diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 9cba25dc88..19d5445c31 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -4947,6 +4947,9 @@ static inline abi_ulong do_shmat(CPUArchState *cpu_env,
 static inline abi_long do_shmdt(abi_ulong shmaddr)
 {
     int i;
+    abi_long rv;
+
+    mmap_lock();
 
     for (i = 0; i < N_SHM_REGIONS; ++i) {
         if (shm_regions[i].in_use && shm_regions[i].start == shmaddr) {
@@ -4955,8 +4958,11 @@ static inline abi_long do_shmdt(abi_ulong shmaddr)
             break;
         }
     }
+    rv = get_errno(shmdt(g2h(shmaddr)));
 
-    return get_errno(shmdt(g2h(shmaddr)));
+    mmap_unlock();
+
+    return rv;
 }
 
 #ifdef TARGET_NR_ipc