[Mesa-dev] [PATCH 1/2] Fix races during _mesa_HashWalk().

[Steinar H. Gunderson]

There is currently no protection against walking a hash (using
_mesa_HashWalk()) and modifying it at the same time, for instance by inserting
or deleting elements. This leads to segfaults in multithreaded code if e.g.
someone calls glTexImage2D (which may have to walk the list of FBOs) while
another thread is calling glDeleteFramebuffers on another thread with the two
contexts sharing lists.

The reason for this is that _mesa_HashWalk() doesn't actually take the mutex
that normally protects the hash; it takes an entirely different mutex.
Thus, walks are only protected against other walks, and there is also no
outer lock taking this. There is an old comment saying that this is to fix
problems with deadlock if the callback needs to take a mutex; we solve this
by changing the mutex to be recursive.

A demonstration Helgrind hit from a real application:

==13412== Possible data race during write of size 8 at 0x3498C6A8 by thread #1
==13412== Locks held: 2, at addresses 0x1AF09530 0x2B3DF400
==13412==    at 0x1F040C99: _mesa_hash_table_remove (hash_table.c:395)
==13412==    by 0x1EE98174: _mesa_HashRemove_unlocked (hash.c:350)
==13412==    by 0x1EE98174: _mesa_HashRemove (hash.c:365)
==13412==    by 0x1EE2372D: _mesa_DeleteFramebuffers (fbobject.c:2669)
==13412==    by 0x6105AA4: movit::ResourcePool::cleanup_unlinked_fbos(void*) (resource_pool.cpp:473)
==13412==    by 0x610615B: movit::ResourcePool::release_fbo(unsigned int) (resource_pool.cpp:442)
[...]
==13412== This conflicts with a previous read of size 8 by thread #20
==13412== Locks held: 2, at addresses 0x1AF09558 0x1AF73318
==13412==    at 0x1F040CD9: _mesa_hash_table_next_entry (hash_table.c:415)
==13412==    by 0x1EE982A8: _mesa_HashWalk (hash.c:426)
==13412==    by 0x1EED6DFD: _mesa_update_fbo_texture.part.33 (teximage.c:2683)
==13412==    by 0x1EED9410: _mesa_update_fbo_texture (teximage.c:3043)
==13412==    by 0x1EED9410: teximage (teximage.c:3073)
==13412==    by 0x1EEDA28F: _mesa_TexImage2D (teximage.c:3105)
==13412==    by 0x166A68: operator() (mixer.cpp:454)

There are many more interactions than just these two possible.

Cc: 11.2 12.0 13.0 <mesa-stable at lists.freedesktop.org>
Signed-off-by: Steinar H. Gunderson <steinar+mesa at gunderson.no>
---
 src/mesa/main/hash.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/src/mesa/main/hash.c b/src/mesa/main/hash.c
index 7d8a5fd..670438a 100644
--- a/src/mesa/main/hash.c
+++ b/src/mesa/main/hash.c
@@ -59,7 +59,6 @@ struct _mesa_HashTable {
    struct hash_table *ht;
    GLuint MaxKey;                        /**< highest key inserted so far */
    mtx_t Mutex;                /**< mutual exclusion lock */
-   mtx_t WalkMutex;            /**< for _mesa_HashWalk() */
    GLboolean InDeleteAll;                /**< Debug check */
    /** Value that would be in the table for DELETED_KEY_VALUE. */
    void *deleted_key_data;
@@ -129,8 +128,11 @@ _mesa_NewHashTable(void)
       }
 
       _mesa_hash_table_set_deleted_key(table->ht, uint_key(DELETED_KEY_VALUE));
-      mtx_init(&table->Mutex, mtx_plain);
-      mtx_init(&table->WalkMutex, mtx_plain);
+      /*
+       * Needs to be recursive, since the callback in _mesa_HashWalk()
+       * is allowed to call _mesa_HashRemove().
+       */
+      mtx_init(&table->Mutex, mtx_recursive);
    }
    else {
       _mesa_error_no_memory(__func__);
@@ -161,7 +163,6 @@ _mesa_DeleteHashTable(struct _mesa_HashTable *table)
    _mesa_hash_table_destroy(table->ht, NULL);
 
    mtx_destroy(&table->Mutex);
-   mtx_destroy(&table->WalkMutex);
    free(table);
 }
 
@@ -401,11 +402,6 @@ _mesa_HashDeleteAll(struct _mesa_HashTable *table,
 
 /**
  * Walk over all entries in a hash table, calling callback function for each.
- * Note: we use a separate mutex in this function to avoid a recursive
- * locking deadlock (in case the callback calls _mesa_HashRemove()) and to
- * prevent multiple threads/contexts from getting tangled up.
- * A lock-less version of this function could be used when the table will
- * not be modified.
  * \param table  the hash table to walk
  * \param callback  the callback function
  * \param userData  arbitrary pointer to pass along to the callback
@@ -422,13 +418,13 @@ _mesa_HashWalk(const struct _mesa_HashTable *table,
 
    assert(table);
    assert(callback);
-   mtx_lock(&table2->WalkMutex);
+   mtx_lock(&table2->Mutex);
    hash_table_foreach(table->ht, entry) {
       callback((uintptr_t)entry->key, entry->data, userData);
    }
    if (table->deleted_key_data)
       callback(DELETED_KEY_VALUE, table->deleted_key_data, userData);
-   mtx_unlock(&table2->WalkMutex);
+   mtx_unlock(&table2->Mutex);
 }
 
 static void
-- 
2.8.0.rc3.226.g39d4020