@@ -19,6 +19,7 @@
 - Prevent duplicated breadcrumbs on tombstone-merged native crash events ([#5888](https://github.com/getsentry/sentry-java/pull/5888))
 - Prevent a class of Session Replay deadlocks by confining lifecycle state changes to Android's main thread ([#5965](https://github.com/getsentry/sentry-java/pull/5965))
 - Symbolicate tombstone native frames for libraries loaded directly from APKs ([#5992](https://github.com/getsentry/sentry-java/pull/5992))
+- Prevent a deadlock between the app start extension and the Android performance event processor ([#6007](https://github.com/getsentry/sentry-java/pull/6007))
 
 ### Features
 
 
@@ -34,6 +34,10 @@ public interface ExtendAppStartListener {
 
   private final @NotNull AppStartMetrics metrics;
   private final @NotNull AutoClosableReentrantLock lock = new AutoClosableReentrantLock();
+  // Serializes the two finish paths against each other. Deliberately separate from `lock`:
+  // finishing re-enters the SDK and that re-entrant path takes `lock`, so the finish cannot run
+  // under `lock` (see finishTransaction). When both are held the order is finishLock, then `lock`.
+  private final @NotNull AutoClosableReentrantLock finishLock = new AutoClosableReentrantLock();
 
   private @Nullable ExtendAppStartListener extendAppStartListener;
   // We hold onto both the span and its transaction because they mean different things and finish
@@ -108,8 +112,12 @@ public void setData(final @NotNull String key, final @Nullable Object value) {
 
   @Override
   public void finishExtendedAppStart() {
-    try (final @NotNull ISentryLifecycleToken ignored = lock.acquire()) {
-      final @Nullable ISpan span = extendedSpan;
+    try (final @NotNull ISentryLifecycleToken ignoredFinish = finishLock.acquire()) {
+      final @Nullable ISpan span;
+      try (final @NotNull ISentryLifecycleToken ignored = lock.acquire()) {
+        span = extendedSpan;
+      }
+      // Finishing runs outside `lock`, see the note on finishTransaction.
       if (span != null && !span.isFinished()) {
         span.finish(SpanStatus.OK);
       }
@@ -145,10 +153,19 @@ public boolean isExtended() {
   }
 
   public void finishTransaction(final @NotNull SentryDate endTimestamp) {
-    try (final @NotNull ISentryLifecycleToken ignored = lock.acquire()) {
-      final @Nullable ITransaction transaction = extendedTransaction;
+    // Finishing has to run outside `lock`: it captures the transaction synchronously, which runs
+    // PerformanceAndroidEventProcessor, which calls back into isExtended()/getExtendedEndTime()
+    // while holding its own lock. Holding `lock` across the call would let the two be taken in
+    // opposite orders and deadlock. finishLock still serializes this against
+    // finishExtendedAppStart, so the end-time clamp below and the finish stay atomic.
+    try (final @NotNull ISentryLifecycleToken ignoredFinish = finishLock.acquire()) {
+      final @Nullable ITransaction transaction;
+      final @Nullable ISpan span;
+      try (final @NotNull ISentryLifecycleToken ignored = lock.acquire()) {
+        transaction = extendedTransaction;
+        span = extendedSpan;
+      }
       if (transaction != null && !transaction.isFinished()) {
-        final @Nullable ISpan span = extendedSpan;
         final @Nullable SentryDate spanEnd = span == null ? null : span.getFinishDate();
         final @NotNull SentryDate end =
             spanEnd != null && spanEnd.isAfter(endTimestamp) ? spanEnd : endTimestamp;
 
@@ -8,6 +8,9 @@ import io.sentry.SentryLongDate
 import io.sentry.SentryNanotimeDate
 import io.sentry.SpanStatus
 import io.sentry.android.core.performance.AppStartMetrics
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicBoolean
 import java.util.concurrent.atomic.AtomicInteger
 import kotlin.test.Test
 import kotlin.test.assertEquals
@@ -17,6 +20,7 @@ import kotlin.test.assertSame
 import kotlin.test.assertTrue
 import org.junit.runner.RunWith
 import org.mockito.kotlin.any
+import org.mockito.kotlin.doAnswer
 import org.mockito.kotlin.mock
 import org.mockito.kotlin.never
 import org.mockito.kotlin.verify
@@ -34,6 +38,26 @@ class AppStartExtensionTest {
     return AppStartExtension(metrics)
   }
 
