Hi Thomas, Just a few comments: [...] > +rtmutex > +======= > + > +RT-mutexes are mutexes with support for priority inheritance (PI). > + > +PI has limitations on non PREEMPT_RT enabled kernels due to preemption and > +interrupt disabled sections. > + > +On a PREEMPT_RT enabled kernel most of these sections are fully > +preemptible. This is possible because PREEMPT_RT forces most executions > +into task context, especially interrupt handlers and soft interrupts, which > +allows to substitute spinlock_t and rwlock_t with RT-mutex based > +implementations. > + > + > +raw_spinlock_t and spinlock_t > +============================= > + > +raw_spinlock_t > +-------------- > + > +raw_spinlock_t is a strict spinning lock implementation regardless of the > +kernel configuration including PREEMPT_RT enabled kernels. > + > +raw_spinlock_t is to be used only in real critical core code, low level > +interrupt handling and places where protecting (hardware) state is required > +to be safe against preemption and eventually interrupts. > + > +Another reason to use raw_spinlock_t is when the critical section is tiny > +to avoid the overhead of spinlock_t on a PREEMPT_RT enabled kernel in the > +contended case. > + > +spinlock_t > +---------- > + > +The semantics of spinlock_t change with the state of CONFIG_PREEMPT_RT. > + > +On a non PREEMPT_RT enabled kernel spinlock_t is mapped to raw_spinlock_t > +and has exactly the same semantics. > + > +spinlock_t and PREEMPT_RT > +------------------------- > + > +On a PREEMPT_RT enabled kernel spinlock_t is mapped to a separate > +implementation based on rt_mutex which changes the semantics: > + > + - Preemption is not disabled > + > + - The hard interrupt related suffixes for spin_lock / spin_unlock > + operations (_irq, _irqsave / _irqrestore) do not affect the CPUs > + interrupt disabled state > + > + - The soft interrupt related suffix (_bh()) is still disabling the > + execution of soft interrupts, but contrary to a non PREEMPT_RT enabled > + kernel, which utilizes the preemption count, this is achieved by a per > + CPU bottom half locking mechanism. > + > +All other semantics of spinlock_t are preserved: > + > + - Migration of tasks which hold a spinlock_t is prevented. On a non > + PREEMPT_RT enabled kernel this is implicit due to preemption disable. > + PREEMPT_RT has a separate mechanism to achieve this. This ensures that > + pointers to per CPU variables stay valid even if the task is preempted. > + > + - Task state preservation. The task state is not affected when a lock is > + contended and the task has to schedule out and wait for the lock to > + become available. The lock wake up restores the task state unless there > + was a regular (not lock related) wake up on the task. This ensures that > + the task state rules are always correct independent of the kernel > + configuration. > + > +rwlock_t > +======== > + > +rwlock_t is a multiple readers and single writer lock mechanism. > + > +On a non PREEMPT_RT enabled kernel rwlock_t is implemented as a spinning > +lock and the suffix rules of spinlock_t apply accordingly. The > +implementation is fair and prevents writer starvation. > You mentioned writer starvation, but I think it would be good to also mention that rwlock_t on a non-PREEMPT_RT kernel also does not have _reader_ starvation problem, since it uses queued implementation. This fact is worth mentioning here, since further below you explain that an rwlock in PREEMPT_RT does have reader starvation problem. > +rwlock_t and PREEMPT_RT > +----------------------- > + > +On a PREEMPT_RT enabled kernel rwlock_t is mapped to a separate > +implementation based on rt_mutex which changes the semantics: > + > + - Same changes as for spinlock_t > + > + - The implementation is not fair and can cause writer starvation under > + certain circumstances. The reason for this is that a writer cannot grant > + its priority to multiple readers. Readers which are blocked on a writer > + fully support the priority inheritance protocol. Is it hard to give priority to multiple readers because the number of readers to give priority to could be unbounded? > + > + > +PREEMPT_RT caveats > +================== > + > +spinlock_t and rwlock_t > +----------------------- > + > +The substitution of spinlock_t and rwlock_t on PREEMPT_RT enabled kernels > +with RT-mutex based implementations has a few implications. > + > +On a non PREEMPT_RT enabled kernel the following code construct is > +perfectly fine:: > + > + local_irq_disable(); > + spin_lock(&lock); > + > +and fully equivalent to:: > + > + spin_lock_irq(&lock); > + > +Same applies to rwlock_t and the _irqsave() suffix variant. > + > +On a PREEMPT_RT enabled kernel this breaks because the RT-mutex > +substitution expects a fully preemptible context. > + > +The preferred solution is to use :c:func:`spin_lock_irq()` or > +:c:func:`spin_lock_irqsave()` and their unlock counterparts. > + > +PREEMPT_RT also offers a local_lock mechanism to substitute the > +local_irq_disable/save() constructs in cases where a separation of the > +interrupt disabling and the locking is really unavoidable. This should be > +restricted to very rare cases. It would also be nice to mention where else local_lock() can be used, such as protecting per-cpu variables without disabling preemption. Could we add a section on protecting per-cpu data? (Happy to do that and send a patch if you prefer). > +raw_spinlock_t > +-------------- > + > +Locking of a raw_spinlock_t disables preemption and eventually interrupts. > +Therefore code inside the critical region has to be careful to avoid calls > +into code which takes a regular spinlock_t or rwlock_t. A prime example is > +memory allocation. > + > +On a non PREEMPT_RT enabled kernel the following code construct is > +perfectly fine code:: > + > + raw_spin_lock(&lock); > + p = kmalloc(sizeof(*p), GFP_ATOMIC); > + > +On a PREEMPT_RT enabled kernel this breaks because the memory allocator is > +fully preemptible and therefore does not support allocations from truly > +atomic contexts. > + > +Contrary to that the following code construct is perfectly fine on > +PREEMPT_RT as spin_lock() does not disable preemption:: > + > + spin_lock(&lock); > + p = kmalloc(sizeof(*p), GFP_ATOMIC); > + > +Most places which use GFP_ATOMIC allocations are safe on PREEMPT_RT as the > +execution is forced into thread context and the lock substitution is > +ensuring preemptibility. > + > + > +bit spinlocks > +------------- > + > +Bit spinlocks are problematic for PREEMPT_RT as they cannot be easily > +substituted by an RT-mutex based implementation for obvious reasons. > + > +The semantics of bit spinlocks are preserved on a PREEMPT_RT enabled kernel > +and the caveats vs. raw_spinlock_t apply. > + > +Some bit spinlocks are substituted by regular spinlock_t for PREEMPT_RT but > +this requires conditional (#ifdef'ed) code changes at the usage side while > +the spinlock_t substitution is simply done by the compiler and the > +conditionals are restricted to header files and core implementation of the > +locking primitives and the usage sites do not require any changes. > + > + > +Lock type nesting rules > +======================= > + > +The most basic rules are: > + > + - Lock types of the same lock category (sleeping, spinning) can nest > + arbitrarily as long as they respect the general lock ordering rules to > + prevent deadlocks. > + > + - Sleeping lock types cannot nest inside spinning lock types. > + > + - Spinning lock types can nest inside sleeping lock types. > + > +These rules apply in general independent of CONFIG_PREEMPT_RT. > + > +As PREEMPT_RT changes the lock category of spinlock_t and rwlock_t from > +spinning to sleeping this has obviously restrictions how they can nest with > +raw_spinlock_t. > + > +This results in the following nest ordering: > + > + 1) Sleeping locks > + 2) spinlock_t and rwlock_t > + 3) raw_spinlock_t and bit spinlocks > + > +Lockdep is aware of these constraints to ensure that they are respected. > + > + > +Owner semantics > +=============== > + > +Most lock types in the Linux kernel have strict owner semantics, i.e. the > +context (task) which acquires a lock has to release it. > + > +There are two exceptions: > + > + - semaphores > + - rwsems > + > +semaphores have no strict owner semantics for historical reasons. They are > +often used for both serialization and waiting purposes. That's generally > +discouraged and should be replaced by separate serialization and wait > +mechanisms. > + > +rwsems have grown interfaces which allow non owner release for special > +purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT > +substitutes all locking primitives except semaphores with RT-mutex based > +implementations to provide priority inheritance for all lock types except > +the truly spinning ones. Priority inheritance on ownerless locks is > +obviously impossible. > + > +For now the rwsem non-owner release excludes code which utilizes it from > +being used on PREEMPT_RT enabled kernels. I could not parse the last sentence here, but I think you meant "For now, PREEMPT_RT enabled kernels disable code that perform a non-owner release of an rwsem". Correct me if I'm wrong. Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> thanks, - Joel > In same cases this can be > +mitigated by disabling portions of the code, in other cases the complete > +functionality has to be disabled until a workable solution has been found. >
