runtime: fix possible deadlock in in_mem_accounts_index

[BurtonQin]

Problem

There is a possible deadlock in in_mem_accounts_index.

self.map() returns &RwLock<...>.

The first Read lock in fn items:

solana/runtime/src/in_mem_accounts_index.rs

Lines 143 to 150 in a5f290a

	let map = self.map().read().unwrap();
	let mut result = Vec::with_capacity(map.len());
	map.iter().for_each(|(k, v)| {
	if range.contains(k) {
	result.push((*k, Arc::clone(v)));
	}
	});
	self.hold_range_in_memory(range, false);

solana/runtime/src/in_mem_accounts_index.rs

Lines 844 to 854 in a5f290a

	pub fn hold_range_in_memory<R>(&self, range: &R, start_holding: bool)
	where
	R: RangeBounds<Pubkey> + Debug,
	{
	let evictions_guard = EvictionsGuard::lock(self);
	if !start_holding || !self.add_hold_range_in_memory_if_already_held(range, &evictions_guard)
	{
	if start_holding {
	// put everything in the cache and it will be held there
	self.put_range_in_cache(&Some(range), &evictions_guard);

The second Write lock in fn put_range_in_cache:

solana/runtime/src/in_mem_accounts_index.rs

Lines 861 to 871 in a5f290a

	fn put_range_in_cache<R>(&self, range: &Option<&R>, _evictions_guard: &EvictionsGuard)
	where
	R: RangeBounds<Pubkey>,
	{
	assert!(self.get_stop_evictions()); // caller should be controlling the lifetime of how long this needs to be present
	let m = Measure::start("range");
	let mut added_to_mem = 0;
	// load from disk
	if let Some(disk) = self.bucket.as_ref() {
	let mut map = self.map().write().unwrap();

Summary of Changes

Add drop(map) before calling fn hold_range_in_memory

Fixes #

[brooksprumo]

Good catch! Thank you for fixing!