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Subject: Re: [OMPI devel] bug in mca framework?
From: Joshua Ladd (joshual_at_[hidden])
Date: 2013-12-17 10:33:48


I believe Devendar Bureddy nailed the root cause. I am providing his excellent analysis below:

>From Devendar:

with curiosity i looked at this issue. here's my 2 cents
I think issue is because of BTL components is opened&closed twice(ompi_init, yoda) which leading to incorrect usage of var groups. The following sequence of events creating invalid memory

1) all openib component parameters registered in ompi_mpi_init
main > start_pes> shmem_init -> oshmem_shmem_init -> ompi_mpi_init -> mca_base_framework_open -> mca_pml_base_open ..... mca_bml_base_open... -> btl_openib_component_register()

* for all string variables it allocated a memory block (var->mbv_storage = PTR)

At this time a new var group id:114 (of parent group id: 112) is created for all openib component variables.

2) This var group is de-registered in ompi_mpi_init. It marks all variables as invalid. but, the group&vars is still exist
main > start_pes> shmem_init -> oshmem_shmem_init -> mca_pml_base_select -> mca_base_components_close -> ... -> mca_bml_base_close -> mca_base_framework_close -> mca_base_var_group_deregister(groupid: 114) * all string variables memory is deallocated ( set var->mbv_storage = NULL;)

3) because of step 2). btl_openib.so shared lib dlclosed

4) Now we are reopening openib in yoda and registering the openib variables again.
main > start_pes> shmem_init > oshmem_shmem_init -> _shmem_init -> mca_base_framework_open -> mca_spml_base_open> mca_spml_yoda_component_open-> ..... mca_bml_base_open... -> btl_openib_component_register -> register_variables()

* In register_variables(), var_find() finds this variable( from the same old group: 114) and reset the variables.
* For string variables, it allocated the buffers again ( (var->mbv_storage = PTR)
* note that group:114 is not belongs to yoda component.

5) In yoda component close, it never finds above group(114) because this is not belongs to this component. So, do not call mca_base_var_group_deregister() again on the var group. string var memory is not deallocated.
main > start_pes> shmem_init > oshmem_shmem_init -> _shmem_init -> mca_spml_base_select ->..> mca_spml_yoda_component_close -> mca_bml_base_close -> mca_base_var_group_find().

6) because of step 5), the btl_openib.so is dlclosed(). This step invalidates, all openib string vars memory ( var->mbv_storage = PTR) allocated in step 4)

7) in ompi_mpi_finalize(), it will loop through all vars and finalizes and deallocate the string var memory (var->mbv_storage = PTR)
ompi_mpi_finalize >...> mca_base_var_finalize * var->mbv_storage = PTR is invalid at this stage and causing the SEGFAULT.

This also explains why Dinar's patch, kostul_fix.patch (http://bgate.mellanox.com/redmine/attachments/1643/kostul_fix.patch), resolves the issue. His patch prevents you from finding the invalid already opened params.
So, I see in a lot of these registration functions the signature has an entry for the project name, but now, NULL, is always passed. I see a note by Nathan in

../opal/mca/base/mca_base_var.c +1311
{
/* XXX -- component_update -- We will stash the project name in the component */
return mca_base_var_register (NULL, component->mca_type_name,

Seems knowing the project name, oshmem, would allow us to distinguish between the different BMLs.

Nathan, please advise.

Josh

-----Original Message-----
From: devel [mailto:devel-bounces_at_[hidden]] On Behalf Of Nathan Hjelm
Sent: Monday, December 16, 2013 12:44 PM
To: Open MPI Developers
Subject: Re: [OMPI devel] bug in mca framework?

On Mon, Dec 16, 2013 at 05:21:05PM +0000, Joshua Ladd wrote:
> After speaking with Igor Ivanov about this this morning, he summarized his findings as follows:
>
> 1. Valgrind comes up clean.

Thats good to hear but unfortunate since this seems really like a stomping-on-memory problem.

> 2. The issue is not reproduced with a static build.

This is a red-herring. The variable itself contains garbage. The mbv_storage pointer looked like it was on the stack, the name was not valid, etc. Not sure how we got an mca_base_var_t into that state since the only time we touch anything in them is in mca_base_var_finalize. That functions cleans up all of the state to two calls to it should be harmless.

> 3. A bisection study reveals that problems first appear after commit:
> https://svn.open-mpi.org/trac/ompi/changeset/28800/trunk/opal/mca/base
> /mca_base_var.c

Possibly also a coincidence. That commit only 1) moves the group stuff into its own file, and 2) adds the mca_base_pvar interface. Its possible I messed something up in the rest of the code but unlikely. I will take another look though.

-Nathan

>
>
> Josh
>
> -----Original Message-----
> From: devel [mailto:devel-bounces_at_[hidden]] On Behalf Of Jeff
> Squyres (jsquyres)
> Sent: Monday, December 16, 2013 12:15 PM
> To: Open MPI Developers
> Subject: Re: [OMPI devel] bug in mca framework?
>
> It might be worthwhile to run this through valgrind and see if something is being freed incorrectly...?
>
>
> On Dec 16, 2013, at 12:11 PM, Nathan Hjelm <hjelmn_at_[hidden]> wrote:
>
> > I took a look at the stacktraces last week and could not identify
> > where the bug is. I will dig deeper this week and see if I can come up with the correct fix.
> >
> > -Nathan
> >
> > On Mon, Dec 09, 2013 at 03:17:36PM +0200, Mike Dubman wrote:
> >> Nathan,
> >> Could you please comment on the Igor`s observations?
> >> Thanks
> >>
> >> On Wed, Dec 4, 2013 at 4:44 PM, Igor Ivanov <igor.ivanov_at_[hidden]>
> >> wrote:
> >>
> >> On 04.12.2013 17:56, Jeff Squyres (jsquyres) wrote:
> >>
> >> On Dec 4, 2013, at 2:52 AM, Igor Ivanov <Igor.Ivanov_at_[hidden]>
> >> wrote:
> >>
> >> It is the first mca variable with type as string from btl/openib as
> >> 'device_param_files'. Actually you can disable it and get failure on
> >> the second.
> >>
> >> Description of case we see:
> >> 1. openib mca variables are registered during startup as stage at
> >> select component phase;
> >> 2. but a winner is cm component and openib mca variables are
> >> deregistered as part of mca group;
> >> 3. mca variables are not removed from global mca array but they
> >> marked as invalid and memory for string is freed;
> >> 4. shmem needs openib for yoda and does bml initialization;
> >> 5. openib mca variables are registered againusing light mode as
> >> searching itself in global array and refreshing their
> >> fields again;
> >>
> >> Can you explain what you mean by step 5? I.e., what does "using light
> >> mode" mean? Is the openib component register function invoked again?
> >>
> >> It is correct, it is called twice. "light mode" means that
> >> mca_base_var_register() does not allocate mca variable object again, it
> >> seeks this variable in global array and finding it updates fields in
> >> mca_base_var_t structure (at least mbv_storage).
> >>
> >> 6. for unknown reason bml finalization does not clean these vars as
> >> it is done in step 2;
> >> 7. mca_btl_openib.so is unloaded;
> >> 8. opal_finalize() destroys mca variables form global array,
> >> observes openib`s variable, try destroy using non accessed
> >> address;
> >>
> >> So a code that is under discussion fixes step 6.
> >>
> >> Nathan: it sounds like an MCA var (and entire group) is registered,
> >> unregistered, and then registered again. Does the MCA var system get
> >> confused here when it tries to unregister the group a 2nd time?
> >>
> >> Probably issue relates incorrect recognition if variable valid/invalid
> >> during second call of mca_base_var_deregister().
> >>
> >> _______________________________________________
> >> devel mailing list
> >> devel_at_[hidden]
> >> http://www.open-mpi.org/mailman/listinfo.cgi/devel
> >
> >> _______________________________________________
> >> devel mailing list
> >> devel_at_[hidden]
> >> http://www.open-mpi.org/mailman/listinfo.cgi/devel
> >
> > _______________________________________________
> > devel mailing list
> > devel_at_[hidden]
> > http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to:
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