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    SubjectRe: [Bug #14015] pty regressed again, breaking expect and gcc's testsuite
    Linus Torvalds <> writes:

    > On Tue, 1 Sep 2009, Rafael J. Wysocki wrote:
    >> On Tuesday 01 September 2009, Mikael Pettersson wrote:
    >> >
    >> > Starting with 2.6.31-rc8 and reverting
    >> >
    >> > 85dfd81dc57e8183a277ddd7a56aa65c96f3f487 pty: fix data loss when stopped (^S/^Q)
    >> > d945cb9cce20ac7143c2de8d88b187f62db99bdc pty: Rework the pty layer to use the normal buffering logic
    >> >
    >> > in that order gives me a kernel that works on both x86 and powerpc64.
    >> >
    >> > So the bug is definitely limited to the pty buffering logic change.
    >> Thanks a lot for this information, adding somme CCs to the list.
    > Mikael, is there any way to get the gcc testsuite to show the "expected"
    > vs "result" cases when the failures occur, so that we can see what the
    > pattern is ("it drops one character every 8kB" or something like that).
    > However, I get the feeling that it's really the same bug that
    > OGAWA-san already fixed - and that his fix just doesn't always do a 100%
    > of the job.
    > So what Ogawa did was to make sure that we flush any pending data whenever
    > we;re checking "do we have any data left". He did that by calling out to
    > tty_flush_to_ldisc(), which should flush the data through to the ldisc.
    > The keyword here being "should". In flush_to_ldisc(), we have at least one
    > case where we say "we'll delay it a bit more":
    > if (!tty->receive_room) {
    > schedule_delayed_work(&tty->, 1);
    > break;
    > }
    > and while I think this _should_ be ok (because if there is no
    > receive-room, then we'll hopefully always return non-zero from
    > "input_available_p()". However, we do have this really odd case that the
    > reader side will do "n_tty_set_room()" onlyl _after_ having checked for
    > input_available_p(), and so maybe we do sometimes trigger the case that
    > - input_available_p() tries to flush to the input buffer before checking
    > how much data is available, by calling 'tty_flush_to_ldisc()'
    > - but 'tty_flush_to_ldisc()' won't do anything, because tty->receive_room
    > is zero.
    > - so now input_available_p will say "I don't have any data", even though
    > there was data in the write buffers.
    > - we'll notice that the other end has hung up, and return EOF/EIO.
    > - which is very WRONG, because the other end may have hung up, but before
    > it did that, it wrote data that is still in the write queues, and we
    > should have returned that data.
    > Anyway, I'm not at all sure that the "receive_room == 0" case can happen
    > at all, but maybe it can. Ogawa-san?

    If I'm not missing, I think it doesn't have big change with old
    code. But I would need to check more deeply.

    Um.., If "receive_room == 0 && tty->read_cnt == 0" is possible, I wonder
    why reverting buffer handling fixes the problem.

    Well, anyway, I'd like to reproduce this on my machine. Could you tell
    me the version of tools? I guess gcc testsuite using the gcc's source
    (svn revision?), expect, dejagnu, tcl. (BTW, I'm using debian
    testing. If it can be reproduced on kvm, I can install distro version
    which you are using)

    OGAWA Hirofumi <>

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      Last update: 2009-09-03 13:33    [W:0.046 / U:1.924 seconds]
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