Re: Re: [Linux-kernel-mentees] [PATCH] Doc: whatisRCU: Add more Markup

Phong Tran
2019-11-06T13:16:44Z

On 11/6/19 4:45 PM, Amol Grover wrote:
> On Wed, Nov 06, 2019 at 04:42:34AM +0700, Phong Tran wrote:
>> o Adding more crossrefs.
>> o Bold some words.
>> o Add header levels.
>>
>> Signed-off-by: Phong Tran <tranmanphong@gmail.com>
>> ---
>> Documentation/RCU/whatisRCU.rst | 67 ++++++++++++++++++++-------------
>> 1 file changed, 41 insertions(+), 26 deletions(-)
>>
>> diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
>> index ae40c8bcc56c..3e24e0155a91 100644
>> --- a/Documentation/RCU/whatisRCU.rst
>> +++ b/Documentation/RCU/whatisRCU.rst
>> @@ -150,7 +150,7 @@ later. See the kernel docbook documentation for more info, or look directly
>> at the function header comments.
>>
>> rcu_read_lock()
>> -
>> +^^^^^^^^^^^^^^^
>> void rcu_read_lock(void);
>>
>> Used by a reader to inform the reclaimer that the reader is
>> @@ -164,7 +164,7 @@ rcu_read_lock()
>> longer-term references to data structures.
>>
>> rcu_read_unlock()
>> -
>> +^^^^^^^^^^^^^^^^^
>> void rcu_read_unlock(void);
>>
>> Used by a reader to inform the reclaimer that the reader is
>> @@ -172,13 +172,13 @@ rcu_read_unlock()
>> read-side critical sections may be nested and/or overlapping.
>>
>> synchronize_rcu()
>> -
>> +^^^^^^^^^^^^^^^^^
>> void synchronize_rcu(void);
>>
>> Marks the end of updater code and the beginning of reclaimer
>> code. It does this by blocking until all pre-existing RCU
>> read-side critical sections on all CPUs have completed.
>> - Note that synchronize_rcu() will -not- necessarily wait for
>> + Note that synchronize_rcu() will **not** necessarily wait for
>> any subsequent RCU read-side critical sections to complete.
>> For example, consider the following sequence of events::
>>
>> @@ -196,7 +196,7 @@ synchronize_rcu()
>> any that begin after synchronize_rcu() is invoked.
>>
>> Of course, synchronize_rcu() does not necessarily return
>> - -immediately- after the last pre-existing RCU read-side critical
>> + **immediately** after the last pre-existing RCU read-side critical
>> section completes. For one thing, there might well be scheduling
>> delays. For another thing, many RCU implementations process
>> requests in batches in order to improve efficiencies, which can
>> @@ -225,10 +225,10 @@ synchronize_rcu()
>> checklist.txt for some approaches to limiting the update rate.
>>
>> rcu_assign_pointer()
>> -
>> +^^^^^^^^^^^^^^^^^^^^
>> void rcu_assign_pointer(p, typeof(p) v);
>>
>> - Yes, rcu_assign_pointer() -is- implemented as a macro, though it
>> + Yes, rcu_assign_pointer() **is** implemented as a macro, though it
>> would be cool to be able to declare a function in this manner.
>> (Compiler experts will no doubt disagree.)
>>
>> @@ -245,7 +245,7 @@ rcu_assign_pointer()
>> the _rcu list-manipulation primitives such as list_add_rcu().
>>
>> rcu_dereference()
>> -
>> +^^^^^^^^^^^^^^^^^
>> typeof(p) rcu_dereference(p);
>>
>> Like rcu_assign_pointer(), rcu_dereference() must be implemented
>> @@ -280,8 +280,8 @@ rcu_dereference()
>> unnecessary overhead on Alpha CPUs.
>>
>> Note that the value returned by rcu_dereference() is valid
>> - only within the enclosing RCU read-side critical section [1].
>> - For example, the following is -not- legal::
>> + only within the enclosing RCU read-side critical section [1]_.
>> + For example, the following is **not** legal::
>>
>> rcu_read_lock();
>> p = rcu_dereference(head.next);
>> @@ -304,9 +304,11 @@ rcu_dereference()
>> at any time, including immediately after the rcu_dereference().
>> And, again like rcu_assign_pointer(), rcu_dereference() is
>> typically used indirectly, via the _rcu list-manipulation
>> - primitives, such as list_for_each_entry_rcu() [2].
>> + primitives, such as list_for_each_entry_rcu() [2]_.
>> +
>> + .. [1]
>
> Hey Phong,
> I just checked the patch but this change doesn't seem quite right.
> It just creates an empty footnote. Something on the lines of
> .. [1] The variant rcu_dereference_protected()...
> should work.
>
thanks, Amol.
fixed it. The patch sent out amend to 7c12b3764c47

https://lkml.org/lkml/2019/11/6/513

>>
>> - [1] The variant rcu_dereference_protected() can be used outside
>> + The variant rcu_dereference_protected() can be used outside
>> of an RCU read-side critical section as long as the usage is
>> protected by locks acquired by the update-side code. This variant
>> avoids the lockdep warning that would happen when using (for
>> @@ -319,7 +321,9 @@ rcu_dereference()
>> a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst
>> and the API's code comments for more details and example usage.
>>
>> - [2] If the list_for_each_entry_rcu() instance might be used by
>> + .. [2]
>
> Similarly for this.
>
Fixed it

