[DPDK/ethdev Bug 1875] mlx5: pthread mutex in shared memory missing PTHREAD_PROCESS_SHARED

[bugzilla]

http://bugs.dpdk.org/show_bug.cgi?id=1875

            Bug ID: 1875
           Summary: mlx5: pthread mutex in shared memory missing
                    PTHREAD_PROCESS_SHARED
           Product: DPDK
           Version: 25.11
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: ethdev
          Assignee: dev@dpdk.org
          Reporter: step...@networkplumber.org
  Target Milestone: ---

This is the mlx5 subset of bugzilla 662

The mlx5 specific analysis was:

## MLX5 Driver PTHREAD_PROCESS_SHARED Issue

### Summary

The mlx5 driver has pthread mutexes in shared memory that are initialized
without `PTHREAD_PROCESS_SHARED`, which causes undefined behavior when used
across multiple processes.

### Affected Mutexes

1. **`sh->txpp.mutex`** (`drivers/net/mlx5/mlx5.c`, line 1846)
   - Location: `struct mlx5_dev_txpp` inside `struct mlx5_dev_ctx_shared`
   - Purpose: Pacing create/destroy mutex for Tx packet pacing

2. **`sh->lwm_config_lock`** (`drivers/net/mlx5/mlx5.c`, line 1963)
   - Location: `struct mlx5_dev_ctx_shared`
   - Purpose: Low watermark configuration lock

### Why This Is a Problem

The `mlx5_dev_ctx_shared` structure is allocated via `mlx5_malloc(MLX5_MEM_ZERO
| MLX5_MEM_RTE, ...)` which uses hugepage-backed shared memory. This shared
context is accessible by both primary and secondary processes.

While the shared context is only **allocated** in the primary process (enforced
by assertion at line 1826), secondary processes can obtain a reference to this
shared context and potentially call operations that lock these mutexes.

Per POSIX, mutexes in shared memory accessed by multiple processes must be
initialized with `PTHREAD_PROCESS_SHARED` attribute. Without this,
synchronization between processes is undefined behavior.

### Note

A third mutex, `mlx5_dev_ctx_list_mutex` (line 201), is a static global
variable and is **not affected** since each process has its own copy.

### Suggested Fix

Initialize the mutexes with `PTHREAD_PROCESS_SHARED`:

```c
static void
mlx5_init_shared_mutex(pthread_mutex_t *mutex)
{
    pthread_mutexattr_t attr;

    pthread_mutexattr_init(&attr);
    pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
    pthread_mutex_init(mutex, &attr);
    pthread_mutexattr_destroy(&attr);
}
```

Then replace:
```c
pthread_mutex_init(&sh->txpp.mutex, NULL);
pthread_mutex_init(&priv->sh->lwm_config_lock, NULL);
```

With:
```c
mlx5_init_shared_mutex(&sh->txpp.mutex);
mlx5_init_shared_mutex(&priv->sh->lwm_config_lock);
```

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