Background: NSS's PORT_FreeArena destroys arena pools. Normally it takes the memory blocks (called arenas) from the arena pool and puts them onto a free list of arenas, from which they are subsequently taken and reused. But that's bad news for leak detection systems that try to report the stack that allocated a leaked block of memory. The recycling of arenas in the arena free list causes leaked arenas to appear to have been allocated by the first stack that originally allocated the arena from the heap, no matter how many times it was "freed" to the arena free list and then subsequently reused from there. The stacks reported by the leak detection systems show the wrong stack for the allocator. So PORT_FreeArena honors an environment variable named NSS_DISABLE_ARENA_FREE_LIST. When that variable is defined, regardless of the string value, PORT_FreeArena frees arenas back to the heap by calling PL_FreeArenaPool, rather than putting the arenas onto the arena free list and recycling them by calling PL_FinishArenaPool. NSS also has a second implementation of arena pools, known as nssArenas. nssArenas and PORT_Arenas share the same arena free list. The problem: Unlike PORT_FreeArena, the function nssArena_Destroy pays no attention to NSS_DISABLE_ARENA_FREE_LIST. nssArena_Destroy always calls PL_FinishArenaPool and never calls PL_FreeArenaPool. So, even when NSS_DISABLE_ARENA_FREE_LIST is defined, nssArenas confuse memory leak checkers. The proposed fix: Change nssArena_Destroy (and nssArena_Create) to pay attention to NSS_DISABLE_ARENA_FREE_LIST and call PL_FinishArenaPool or PL_FreeArenaPool depending on its presence or absence, just as PORT_FreeArena does.
Sigh. I remembered it exactly backwards. PL_FinishArenaPool really frees the arenas in the pool PL_FreeArenaPool uses the arena free list (despite the name). So, nssArena_Destroy (and nssArena_Create) NEVER use the free list.
Status: NEW → RESOLVED
Last Resolved: 11 years ago
Resolution: --- → INVALID
Nelson, Is there any reason why we would never want the nssArena to use the free list ?
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