Re: [patch V3 13/20] Documentation: Add lock ordering and nesting documentation

Paul E. McKenney
2020-03-25T00:28:19Z

On Wed, Mar 25, 2020 at 12:13:34AM +0100, Thomas Gleixner wrote:
> Paul,
>
> "Paul E. McKenney" <paulmck@kernel.org> writes:
> > On Sat, Mar 21, 2020 at 12:25:57PM +0100, Thomas Gleixner wrote:
> > In the normal case where the task sleeps through the entire lock
> > acquisition, the sequence of events is as follows:
> >
> > state = UNINTERRUPTIBLE
> > lock()
> > block()
> > real_state = state
> > state = SLEEPONLOCK
> >
> > lock wakeup
> > state = real_state == UNINTERRUPTIBLE
> >
> > This sequence of events can occur when the task acquires spinlocks
> > on its way to sleeping, for example, in a call to wait_event().
> >
> > The non-lock wakeup can occur when a wakeup races with this wait_event(),
> > which can result in the following sequence of events:
> >
> > state = UNINTERRUPTIBLE
> > lock()
> > block()
> > real_state = state
> > state = SLEEPONLOCK
> >
> > non lock wakeup
> > real_state = RUNNING
> >
> > lock wakeup
> > state = real_state == RUNNING
> >
> > Without this real_state subterfuge, the wakeup might be lost.
>
> I added this with a few modifications which reflect the actual
> implementation. Conceptually the same.

Looks good!

> > rwsems have grown special-purpose interfaces that allow non-owner release.
> > This non-owner release prevents PREEMPT_RT from substituting RT-mutex
> > implementations, for example, by defeating priority inheritance.
> > After all, if the lock has no owner, whose priority should be boosted?
> > As a result, PREEMPT_RT does not currently support rwsem, which in turn
> > means that code using it must therefore be disabled until a workable
> > solution presents itself.
> >
> > [ Note: Not as confident as I would like to be in the above. ]
>
> I'm not confident either especially not after looking at the actual
> code.
>
> In fact I feel really stupid because the rw_semaphore reader non-owner
> restriction on RT simply does not exist anymore and my history biased
> memory tricked me.

I guess I am glad that it is not just me. ;-)

> The first rw_semaphore implementation of RT was simple and restricted
> the reader side to a single reader to support PI on both the reader and
> the writer side. That obviosuly did not scale well and made mmap_sem
> heavy use cases pretty unhappy.
>
> The short interlude with multi-reader boosting turned out to be a failed
> experiment - Steven might still disagree though :)
>
> At some point we gave up and I myself (sic!) reimplemented the RT
> variant of rw_semaphore with a reader biased mechanism.
>
> The reader never holds the underlying rt_mutex accross the read side
> critical section. It merily increments the reader count and drops it on
> release.
>
> The only time a reader takes the rt_mutex is when it blocks on a
> writer. Writers hold the rt_mutex across the write side critical section
> to allow incoming readers to boost them. Once the writer releases the
> rw_semaphore it unlocks the rt_mutex which is then handed off to the
> readers. They increment the reader count and then drop the rt_mutex
> before continuing in the read side critical section.
>
> So while I changed the implementation it did obviously not occur to me
> that this also lifted the non-owner release restriction. Nobody else
> noticed either. So we kept dragging this along in both memory and
> implementation. Both will be fixed now :)
>
> The owner semantics of down/up_read() are only enforced by lockdep. That
> applies to both RT and !RT. The up/down_read_non_owner() variants are
> just there to tell lockdep about it.
>
> So, I picked up your other suggestions with slight modifications and
> adjusted the owner, semaphore and rw_semaphore docs accordingly.
>
> Please have a close look at the patch below (applies on tip core/locking).
>
> Thanks,
>
> tglx, who is searching a brown paperbag

Sorry, used all the ones here over the past few days. :-/

Please see below for a wordsmithing patch to be applied on top of
or merged into the patch in your email.