>> +
>> + If the list_for_each_entry_rcu() instance might be used by
>> update-side code as well as by RCU readers, then an additional
>> lockdep expression can be added to its list of arguments.
>> For example, given an additional "lock_is_held(&mylock)" argument,
>> @@ -459,22 +463,22 @@ uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`,
>>
>> So, to sum up:
>>
>> -o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
>> +- Use rcu_read_lock() and rcu_read_unlock() to guard RCU
>> read-side critical sections.
>>
>> -o Within an RCU read-side critical section, use rcu_dereference()
>> +- Within an RCU read-side critical section, use rcu_dereference()
>> to dereference RCU-protected pointers.
>>
>> -o Use some solid scheme (such as locks or semaphores) to
>> +- Use some solid scheme (such as locks or semaphores) to
>> keep concurrent updates from interfering with each other.
>>
>> -o Use rcu_assign_pointer() to update an RCU-protected pointer.
>> +- Use rcu_assign_pointer() to update an RCU-protected pointer.
>> This primitive protects concurrent readers from the updater,
>> - -not- concurrent updates from each other! You therefore still
>> + **not** concurrent updates from each other! You therefore still
>> need to use locking (or something similar) to keep concurrent
>> rcu_assign_pointer() primitives from interfering with each other.
>>
>> -o Use synchronize_rcu() -after- removing a data element from an
>> +- Use synchronize_rcu() **after** removing a data element from an
>> RCU-protected data structure, but -before- reclaiming/freeing
>
> And this -before- is feeling lonely aswell.
>
okay, fixed.

Regards,
Phong.

>
> Thanks
> Amol
>
>> the data element, in order to wait for the completion of all
>> RCU read-side critical sections that might be referencing that
>> @@ -566,7 +570,7 @@ namely foo_reclaim().
>> The summary of advice is the same as for the previous section, except
>> that we are now using call_rcu() rather than synchronize_rcu():
>>
>> -o Use call_rcu() -after- removing a data element from an
>> +- Use call_rcu() **after** removing a data element from an
>> RCU-protected data structure in order to register a callback
>> function that will be invoked after the completion of all RCU
>> read-side critical sections that might be referencing that
>> @@ -603,7 +607,7 @@ more details on the current implementation as of early 2004.
>>
>>
>> 5A. "TOY" IMPLEMENTATION #1: LOCKING
>> -
>> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> This section presents a "toy" RCU implementation that is based on
>> familiar locking primitives. Its overhead makes it a non-starter for
>> real-life use, as does its lack of scalability. It is also unsuitable
>> @@ -671,6 +675,8 @@ that the only thing that can block rcu_read_lock() is a synchronize_rcu().
>> But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
>> so there can be no deadlock cycle.
>>
>> +.. _quiz_1:
>> +
>> Quick Quiz #1:
>> Why is this argument naive? How could a deadlock
>> occur when using this algorithm in a real-world Linux
>> @@ -679,7 +685,7 @@ Quick Quiz #1:
>> :ref:`Answers to Quick Quiz <8_whatisRCU>`
>>
>> 5B. "TOY" EXAMPLE #2: CLASSIC RCU
>> -
>> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> This section presents a "toy" RCU implementation that is based on
>> "classic RCU". It is also short on performance (but only for updates) and
>> on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
>> @@ -710,14 +716,14 @@ CPU in turn. The run_on() primitive can be implemented straightforwardly
>> in terms of the sched_setaffinity() primitive. Of course, a somewhat less
>> "toy" implementation would restore the affinity upon completion rather
>> than just leaving all tasks running on the last CPU, but when I said
>> -"toy", I meant -toy-!
>> +"toy", I meant **toy**!
>>
>> So how the heck is this supposed to work???
>>
>> Remember that it is illegal to block while in an RCU read-side critical
>> section. Therefore, if a given CPU executes a context switch, we know
>> that it must have completed all preceding RCU read-side critical sections.
>> -Once -all- CPUs have executed a context switch, then -all- preceding
>> +Once **all** CPUs have executed a context switch, then **all** preceding
>> RCU read-side critical sections will have completed.
>>
>> So, suppose that we remove a data item from its structure and then invoke
>> @@ -725,12 +731,16 @@ synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
>> that there are no RCU read-side critical sections holding a reference
>> to that data item, so we can safely reclaim it.
>>
>> +.. _quiz_2:
>> +
>> Quick Quiz #2:
>> Give an example where Classic RCU's read-side
>> - overhead is -negative-.
>> + overhead is **negative**.
>>
>> :ref:`Answers to Quick Quiz <8_whatisRCU>`
>>
>> +.. _quiz_3:
>> +
>> Quick Quiz #3:
>> If it is illegal to block in an RCU read-side
>> critical section, what the heck do you do in
>> @@ -1076,9 +1086,11 @@ Answer:
>> approach where tasks in RCU read-side critical sections
>> cannot be blocked by tasks executing synchronize_rcu().
>>
>> +:ref:`Back to Quick Quiz #1 <quiz_1>`
>> +
>> Quick Quiz #2:
>> