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Avoid crashing when LRU cache keys change. #96930

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Avoid crashing when LRU cache keys change. #96930

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1 change: 1 addition & 0 deletions third_party/xla/xla/pjrt/BUILD
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Original file line number Diff line number Diff line change
Expand Up @@ -860,6 +860,7 @@ cc_library(
]),
deps = [
"@com_google_absl//absl/container:node_hash_map",
"@com_google_absl//absl/log:check",
"@local_tsl//tsl/platform:logging",
],
)
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12 changes: 9 additions & 3 deletions third_party/xla/xla/pjrt/lru_cache.h
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Expand Up @@ -20,6 +20,7 @@ limitations under the License.
#include <unordered_map>

#include "absl/container/node_hash_map.h"
#include "absl/log/check.h"
#include "tsl/platform/logging.h"

namespace xla {
Expand Down Expand Up @@ -180,12 +181,17 @@ Value LRUCache<Key, Value, Hash, Eq>::GetOrCreateIfAbsent(
// Evict an LRU entry if we are over capacity.
if (lru_list_->size_ > lru_list_->capacity_) {
Entry* to_remove = static_cast<Entry*>(lru_head.next);
to_remove->next->prev = &lru_head;
lru_head.next = to_remove->next;
// Extract instead of erase in case the kv pair contains python objects
// whose destruction could call back into this code. Extract causes the
// dtor to be delayed until the kv pair is fully removed from the map.
to_remove->container->entries_.extract(*to_remove->key);
//
// TODO: mwhittaker - Add a check that node is equal to to_remove. If it's
// not, then the user has probably mutated a key, which is undefined
// behavior.
auto node = to_remove->container->entries_.extract(*to_remove->key);
CHECK(!node.empty());
node.mapped().next->prev = node.mapped().prev;
node.mapped().prev->next = node.mapped().next;
--lru_list_->size_;
}
return v;
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47 changes: 47 additions & 0 deletions third_party/xla/xla/pjrt/lru_cache_test.cc
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Expand Up @@ -15,6 +15,7 @@ limitations under the License.

#include "xla/pjrt/lru_cache.h"

#include <cstdint>
#include <random>

#include "xla/hlo/testlib/test.h"
Expand Down Expand Up @@ -115,5 +116,51 @@ TEST(LRUCache, RandomInsertions) {
}
}

struct Int {
int val;
};

struct IntHash {
uint64_t operator()(Int*) const { return 0; }
};

struct IntEq {
bool operator()(Int* x, Int* y) const { return x->val == y->val; }
};

TEST(LRUCache, ChangingKeys) {
// This test checks the behavior of LRUCache in the face of changing keys.
// Technically, changing keys leads to undefined behavior, but practically, we
// are checking that this does not crash. The workload itself is written
// pathologically to trigger a previous bug.
//
// The following two blocks differ only in whether they assign b.val to a.val
// or vice versa. One of the two used to crash.

{
LRUCache<Int*, int, IntHash, IntEq>::LRUList list(2);
LRUCache<Int*, int, IntHash, IntEq> cache(&list);
Int a{1};
Int b{2};
Int c{3};
cache.GetOrCreateIfAbsent(&a, [](Int*) { return 1; });
cache.GetOrCreateIfAbsent(&b, [](Int*) { return 2; });
a.val = b.val;
cache.GetOrCreateIfAbsent(&c, [](Int*) { return 3; });
}

{
LRUCache<Int*, int, IntHash, IntEq>::LRUList list(2);
LRUCache<Int*, int, IntHash, IntEq> cache(&list);
Int a{1};
Int b{2};
Int c{3};
cache.GetOrCreateIfAbsent(&a, [](Int*) { return 1; });
cache.GetOrCreateIfAbsent(&b, [](Int*) { return 2; });
b.val = a.val;
cache.GetOrCreateIfAbsent(&c, [](Int*) { return 3; });
}
}

} // namespace
} // namespace xla