[PATCH RFC] fork: reduce chances for "address space is already occupied" errors

[Corinna Vinschen]

On Mar 26 19:25, Corinna Vinschen wrote:
> Hi Michael,
> 
> 
> Redirected to cygwin-patches...
> 
> 
> On Mar 26 18:10, Michael Haubenwallner wrote:
> > Hi Corinna,
> > 
> > as I do still encounter fork errors (address space needed by <dll> is
> > already occupied) with dynamically loaded dlls (but unrelated to
> > replaced dlls), one of them repeating even upon multiple retries,
> 
> Why didn't rebase fix that?

Btw., is that 32 or 64 bit?  Both?


Corinna


> >  I'm
> > coming up with attached patch.
> > 
> > What do you think about it?
> 
> I'm not opposed to this patch but I don't quite follow the description.
> threadinterface->Init only creates three event objects.  From what I can
> tell, Events are stored in Paged and Nonpaged Pools, so they don't
> affect the processes VM.  What am I missing?
> 
> 
> Thanks,
> Corinna
> 
> 
> > 
> > Thanks!
> > /haubi/
> 
> > >From dfc28bcbb7ed55fe33ddb8d15e761b4d5b4815f8 Mon Sep 17 00:00:00 2001
> > From: Michael Haubenwallner <michael.haubenwallner@ssi-schaefer.com>
> > Date: Tue, 26 Mar 2019 17:38:36 +0100
> > Subject: [PATCH] Cygwin: fork: reserve dynloaded dll areas earlier
> > 
> > In dll_crt0_0, both threadinterface->Init and sigproc_init allocate
> > windows object handles using unpredictable memory regions, which may
> > collide with dynamically loaded dlls when they were relocated.
> > ---
> >  winsup/cygwin/dcrt0.cc | 6 ++++++
> >  winsup/cygwin/fork.cc  | 6 ------
> >  2 files changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/winsup/cygwin/dcrt0.cc b/winsup/cygwin/dcrt0.cc
> > index 11edcdf0d..fb726a739 100644
> > --- a/winsup/cygwin/dcrt0.cc
> > +++ b/winsup/cygwin/dcrt0.cc
> > @@ -632,6 +632,12 @@ child_info_fork::handle_fork ()
> >  
> >    if (fixup_mmaps_after_fork (parent))
> >      api_fatal ("recreate_mmaps_after_fork_failed");
> > +
> > +  /* We need to occupy the address space for dynamically loaded dlls
> > +     before we allocate any dynamic object, or we may end up with
> > +     error "address space needed by <dll> is already occupied"
> > +     for no good reason (seen with some relocated dll). */
> > +  dlls.reserve_space ();
> >  }
> >  
> >  bool
> > diff --git a/winsup/cygwin/fork.cc b/winsup/cygwin/fork.cc
> > index 74ee9acf4..7e1c08990 100644
> > --- a/winsup/cygwin/fork.cc
> > +++ b/winsup/cygwin/fork.cc
> > @@ -136,12 +136,6 @@ frok::child (volatile char * volatile here)
> >  {
> >    HANDLE& hParent = ch.parent;
> >  
> > -  /* NOTE: Logically this belongs in dll_list::load_after_fork, but by
> > -     doing it here, before the first sync_with_parent, we can exploit
> > -     the existing retry mechanism in hopes of getting a more favorable
> > -     address space layout next time. */
> > -  dlls.reserve_space ();
> > -
> >    sync_with_parent ("after longjmp", true);
> >    debug_printf ("child is running.  pid %d, ppid %d, stack here %p",
> >  		myself->pid, myself->ppid, __builtin_frame_address (0));
> > -- 
> > 2.17.0
> > 
> > 
> 
> > 
> > --
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> 
> 
> -- 
> Corinna Vinschen
> Cygwin Maintainer



-- 
Corinna Vinschen
Cygwin Maintainer
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