diff --git a/cpu-exec.c b/cpu-exec.c
index 3e408865a8..4188fed3c6 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -150,11 +150,13 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 #if defined(DEBUG_DISAS)
     if (qemu_loglevel_mask(CPU_LOG_TB_CPU)
         && qemu_log_in_addr_range(itb->pc)) {
+        qemu_log_lock();
 #if defined(TARGET_I386)
         log_cpu_state(cpu, CPU_DUMP_CCOP);
 #else
         log_cpu_state(cpu, 0);
 #endif
+        qemu_log_unlock();
     }
 #endif /* DEBUG_DISAS */
 
diff --git a/exec.c b/exec.c
index 4d085812ca..b1094c0cd2 100644
--- a/exec.c
+++ b/exec.c
@@ -911,11 +911,13 @@ void cpu_abort(CPUState *cpu, const char *fmt, ...)
     fprintf(stderr, "\n");
     cpu_dump_state(cpu, stderr, fprintf, CPU_DUMP_FPU | CPU_DUMP_CCOP);
     if (qemu_log_separate()) {
+        qemu_log_lock();
         qemu_log("qemu: fatal: ");
         qemu_log_vprintf(fmt, ap2);
         qemu_log("\n");
         log_cpu_state(cpu, CPU_DUMP_FPU | CPU_DUMP_CCOP);
         qemu_log_flush();
+        qemu_log_unlock();
         qemu_log_close();
     }
     va_end(ap2);
diff --git a/include/qemu/log.h b/include/qemu/log.h
index 00bf37fc0f..a50e994c21 100644
--- a/include/qemu/log.h
+++ b/include/qemu/log.h
@@ -51,6 +51,22 @@ static inline bool qemu_loglevel_mask(int mask)
     return (qemu_loglevel & mask) != 0;
 }
 
+/* Lock output for a series of related logs.  Since this is not needed
+ * for a single qemu_log / qemu_log_mask / qemu_log_mask_and_addr, we
+ * assume that qemu_loglevel_mask has already been tested, and that
+ * qemu_loglevel is never set when qemu_logfile is unset.
+ */
+
+static inline void qemu_log_lock(void)
+{
+    qemu_flockfile(qemu_logfile);
+}
+
+static inline void qemu_log_unlock(void)
+{
+    qemu_funlockfile(qemu_logfile);
+}
+
 /* Logging functions: */
 
 /* main logging function
diff --git a/include/sysemu/os-posix.h b/include/sysemu/os-posix.h
index 3cfedbc28b..b0a6c0695b 100644
--- a/include/sysemu/os-posix.h
+++ b/include/sysemu/os-posix.h
@@ -87,4 +87,16 @@ void *qemu_alloc_stack(size_t *sz);
  */
 void qemu_free_stack(void *stack, size_t sz);
 
+/* POSIX and Mingw32 differ in the name of the stdio lock functions.  */
+
+static inline void qemu_flockfile(FILE *f)
+{
+    flockfile(f);
+}
+
+static inline void qemu_funlockfile(FILE *f)
+{
+    funlockfile(f);
+}
+
 #endif
diff --git a/include/sysemu/os-win32.h b/include/sysemu/os-win32.h
index 17aad3b20f..ff18b23db1 100644
--- a/include/sysemu/os-win32.h
+++ b/include/sysemu/os-win32.h
@@ -103,6 +103,21 @@ static inline char *realpath(const char *path, char *resolved_path)
     return resolved_path;
 }
 
+/* ??? Mingw appears to export _lock_file and _unlock_file as the functions
+ * with which to lock a stdio handle.  But something is wrong in the markup,
+ * either in the header or the library, such that we get undefined references
+ * to "_imp___lock_file" etc when linking.  Since we seem to have no other
+ * alternative, and the usage within the logging functions isn't critical,
+ * ignore FILE locking.
+ */
+
+static inline void qemu_flockfile(FILE *f)
+{
+}
+
+static inline void qemu_funlockfile(FILE *f)
+{
+}
 
 /* We wrap all the sockets functions so that we can
  * set errno based on WSAGetLastError()
diff --git a/target-alpha/translate.c b/target-alpha/translate.c
index 03e47765ed..114927b751 100644
--- a/target-alpha/translate.c
+++ b/target-alpha/translate.c
@@ -2994,9 +2994,11 @@ void gen_intermediate_code(CPUAlphaState *env, struct TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, ctx.pc - pc_start, 1);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
index ded924a0a9..de48747376 100644
--- a/target-arm/translate-a64.c
+++ b/target-arm/translate-a64.c
@@ -11420,11 +11420,13 @@ done_generating:
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM) &&
         qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc->pc - pc_start,
                          4 | (bswap_code(dc->sctlr_b) ? 2 : 0));
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
     tb->size = dc->pc - pc_start;
diff --git a/target-arm/translate.c b/target-arm/translate.c
index 718f7d06f3..0ad9070b45 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -11963,11 +11963,13 @@ done_generating:
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM) &&
         qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc->pc - pc_start,
                          dc->thumb | (dc->sctlr_b << 1));
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
     tb->size = dc->pc - pc_start;
diff --git a/target-cris/translate.c b/target-cris/translate.c
index 8d4c864526..b91042743f 100644
--- a/target-cris/translate.c
+++ b/target-cris/translate.c
@@ -3290,12 +3290,14 @@ void gen_intermediate_code(CPUCRISState *env, struct TranslationBlock *tb)
 #if !DISAS_CRIS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("--------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc->pc - pc_start,
                          env->pregs[PR_VR]);
         qemu_log("\nisize=%d osize=%d\n",
                  dc->pc - pc_start, tcg_op_buf_count());
+        qemu_log_unlock();
     }
 #endif
 #endif
diff --git a/target-i386/translate.c b/target-i386/translate.c
index 927b366534..324103c885 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -2432,11 +2432,13 @@ static void gen_unknown_opcode(CPUX86State *env, DisasContext *s)
 
     if (qemu_loglevel_mask(LOG_UNIMP)) {
         target_ulong pc = s->pc_start, end = s->pc;
+        qemu_log_lock();
         qemu_log("ILLOPC: " TARGET_FMT_lx ":", pc);
         for (; pc < end; ++pc) {
             qemu_log(" %02x", cpu_ldub_code(env, pc));
         }
         qemu_log("\n");
+        qemu_log_unlock();
     }
 }
 
@@ -8470,6 +8472,7 @@ done_generating:
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
         int disas_flags;
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
 #ifdef TARGET_X86_64
@@ -8480,6 +8483,7 @@ done_generating:
             disas_flags = !dc->code32;
         log_target_disas(cs, pc_start, pc_ptr - pc_start, disas_flags);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 
diff --git a/target-lm32/translate.c b/target-lm32/translate.c
index 842af63a98..692882f447 100644
--- a/target-lm32/translate.c
+++ b/target-lm32/translate.c
@@ -1148,10 +1148,12 @@ void gen_intermediate_code(CPULM32State *env, struct TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("\n");
         log_target_disas(cs, pc_start, dc->pc - pc_start, 0);
         qemu_log("\nisize=%d osize=%d\n",
                  dc->pc - pc_start, tcg_op_buf_count());
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-m68k/translate.c b/target-m68k/translate.c
index ee0ffe3e07..9ad974f86a 100644
--- a/target-m68k/translate.c
+++ b/target-m68k/translate.c
@@ -3549,10 +3549,12 @@ void gen_intermediate_code(CPUM68KState *env, TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc->pc - pc_start, 0);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
     tb->size = dc->pc - pc_start;
diff --git a/target-microblaze/translate.c b/target-microblaze/translate.c
index 5a4a8b9a81..5274191ce3 100644
--- a/target-microblaze/translate.c
+++ b/target-microblaze/translate.c
@@ -1813,12 +1813,14 @@ void gen_intermediate_code(CPUMBState *env, struct TranslationBlock *tb)
 #if !SIM_COMPAT
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("--------------\n");
 #if DISAS_GNU
         log_target_disas(cs, pc_start, dc->pc - pc_start, 0);
 #endif
         qemu_log("\nisize=%d osize=%d\n",
                  dc->pc - pc_start, tcg_op_buf_count());
+        qemu_log_unlock();
     }
 #endif
 #endif
diff --git a/target-mips/translate.c b/target-mips/translate.c
index 55c2ca0c7b..d8dde7a2f5 100644
--- a/target-mips/translate.c
+++ b/target-mips/translate.c
@@ -20043,9 +20043,11 @@ done_generating:
     LOG_DISAS("\n");
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, ctx.pc - pc_start, 0);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-openrisc/translate.c b/target-openrisc/translate.c
index a4625f9b6f..229361aed1 100644
--- a/target-openrisc/translate.c
+++ b/target-openrisc/translate.c
@@ -1750,11 +1750,13 @@ void gen_intermediate_code(CPUOpenRISCState *env, struct TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc->pc - pc_start, 0);
         qemu_log("\nisize=%d osize=%d\n",
                  dc->pc - pc_start, tcg_op_buf_count());
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-ppc/translate.c b/target-ppc/translate.c
index 43505a936c..54f35e9904 100644
--- a/target-ppc/translate.c
+++ b/target-ppc/translate.c
@@ -7211,9 +7211,11 @@ void gen_intermediate_code(CPUPPCState *env, struct TranslationBlock *tb)
         int flags;
         flags = env->bfd_mach;
         flags |= ctx.le_mode << 16;
+        qemu_log_lock();
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, ctx.nip - pc_start, flags);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-s390x/translate.c b/target-s390x/translate.c
index 1a07d70b21..02bc7058fd 100644
--- a/target-s390x/translate.c
+++ b/target-s390x/translate.c
@@ -5432,9 +5432,11 @@ void gen_intermediate_code(CPUS390XState *env, struct TranslationBlock *tb)
 #if defined(S390X_DEBUG_DISAS)
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc.pc - pc_start, 1);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-sh4/translate.c b/target-sh4/translate.c
index ca80cf70ca..c89a14733f 100644
--- a/target-sh4/translate.c
+++ b/target-sh4/translate.c
@@ -1927,9 +1927,11 @@ void gen_intermediate_code(CPUSH4State * env, struct TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
 	qemu_log("IN:\n");	/* , lookup_symbol(pc_start)); */
         log_target_disas(cs, pc_start, ctx.pc - pc_start, 0);
 	qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-sparc/translate.c b/target-sparc/translate.c
index a13b76ebd9..2205f89837 100644
--- a/target-sparc/translate.c
+++ b/target-sparc/translate.c
@@ -5796,10 +5796,12 @@ void gen_intermediate_code(CPUSPARCState * env, TranslationBlock * tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("--------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, last_pc + 4 - pc_start, 0);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-tilegx/translate.c b/target-tilegx/translate.c
index 11c9732389..9c734eeba3 100644
--- a/target-tilegx/translate.c
+++ b/target-tilegx/translate.c
@@ -2391,6 +2391,7 @@ void gen_intermediate_code(CPUTLGState *env, struct TranslationBlock *tb)
     TCGV_UNUSED_I64(dc->zero);
 
