Re: [Patch][v5] libgomp/nvptx: Prepare for reverse-offload callback handling

[Thomas Schwinge <thomas@codesourcery.com>]

Hi Tobias!

On 2022-10-24T21:05:46+0200, I wrote:
> On 2022-10-24T16:07:25+0200, Jakub Jelinek via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>> On Wed, Oct 12, 2022 at 10:55:26AM +0200, Tobias Burnus wrote:
>>> libgomp/nvptx: Prepare for reverse-offload callback handling
>
>> Ok, thanks.
>
> Per commit r13-3460-g131d18e928a3ea1ab2d3bf61aa92d68a8a254609
> "libgomp/nvptx: Prepare for reverse-offload callback handling",
> I'm seeing a lot of libgomp execution test regressions.  Random
> example, 'libgomp.c-c++-common/error-1.c':
>
>     [...]
>       GOMP_OFFLOAD_run: kernel main$_omp_fn$0: launch [(teams: 1), 1, 1] [(lanes: 32), (threads: 8), 1]
>
>     Thread 1 "a.out" received signal SIGSEGV, Segmentation fault.
>     0x00007ffff793b87d in GOMP_OFFLOAD_run (ord=<optimized out>, tgt_fn=<optimized out>, tgt_vars=<optimized out>, args=<optimized out>) at [...]/source-gcc/libgomp/plugin/plugin-nvptx.c:2127
>     2127            if (__atomic_load_n (&ptx_dev->rev_data->fn, __ATOMIC_ACQUIRE) != 0)
>     (gdb) print ptx_dev
>     $1 = (struct ptx_device *) 0x6a55a0
>     (gdb) print ptx_dev->rev_data
>     $2 = (struct rev_offload *) 0xffffffff00000000
>     (gdb) print ptx_dev->rev_data->fn
>     Cannot access memory at address 0xffffffff00000000
>
> Why is it even taking this 'if (reverse_offload)' code path, which isn't
> applicable to this test case (as far as I understand)?  (Well, the answer
> is 'bool reverse_offload = ptx_dev->rev_data != NULL;', but why is that?)

Well.

    --- a/libgomp/plugin/plugin-nvptx.c
    +++ b/libgomp/plugin/plugin-nvptx.c

    @@ -329,6 +332,7 @@ struct ptx_device
           pthread_mutex_t lock;
         } omp_stacks;

    +  struct rev_offload *rev_data;
       struct ptx_device *next;
     };

... but as far as I can tell, this is never initialized in
'nvptx_open_device', which does 'ptx_dev = GOMP_PLUGIN_malloc ([...]);'.
Would the following be the correct fix (currently testing)?

    --- libgomp/plugin/plugin-nvptx.c
    +++ libgomp/plugin/plugin-nvptx.c
    @@ -546,6 +546,8 @@ nvptx_open_device (int n)
       ptx_dev->omp_stacks.size = 0;
       pthread_mutex_init (&ptx_dev->omp_stacks.lock, NULL);

    +  ptx_dev->rev_data = NULL;
    +
       return ptx_dev;
     }



Grüße
 Thomas
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