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Date: Mon, 30 Aug 2021 09:13:14 +0900
To: cygwin AT cygwin DOT com
Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Sun, 29 Aug 2021 17:04:56 -0400
Ken Brown wrote:
> On 8/29/2021 5:07 AM, Takashi Yano via Cygwin wrote:
> > On Sat, 28 Aug 2021 18:41:02 +0900
> > Takashi Yano wrote:
> >> On Sat, 28 Aug 2021 10:43:27 +0200
> >> Corinna Vinschen wrote:
> >>> On Aug 28 02:21, Takashi Yano via Cygwin wrote:
> >>>> On Fri, 27 Aug 2021 12:00:50 -0400
> >>>> Ken Brown wrote:
> >>>>> Two years ago I thought I needed nt_create to avoid problems when calling
> >>>>> set_pipe_non_blocking.  Are you saying that's not an issue?  Is
> >>>>> set_pipe_non_blocking unnecessary?  Is that the point of your modification to
> >>>>> raw_read?
> >>>>
> >>>> Yes. Instead of making windows read function itself non-blocking,
> >>>> it is possible to check if the pipe can be read before read using
> >>>> PeekNamedPipe(). If the pipe cannot be read right now, EAGAIN is
> >>>> returned.
> >>>
> >>> The problem is this:
> >>>
> >>>    if (PeekNamedPipe())
> >>>      ReadFile(blocking);
> >>>
> >>> is not atomic.  I. e., if PeekNamedPipe succeeds, nothing keeps another
> >>> thread from draining the pipe between the PeekNamedPipe and the ReadFile
> >>> call.  And as soon as ReadFile runs, it hangs indefinitely and we can't
> >>> stop it via a signal.
> >>
> >> Hmm, you are right. Mutex guard seems to be necessary like pty code
> >> if we go this way.
> > 
> > I have found that set_pipe_non_blocking() succeeds for both read and
> > write pipes if the write pipe is created by CreateNamedPipe() and the
> > read pipe is created by CreateFile() contrary to the current create()
> > code. Therefore, not only nt_create() but also PeekNamedPipe() become
> > unnecessary.
> > 
> > Please see the revised patch attached.
> 
> I haven't had a chance to test this myself yet, but occurs to me that we might 
> have a different problem after this patch: Does the write handle that we get 
> from CreateNamedPipe() have FILE_READ_ATTRIBUTES access?

I have just checked this, and the answer is "No". Due to this problem,
NtQueryInformationFile() call in select() fails on the write pipe.

It seems that we need more consideration...

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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