wrote:
    >> On Fri, May 28, 2010 at 6:06 PM, Changli Gao <xiaosuo@gmail.com> wrote:
    >> > On Fri, May 28, 2010 at 5:40 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:
    >> >> On Wed, 26 May 2010, Changli Gao wrote:
    >> >>> remove flags paramter of do_splice_direct().
    >> >>>
    >> >>> the flags parameter of do_splice_direct() doesn't mean non-block read of
    >> >>> in file,
    >> >>
    >> >> Actually, it does.  Look at the SPLICE_F_NONBLOCK usage in
    >> >> __generic_splice_file_read().
    >> >>
    >> >
    >> > Oh, I checked the code again. You're right. However, why don't we
    >> > check the flags of in file instead in __generic_file_splice_read()?
    >> >
    >>
    >> I am afraid that __generic_file_splice_read() should not use SPLICE_F_NONBLOCK.
    >>
    >> Here is the comment from the commit 42324c62704365d6a3e89138dea55909d2f26afe :
    >>
    >> <<EOF
    >> Linus introduced  SPLICE_F_NONBLOCK in commit
    >> 29e350944fdc2dfca102500790d8ad6d6ff4f69d
    >> (splice: add SPLICE_F_NONBLOCK flag )
    >>
    >>   It doesn't make the splice itself necessarily nonblocking (because the
    >>   actual file descriptors that are spliced from/to may block unless they
    >>   have the O_NONBLOCK flag set), but it makes the splice pipe operations
    >>   nonblocking.
    >>
    >> Linus intention was clear : let SPLICE_F_NONBLOCK control the splice
    >> pipe mode only
    >> EOF
    >>
    >> And I have greped the whole code:
    >>
    >
    >
    >> kernel/trace/trace.c:           if (flags & SPLICE_F_NONBLOCK)
    >>
    >> besides the __generic_file_splice_read(), there are two users: relay
    >> and trace. I believe they all misuse this flag.
    >>
    >
    >
    > The trace splice never blocks anyway. Since we have no good method to
    > wake up a waiter. That is, we never know if it is safe to call wakeup,
    > because a tracepoint may be in a section where the rq lock is held.
    >
    > The only thing that passing SPLICE_F_NONBLOCK does in that code is to
    > return -EAGAIN when there is nothing in the buffer. Otherwise, it
    > returns 0.
    >

    But why not use (file->f_flags & O_NONBLOCK) instead, as
    tracing_splice_read_pipe() does?


    --
    Regards,
    Changli Gao(xiaosuo@gmail.com)
    --
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