Fix LP32 large pid detection.

Bug: http://b/68046352
Test: ran tests
Change-Id: I89cb99173ca77e9457e677187430b61cedb55c04

docs/32-bit-abi.md

@@ -57,3 +57,14 @@ On 32-bit Android, `time_t` is 32-bit. The header `<time64.h>` and type
 Android all use the 32-bit `time_t`.
 
 In the 64-bit ABI, `time_t` is 64-bit.
+
+`pthread_mutex_t` is too small for large pids
+---------------------------------------------
+
+This doesn't generally affect Android devices, because on devices
+`/proc/sys/kernel/pid_max` is usually too small to hit the 16-bit limit,
+but 32-bit bionic's `pthread_mutex` is a total of 32 bits, leaving just
+16 bits for the owner thread id. This means bionic isn't able to support
+mutexes for tids that don't fit in 16 bits. This typically manifests as
+a hang in `pthread_mutex_lock` if the libc startup code doesn't detect
+this condition and abort.

libc/bionic/libc_init_dynamic.cpp

@@ -77,11 +77,14 @@ static void __libc_preinit_impl(KernelArgumentBlock& args) {
   netdClientInit();
 }
 
-// We flag the __libc_preinit function as a constructor to ensure
-// that its address is listed in libc.so's .init_array section.
-// This ensures that the function is called by the dynamic linker
-// as soon as the shared library is loaded.
-__attribute__((constructor)) static void __libc_preinit() {
+// We flag the __libc_preinit function as a constructor to ensure that
+// its address is listed in libc.so's .init_array section.
+// This ensures that the function is called by the dynamic linker as
+// soon as the shared library is loaded.
+// We give this constructor priority 1 because we want libc's constructor
+// to run before any others (such as the jemalloc constructor), and lower
+// is better (http://b/68046352).
+__attribute__((constructor(1))) static void __libc_preinit() {
   // Read the kernel argument block pointer from TLS.
   void** tls = __get_tls();
   KernelArgumentBlock** args_slot = &reinterpret_cast<KernelArgumentBlock**>(tls)[TLS_SLOT_BIONIC_PREINIT];