Potential Locking Pitfalls

Mutexes are not re-entrant by the same thread. If you already own the mutex, attempting to claim it again leads to this panic:

panic: recursive mutex_enter. mutex %x caller %x

Releasing a mutex that the current thread does not hold causes this panic:

panic: mutex_adaptive_exit: mutex not held by thread

The following panic occurs only on uniprocessors:

panic: lock_set: lock held and only one CPU

It indicates that a spin mutex is held and will spin forever, because there is no other CPU to release it. This could happen because the driver forgot to release the mutex on one code path, or blocked while holding it.

A common cause of this panic is that the device's interrupt is high-level and is calling a routine that blocks the interrupt handler while holding a spin mutex. This is obvious if the driver explicitly calls cv_wait(9F), but might not be so if the driver is blocking while grabbing an adaptive mutex with mutex_enter(9F).