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




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?

>  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@xxxxxxxxxxxxxxxx>
> 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

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