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50410cb1d5
If cancellation points are enabled, then the following logic is activated in sem_wait(). This causes ECANCELED to be returned every time that sem_wait is called.
int sem_wait(FAR sem_t *sem)
{
...
/* sem_wait() is a cancellation point */
if (enter_cancellation_point())
{
#ifdef CONFIG_CANCELLATION_POINTS
/* If there is a pending cancellation, then do not perform
* the wait. Exit now with ECANCELED.
*/
errcode = ECANCELED;
goto errout_with_cancelpt;
#endif
}
...
Normally this works fine. sem_wait() is the OS API called by the application and will cancel the thread just before it returns to the application. Since it is cancellation point, it should never be called from within the OS.
There there is is one perverse cases where sem_wait() may be nested within another cancellation point. If open() is called, it will attempt to lock a VFS data structure and will eventually call nxmutex_lock(). nxmutex_lock() waits on a semaphore:
int nxmutex_lock(FAR mutex_t *mutex)
{
...
for (; ; )
{
/* Take the semaphore (perhaps waiting) */
ret = _SEM_WAIT(&mutex->sem);
if (ret >= 0)
{
mutex->holder = _SCHED_GETTID();
break;
}
ret = _SEM_ERRVAL(ret);
if (ret != -EINTR && ret != -ECANCELED)
{
break;
}
}
...
}
In the FLAT build, _SEM_WAIT expands to sem_wait(). That causes the error in the logic: It should always expand to nxsem_wait(). That is because sem_wait() is cancellation point and should never be called from with the OS or the C library internally.
The failure occurs because the cancellation point logic in sem_wait() returns -ECANCELED (via _SEM_ERRVAL) because sem_wait() is nested; it needs to return the -ECANCELED error to the outermost cancellation point which is open() in this case. Returning -ECANCELED then causes an infinite loop to occur in nxmutex_lock().
The correct behavior in this case is to call nxsem_wait() instead of sem_wait(). nxsem_wait() is identical to sem_wait() except that it is not a cancelation point. It will return -ECANCELED if the thread is canceled, but only once. So no infinite loop results.
In addition, an nxsem_wait() system call was added to support the call from nxmutex_lock().
This resolves Issue #9695
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| libc | ||
| libdsp | ||
| libm | ||
| libnx | ||
| libxx | ||
| README.txt | ||
README ====== This directory holds NuttX libraries. Libraries in NuttX are very special creatures. They have these properties: 1. They can be shared by both application logic and logic within the OS when using the FLAT build. 2. But in PROTECTED and KERNEL modes, they must be built differently: The copies used by applications and the OS cannot be the same. Rather, separate versions of libraries must be built for the kernel and for applications. 3. When used by the OS, some special care must be taken to assure that the OS logic does not disrupt the user's errno value and that the OS does not create inappropriate cancellation points. For example, sem_wait() is both a cancellation point and modifies the errno value. So within the FLAT build and without kernel version for the PROTECTED and KERNEL builds, the special internal OS interface nxsem_wait() must be used. Within libraries, the macro _SEM_WAIT() (as defined in include/nuttx/semaphore.h) is used instead. The definition of this macro accounts for the different usage environments. NOTE: The libraries under libs/ build differently from other NuttX components: There are no build-related files in the libs/ directory; it is simply a container for other well-known, individual library directories. The upper level Makefile logic is aware of the libraries within the libs/ container. The only real function of the libs/ directory is to prevent the top-level directory from becoming cluttered with individual libraries.