diff --git a/ChangeLog b/ChangeLog
index 602d06e..4ccf7fe 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2019-05-10  NIIBE Yutaka  <gniibe@fsij.org>
+
+	* chopstx.c (chopstx_claim_irq): Check INTR before the call.
+
+	* chopstx-cortex-m.c (chx_disable_intr): Have return value.
+	* chopstx-gnu-linux.c (chx_disable_intr): Likewise.
+
 2019-05-10  NIIBE Yutaka  <gniibe@fsij.org>
 
 	* mcu/sys-stm32f103.c: SYS version 4.0.
diff --git a/chopstx-cortex-m.c b/chopstx-cortex-m.c
index c6d1b3e..23cfe03 100644
--- a/chopstx-cortex-m.c
+++ b/chopstx-cortex-m.c
@@ -179,10 +179,13 @@ chx_clr_intr (uint8_t irq_num)
   NVIC_ICPR (irq_num) = 1 << (irq_num & 0x1f);
 }
 
-static void
+static int
 chx_disable_intr (uint8_t irq_num)
 {
+  int already_disabled = !!(NVIC_ICER (irq_num) & (1 << (irq_num & 0x1f)));
+
   NVIC_ICER (irq_num) = 1 << (irq_num & 0x1f);
+  return already_disabled;
 }
 
 static void
diff --git a/chopstx-gnu-linux.c b/chopstx-gnu-linux.c
index 07f9a7a..9c7f8df 100644
--- a/chopstx-gnu-linux.c
+++ b/chopstx-gnu-linux.c
@@ -2,7 +2,7 @@
  * chopstx-gnu-linux.c - Threads and only threads: Arch specific code
  *                       for GNU/Linux emulation
  *
- * Copyright (C) 2017, 2018 Flying Stone Technology
+ * Copyright (C) 2017, 2018, 2019 Flying Stone Technology
  * Author: NIIBE Yutaka <gniibe@fsij.org>
  *
  * This file is a part of Chopstx, a thread library for embedded.
@@ -89,10 +89,13 @@ chx_clr_intr (uint8_t irq_num)
   (void)irq_num;
 }
 
-static void
+static int
 chx_disable_intr (uint8_t irq_num)
 {
+  int already_disabled = sigismember (&ss_cur, irq_num);
+
   sigaddset (&ss_cur, irq_num);
+  return already_disabled;
 }
 
 static void
diff --git a/chopstx.c b/chopstx.c
index 70217f6..022d821 100644
--- a/chopstx.c
+++ b/chopstx.c
@@ -1005,17 +1005,21 @@ chx_cond_hook (struct chx_px *px, struct chx_poll_head *pd)
 void
 chopstx_claim_irq (chopstx_intr_t *intr, uint8_t irq_num)
 {
+  int intr_before_claim;
+
   intr->type = CHOPSTX_POLL_INTR;
   intr->ready = 0;
   intr->irq_num = irq_num;
 
   chx_cpu_sched_lock ();
   chx_spin_lock (&q_intr.lock);
-  chx_disable_intr (irq_num);
+  intr_before_claim = chx_disable_intr (irq_num);
   chx_clr_intr (irq_num);
   chx_set_intr_prio (irq_num);
   chx_spin_unlock (&q_intr.lock);
   chx_cpu_sched_unlock ();
+  if (intr_before_claim)
+    intr->ready = 1;
 }
 
 
diff --git a/example-cdc-gnu-linux/usb-cdc.c b/example-cdc-gnu-linux/usb-cdc.c
index 90b27c5..aa4c01d 100644
--- a/example-cdc-gnu-linux/usb-cdc.c
+++ b/example-cdc-gnu-linux/usb-cdc.c
@@ -660,31 +660,8 @@ tty_main (void *arg)
   struct usb_dev dev;
   int e;
 
-#if defined(OLDER_SYS_H)
-  /*
-   * Historically (before sys < 3.0), NVIC priority setting for USB
-   * interrupt was done in usb_lld_sys_init.  Thus this code.
-   *
-   * When USB interrupt occurs between usb_lld_init (which assumes
-   * ISR) and chopstx_claim_irq (which clears pending interrupt),
-   * invocation of usb_lld_event_handler won't occur.
-   *
-   * Calling usb_lld_event_handler is no harm even if there were no
-   * interrupts, thus, we call it unconditionally here, just in case
-   * if there is a request.
-   *
-   * We can't call usb_lld_init after chopstx_claim_irq, as
-   * usb_lld_init does its own setting for NVIC.  Calling
-   * chopstx_claim_irq after usb_lld_init overrides that.
-   *
-   */
-  usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
-  chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
-  goto event_handle;
-#else
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
-#endif
 
