From f0d487dce529d46ffe7925e9f8f0435781e49422 Mon Sep 17 00:00:00 2001 From: Petros Angelatos Date: Sat, 8 Apr 2023 19:41:50 +0200 Subject: [PATCH] sync::mpsc: synchronize receiver disconnect with initialization Receiver disconnection relies on the incorrect assumption that `head.index != tail.index` implies that the channel is initialized (i.e `head.block` and `tail.block` point to allocated blocks). However, it can happen that `head.index != tail.index` and `head.block == null` at the same time which leads to a segfault when a channel is dropped in that state. This can happen because initialization is performed in two steps. First, the tail block is allocated and the `tail.block` is set. If that is successful `head.block` is set to the same pointer. Importantly, initialization is skipped if `tail.block` is not null. Therefore we can have the following situation: 1. Thread A starts to send the first value of the channel, observes that `tail.block` is null and begins initialization. It sets `tail.block` to point to a newly allocated block and then gets preempted. `head.block` is still null at this point. 2. Thread B starts to send the second value of the channel, observes that `tail.block` *is not* null and proceeds with writing its value in the allocated tail block and sets `tail.index` to 1. 3. Thread B drops the receiver of the channel which observes that `head.index != tail.index` (0 and 1 respectively), therefore there must be messages to drop. It starts traversing the linked list from `head.block` which is still a null pointer, leading to a segfault. This PR fixes this problem by waiting for initialization to complete when `head.index != tail.index` and the `head.block` is still null. A similar check exists in `start_recv` for similar reasons. Fixes #110001 Signed-off-by: Petros Angelatos --- library/std/src/sync/mpmc/list.rs | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/library/std/src/sync/mpmc/list.rs b/library/std/src/sync/mpmc/list.rs index ec6c0726ac790..406a331a30961 100644 --- a/library/std/src/sync/mpmc/list.rs +++ b/library/std/src/sync/mpmc/list.rs @@ -549,6 +549,18 @@ impl Channel { let mut head = self.head.index.load(Ordering::Acquire); let mut block = self.head.block.load(Ordering::Acquire); + // If we're going to be dropping messages we need to synchronize with initialization + if head >> SHIFT != tail >> SHIFT { + // The block can be null here only if a sender is in the process of initializing the + // channel while another sender managed to send a message by inserting it into the + // semi-initialized channel and advanced the tail. + // In that case, just wait until it gets initialized. + while block.is_null() { + backoff.spin_heavy(); + block = self.head.block.load(Ordering::Acquire); + } + } + unsafe { // Drop all messages between head and tail and deallocate the heap-allocated blocks. while head >> SHIFT != tail >> SHIFT {