Thanx, Paul

------------------------------------------------------------------------

commit e38c64ce8db45e2b0a19082f1e1f988c3b25fb81
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Tue Mar 24 17:23:36 2020 -0700

Documentation: Wordsmith lock ordering and nesting documentation

This commit is strictly wordsmithing with no (intended) semantic
changes.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
index ca7bf84..8eb52e9 100644
--- a/Documentation/locking/locktypes.rst
+++ b/Documentation/locking/locktypes.rst
@@ -94,7 +94,7 @@ interrupt handlers and soft interrupts. This conversion allows spinlock_t
and rwlock_t to be implemented via RT-mutexes.

-sempahore
+semaphore
=========

semaphore is a counting semaphore implementation.
@@ -103,17 +103,17 @@ Semaphores are often used for both serialization and waiting, but new use
cases should instead use separate serialization and wait mechanisms, such
as mutexes and completions.

-sempahores and PREEMPT_RT
+semaphores and PREEMPT_RT
----------------------------

-PREEMPT_RT does not change the sempahore implementation. That's impossible
-due to the counting semaphore semantics which have no concept of owners.
-The lack of an owner conflicts with priority inheritance. After all an
-unknown owner cannot be boosted. As a consequence blocking on semaphores
-can be subject to priority inversion.
+PREEMPT_RT does not change the semaphore implementation because counting
+semaphores have no concept of owners, thus preventing PREEMPT_RT from
+providing priority inheritance for semaphores. After all, an unknown
+owner cannot be boosted. As a consequence, blocking on semaphores can
+result in priority inversion.

-rw_sempahore
+rw_semaphore
============

rw_semaphore is a multiple readers and single writer lock mechanism.
@@ -125,13 +125,13 @@ rw_semaphore complies by default with the strict owner semantics, but there
exist special-purpose interfaces that allow non-owner release for readers.
These work independent of the kernel configuration.

-rw_sempahore and PREEMPT_RT
+rw_semaphore and PREEMPT_RT
---------------------------

-PREEMPT_RT kernels map rw_sempahore to a separate rt_mutex-based
+PREEMPT_RT kernels map rw_semaphore to a separate rt_mutex-based
implementation, thus changing the fairness:

- Because an rw_sempaphore writer cannot grant its priority to multiple
+ Because an rw_semaphore writer cannot grant its priority to multiple
readers, a preempted low-priority reader will continue holding its lock,
thus starving even high-priority writers. In contrast, because readers
can grant their priority to a writer, a preempted low-priority writer will
@@ -158,7 +158,7 @@ critical section is tiny, thus avoiding RT-mutex overhead.
spinlock_t
----------

-The semantics of spinlock_t change with the state of CONFIG_PREEMPT_RT.
+The semantics of spinlock_t change with the state of PREEMPT_RT.

On a non PREEMPT_RT enabled kernel spinlock_t is mapped to raw_spinlock_t
and has exactly the same semantics.
@@ -196,7 +196,7 @@ PREEMPT_RT kernels preserve all other spinlock_t semantics:
kernels leave task state untouched. However, PREEMPT_RT must change
task state if the task blocks during acquisition. Therefore, it saves
the current task state before blocking and the corresponding lock wakeup
- restores it::
+ restores it, as shown below::

task->state = TASK_INTERRUPTIBLE
lock()
@@ -333,7 +333,7 @@ The most basic rules are:

- Spinning lock types can nest inside sleeping lock types.

-These constraints apply both in CONFIG_PREEMPT_RT and otherwise.
+These constraints apply both in PREEMPT_RT and otherwise.

The fact that PREEMPT_RT changes the lock category of spinlock_t and
rwlock_t from spinning to sleeping means that they cannot be acquired while
@@ -344,4 +344,4 @@ holding a raw spinlock. This results in the following nesting ordering:
3) raw_spinlock_t and bit spinlocks

Lockdep will complain if these constraints are violated, both in
-CONFIG_PREEMPT_RT and otherwise.
+PREEMPT_RT and otherwise.