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    SubjectRe: [patch] sys_sync livelock fix
    On Tue, 12 Feb 2002, Andrew Morton wrote:

    > IMO, the SuS definition sucks. We really do want to do our best to
    > ensure that pending writes are committed to disk before sys_sync()
    > returns. As long as that doesn't involve waiting until mid-August.

    The current behaviour allows the system to hang forever waiting for
    sync(2). In practice it does actually wait minutes on a busy system (df
    has --no-sync for that reason) when there is no reason for that to happen.
    I think that not only sucks worse, it's non-standard as well.

    > For example, ext3 users get to enjoy rebooting with `sync ; reboot -f'
    > to get around all those silly shutdown scripts. This very much relies
    > upon the sync waiting upon the I/O.

    Because people count on something broken we should keep the bug? You do
    realize that the sync may NEVER finish? That's not in theory, I have news
    servers which may wait overnight without finishing a "df" iwthout the
    option.

    > I mean, according to SUS, our sys_sync() implementation could be
    >
    > asmlinkage void sys_sync(void)
    > {
    > return;
    > }
    >
    > Because all I/O is already scheduled, thanks to kupdate.

    I think the wording is queued, and I would read that as "on the
    elevator." Your example is a good example of bad practive, since even with
    ext3 a program creating files quickly would lose data, even though the
    directory structure is returned to a known state, without stopping the
    writing processes the results are unknown.

    > But we want sync to be useful.

    No one has proposed otherwise. Unless you think that a possible hang is
    useful, the questions becomes adding all dirty buffers to the elevator,
    then (a) waiting or (b) returning. Either satisfies SuSv2.

    >
    > >
    > > If this were only a performance issue I wouldn't push for prompt
    > > implementation, but anything which can hang the system, particularly in
    > > shutdown, is bad.
    > >
    >
    > If shutdown hangs, it's probably due to something else.

    If you discount the evidence you can prove anything... or disbelieve
    anything.

    --
    bill davidsen <davidsen@tmr.com>
    CTO, TMR Associates, Inc
    Doing interesting things with little computers since 1979.

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