+  private class ReentrantCall(val run: () -> Unit, val succeeded: AtomicBoolean)
+
+  /**
+   * Runs [call] on another thread and records whether it completed while the caller is still inside
+   * the stubbed method. Used to prove a lock is not held across that call.
+   */
+  private fun reentrantCallDuring(call: () -> Unit): ReentrantCall {
+    val succeeded = AtomicBoolean(false)
+    val done = CountDownLatch(1)
+    val run = {
+      Thread {
+          call()
+          done.countDown()
+        }
+        .start()
+      succeeded.set(done.await(2, TimeUnit.SECONDS))
+    }
+    return ReentrantCall(run, succeeded)
+  }
+
   /** Simulates the integration's listener: hands a transaction + span back to the extension. */
   private fun AppStartExtension.registerHandOver(
     txn: ITransaction = mock(),
@@ -123,6 +147,68 @@ class AppStartExtensionTest {
     verify(span, never()).finish(any<SpanStatus>())
   }
 
+  @Test
+  fun `finishExtendedAppStart releases the lock before finishing the span`() {
+    val ext = extension(windowOpen = true)
+    val span = mock<ISpan>()
+    ext.registerHandOver(span = span)
+    ext.extendAppStart()
+
+    // Finishing the real span captures the transaction synchronously, which runs
+    // PerformanceAndroidEventProcessor, which calls back into the extension while holding its own
+    // lock. Holding this lock across the finish lets the two be taken in opposite orders and
+    // deadlock, so require another thread to get through the extension lock during the finish.
+    val reentered = reentrantCallDuring { ext.isExtended }
+    doAnswer { reentered.run() }.whenever(span).finish(any<SpanStatus>())
+
+    ext.finishExtendedAppStart()
+
+    assertTrue(reentered.succeeded.get(), "extension lock was held while finishing the span")
+  }
+
+  @Test
+  fun `finishTransaction releases the lock before finishing the transaction`() {
+    val ext = extension(windowOpen = true)
+    val txn = mock<ITransaction>()
+    ext.registerHandOver(txn = txn)
+    ext.extendAppStart()
+
+    val reentered = reentrantCallDuring { ext.isExtended }
+    doAnswer { reentered.run() }.whenever(txn).finish(any<SpanStatus>(), any())
+
+    ext.finishTransaction(SentryNanotimeDate())
+
+    assertTrue(reentered.succeeded.get(), "extension lock was held while finishing the transaction")
+  }
+
+  @Test
+  fun `finishTransaction and finishExtendedAppStart do not interleave`() {
+    val ext = extension(windowOpen = true)
+    val txn = mock<ITransaction>()
+    val span = mock<ISpan>()
+    ext.registerHandOver(txn = txn, span = span)
+    ext.extendAppStart()
+
+    // The end-time clamp in finishTransaction reads the span's finish date and then finishes the
+    // transaction. If finishExtendedAppStart can finish the span in between, the transaction is
+    // captured with an end earlier than its own child span.
+    val spanFinished = AtomicBoolean(false)
+    val interleaved = AtomicBoolean(false)
+    doAnswer { spanFinished.set(true) }.whenever(span).finish(any<SpanStatus>())
+    doAnswer {
+        val other = Thread { ext.finishExtendedAppStart() }
+        other.start()
+        other.join(1_000)
+        interleaved.set(spanFinished.get())
+      }
+      .whenever(txn)
+      .finish(any<SpanStatus>(), any())
+
+    ext.finishTransaction(SentryNanotimeDate())
+
+    assertFalse(interleaved.get(), "the extended span was finished mid-finishTransaction")
+  }
+
   @Test
   fun `isActive reflects the transaction state`() {
     val ext = extension(windowOpen = true)