   while (1)
     {
diff --git a/example-cdc/usb-cdc.c b/example-cdc/usb-cdc.c
index a4d146c..64c5ac4 100644
--- a/example-cdc/usb-cdc.c
+++ b/example-cdc/usb-cdc.c
@@ -675,24 +675,17 @@ tty_main (void *arg)
 #if defined(OLDER_SYS_H)
   /*
    * Historically (before sys < 3.0), NVIC priority setting for USB
-   * interrupt was done in usb_lld_sys_init.  Thus this code.
-   *
-   * When USB interrupt occurs between usb_lld_init (which assumes
-   * ISR) and chopstx_claim_irq (which clears pending interrupt),
-   * invocation of usb_lld_event_handler won't occur.
-   *
-   * Calling usb_lld_event_handler is no harm even if there were no
-   * interrupts, thus, we call it unconditionally here, just in case
-   * if there is a request.
+   * interrupt was done in usb_lld_sys_init for free standing
+   * application.  Thus this compatibility code.
    *
    * We can't call usb_lld_init after chopstx_claim_irq, as
-   * usb_lld_init does its own setting for NVIC.  Calling
-   * chopstx_claim_irq after usb_lld_init overrides that.
+   * usb_lld_init does its own setting for NVIC, which is incompatible
+   * to Chopstx's interrupt handling.  Calling chopstx_claim_irq after
+   * usb_lld_init overrides that for Chopstx.
    *
    */
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
-  goto event_handle;
 #else
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
diff --git a/example-fs-bb48/usb-cdc.c b/example-fs-bb48/usb-cdc.c
index f96244d..1e44124 100644
--- a/example-fs-bb48/usb-cdc.c
+++ b/example-fs-bb48/usb-cdc.c
@@ -660,24 +660,17 @@ tty_main (void *arg)
 #if defined(OLDER_SYS_H)
   /*
    * Historically (before sys < 3.0), NVIC priority setting for USB
-   * interrupt was done in usb_lld_sys_init.  Thus this code.
-   *
-   * When USB interrupt occurs between usb_lld_init (which assumes
-   * ISR) and chopstx_claim_irq (which clears pending interrupt),
-   * invocation of usb_lld_event_handler won't occur.
-   *
-   * Calling usb_lld_event_handler is no harm even if there were no
-   * interrupts, thus, we call it unconditionally here, just in case
-   * if there is a request.
+   * interrupt was done in usb_lld_sys_init for free standing
+   * application.  Thus this compatibility code.
    *
    * We can't call usb_lld_init after chopstx_claim_irq, as
-   * usb_lld_init does its own setting for NVIC.  Calling
-   * chopstx_claim_irq after usb_lld_init overrides that.
+   * usb_lld_init does its own setting for NVIC, which is incompatible
+   * to Chopstx's interrupt handling.  Calling chopstx_claim_irq after
+   * usb_lld_init overrides that for Chopstx.
    *
    */
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
-  goto event_handle;
 #else
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
diff --git a/example-usb-serial/usb-cdc.c b/example-usb-serial/usb-cdc.c
index 898217b..8e0adfa 100644
--- a/example-usb-serial/usb-cdc.c
+++ b/example-usb-serial/usb-cdc.c
@@ -725,24 +725,17 @@ cdc_main (void *arg)
 #if defined(OLDER_SYS_H)
   /*
    * Historically (before sys < 3.0), NVIC priority setting for USB
-   * interrupt was done in usb_lld_sys_init.  Thus this code.
-   *
-   * When USB interrupt occurs between usb_lld_init (which assumes
-   * ISR) and chopstx_claim_irq (which clears pending interrupt),
-   * invocation of usb_lld_event_handler won't occur.
-   *
-   * Calling usb_lld_event_handler is no harm even if there were no
-   * interrupts, thus, we call it unconditionally here, just in case
-   * if there is a request.
+   * interrupt was done in usb_lld_sys_init for free standing
+   * application.  Thus this compatibility code.
    *
    * We can't call usb_lld_init after chopstx_claim_irq, as
-   * usb_lld_init does its own setting for NVIC.  Calling
-   * chopstx_claim_irq after usb_lld_init overrides that.
+   * usb_lld_init does its own setting for NVIC, which is incompatible
+   * to Chopstx's interrupt handling.  Calling chopstx_claim_irq after
+   * usb_lld_init overrides that for Chopstx.
    *
    */
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
-  goto event_handle;
 #else
   chopstx_claim_irq (&usb_intr, INTR_REQ_USB);
   usb_lld_init (&dev, VCOM_FEATURE_BUS_POWERED);