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)) {
+        qemu_log_lock();
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
     }
     if (!max_insns) {
@@ -2429,7 +2430,10 @@ void gen_intermediate_code(CPUTLGState *env, struct TranslationBlock *tb)
     tb->size = dc->pc - pc_start;
     tb->icount = num_insns;
 
-    qemu_log_mask(CPU_LOG_TB_IN_ASM, "\n");
+    if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)) {
+        qemu_log("\n");
+        qemu_log_unlock();
+    }
 }
 
 void restore_state_to_opc(CPUTLGState *env, TranslationBlock *tb,
diff --git a/target-tricore/translate.c b/target-tricore/translate.c
index 9a50df9a88..36f734a662 100644
--- a/target-tricore/translate.c
+++ b/target-tricore/translate.c
@@ -8789,9 +8789,11 @@ void gen_intermediate_code(CPUTriCoreState *env, struct TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, ctx.pc - pc_start, 0);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 }
diff --git a/target-unicore32/translate.c b/target-unicore32/translate.c
index 09354f92d2..514d460408 100644
--- a/target-unicore32/translate.c
+++ b/target-unicore32/translate.c
@@ -2024,10 +2024,12 @@ done_generating:
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc->pc - pc_start, 0);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
     tb->size = dc->pc - pc_start;
diff --git a/target-xtensa/translate.c b/target-xtensa/translate.c
index fb0fa56f1e..0858c296ea 100644
--- a/target-xtensa/translate.c
+++ b/target-xtensa/translate.c
@@ -3155,10 +3155,12 @@ void gen_intermediate_code(CPUXtensaState *env, TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_IN_ASM)
         && qemu_log_in_addr_range(pc_start)) {
+        qemu_log_lock();
         qemu_log("----------------\n");
         qemu_log("IN: %s\n", lookup_symbol(pc_start));
         log_target_disas(cs, pc_start, dc.pc - pc_start, 0);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
     tb->size = dc.pc - pc_start;
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 2d3e498bc2..aabf94f365 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -412,10 +412,12 @@ void tcg_prologue_init(TCGContext *s)
 
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_OUT_ASM)) {
+        qemu_log_lock();
         qemu_log("PROLOGUE: [size=%zu]\n", prologue_size);
         log_disas(buf0, prologue_size);
         qemu_log("\n");
         qemu_log_flush();
+        qemu_log_unlock();
     }
 #endif
 }
@@ -2542,9 +2544,11 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (unlikely(qemu_loglevel_mask(CPU_LOG_TB_OP)
                  && qemu_log_in_addr_range(tb->pc))) {
+        qemu_log_lock();
         qemu_log("OP:\n");
         tcg_dump_ops(s);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 
@@ -2570,9 +2574,11 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
 #ifdef DEBUG_DISAS
             if (unlikely(qemu_loglevel_mask(CPU_LOG_TB_OP_IND)
                          && qemu_log_in_addr_range(tb->pc))) {
+                qemu_log_lock();
                 qemu_log("OP before indirect lowering:\n");
                 tcg_dump_ops(s);
                 qemu_log("\n");
+                qemu_log_unlock();
             }
 #endif
             /* Replace indirect temps with direct temps.  */
@@ -2590,9 +2596,11 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
 #ifdef DEBUG_DISAS
     if (unlikely(qemu_loglevel_mask(CPU_LOG_TB_OP_OPT)
                  && qemu_log_in_addr_range(tb->pc))) {
+        qemu_log_lock();
         qemu_log("OP after optimization and liveness analysis:\n");
         tcg_dump_ops(s);
         qemu_log("\n");
+        qemu_log_unlock();
     }
 #endif
 
diff --git a/translate-all.c b/translate-all.c
index e6a8b07363..3dd9214904 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -1355,10 +1355,12 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_OUT_ASM) &&
         qemu_log_in_addr_range(tb->pc)) {
+        qemu_log_lock();
         qemu_log("OUT: [size=%d]\n", gen_code_size);
         log_disas(tb->tc_ptr, gen_code_size);
         qemu_log("\n");
         qemu_log_flush();
+        qemu_log_unlock();
     }
 #endif