Squashed commit of the following:
mm/iob: The IOB available notifier is now just a wrapper around the common signal notifier.
sched/signal: Add a generic signal notification facility.
sched/signal/sig_evthread.c: More trivial naming changes.
sched/signal: Rename nxsig_notification() to nxsig_evthread() to make forthcoming naming additions more consistent.
This commit backs out most of commit b4747286b1. That change was added because sem_wait() would sometimes cause cancellation points inappropriated. But with these recent changes, nxsem_wait() is used instead and it is not a cancellation point.
In the OS, all calls to sem_wait() changed to nxsem_wait(). nxsem_wait() does not return errors via errno so each place where nxsem_wait() is now called must not examine the errno variable.
In all OS functions (not libraries), change sem_wait() to nxsem_wait(). This will prevent the OS from creating bogus cancellation points and from modifying the per-task errno variable.
sched/semaphore: Add the function nxsem_wait(). This is a new internal OS interface. It is functionally equivalent to sem_wait() except that (1) it is not a cancellation point, and (2) it does not set the per-thread errno value on return.
libc/semaphore: Add nxsem_getvalue() which is identical to sem_getvalue() except that it never modifies the errno variable. Changed all references to sem_getvalue in the OS to nxsem_getvalue().
sched/semaphore: Rename all internal private functions from sem_xyz to nxsem_xyz. The sem_ prefix is (will be) reserved only for the application semaphore interfaces.
libc/semaphore: Add nxsem_init() which is identical to sem_init() except that it never modifies the errno variable. Changed all references to sem_init in the OS to nxsem_init().
sched/semaphore: Rename sem_tickwait() to nxsem_tickwait() so that it is clear this is an internal OS function.
sched/semaphoate: Rename sem_reset() to nxsem_reset() so that it is clear this is an internal OS function.
Task A holds an IOB. There are no further IOBs. The value of semcount is zero.
Task B calls iob_alloc(). Since there are not IOBs, it calls sem_wait(). The v
alue of semcount is now -1.
Task A frees the IOB. iob_free() adds the IOB to the free list and calls sem_post() this makes Task B ready to run and sets semcount to zero NOT 1. There is one IOB in the free list and semcount is zero. When Task B wakes up it would increment the sem_count back to the correct value.
But an interrupt or another task runs occurs before Task B executes. The interrupt or other tak takes the IOB off of the free list and decrements the semcount. But since semcount is then < 0, this causes the assertion because that is an invalid state in the interrupt handler.
So I think that the root cause is that there the asynchrony between incrementing the semcount. This change separates the list of IOBs: Currently there is only a free list of IOBs. The problem, I believe, is because of asynchronies due sem_post() post cause the semcount and the list content to become out of sync. This change adds a new 'committed' list: When there is a task waiting for an IOB, it will go into the committed list rather than the free list before the semaphore is posted. On the waiting side, when awakened from the semaphore wait, it will expect to find its IOB in the committed list, rather than free list.
In this way, the content of the free list and the value of the semaphore count always remain in sync.
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