Re: [newlib-cygwin] Cygwin: signalfd: implement non-polling select

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Jan 16 07:09, Ryan Johnson wrote:
> On 1/15/2019 4:22 AM, Corinna Vinschen wrote:
> > On Jan 15 10:50, Václav Haisman wrote:
> > > On Mon, 14 Jan 2019 at 17:19, Corinna Vinschen <corinna@sourceware.org> wrote:
> > > > https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=f42776fa781de858a927bc03aa966a0f3096b581
> > > > 
> > > > commit f42776fa781de858a927bc03aa966a0f3096b581
> > > > Author: Corinna Vinschen <corinna@vinschen.de>
> > > > Date:   Mon Jan 14 17:19:37 2019 +0100
> > > > 
> > > >      Cygwin: signalfd: implement non-polling select
> > > > 
> > > >      Allow the signal thread to recognize we're called in consequence of
> > > >      select on a signalfd.  If the signal is part of the wait mask, don't
> > > >      call any signal handler and don't remove the signal from the queue,
> > > >      so a subsequent read (or sigwaitinfo/sigtimedwait) still gets the
> > > >      signal.  Instead, just signal the event object at
> > > >      _cygtls::signalfd_select_wait for the thread running select.
> > > > 
> > > >      The addition of signalfd_select_wait to _cygtls unearthed the alignment
> > > >      problem of the context member again.  To make sure this doesn't get lost,
> > > >      improve the related comment in the header file so that this (hopefully)
> > > >      doesn't get lost (again).
> > > > 
> > > >      Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
> > > > [...]
> > > > 
> > > > diff --git a/winsup/cygwin/cygtls.h b/winsup/cygwin/cygtls.h
> > > > index 39dba13..65a905c 100644
> > > > --- a/winsup/cygwin/cygtls.h
> > > > +++ b/winsup/cygwin/cygtls.h
> > > > @@ -189,8 +189,11 @@ public:
> > > >     stack_t altstack;
> > > >     siginfo_t *sigwait_info;
> > > >     HANDLE signal_arrived;
> > > > +  HANDLE signalfd_select_wait;
> > > >     bool will_wait_for_signal;
> > > > -  long __align;                        /* Needed to align context to 16 byte. */
> > > > +  /* context MUST be aligned to 16 byte, otherwise RtlCaptureContext fails.
> > > > +     If you prepend cygtls members here, make sure context stays 16 byte
> > > > +     aligned. */
> > > >     ucontext_t context;
> > > > [...]
> > > Would it not better to do this with `ucontext_t context
> > > __attribute__((__aligned__(16)));` instead?
> > In theory yes, but it doesn't work.  The simple perl parser in
> > gentls_offsets doesn't understand __attribute__ and screws up,
> > so generating tlsoffsets{64}.h fails.
> 
> Why not introduce a typedef in some convenient location the perl script
> doesn't examine, e.g.
> 
> typedef ucontext_t __attribute__((aligned(16))) aligned_ucontext_t;
> 
> And then use that in the struct?
> 
> Ryan

Great idea... it just doesn't work either, same as
https://cygwin.com/ml/cygwin-developers/2019-01/msg00006.html


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

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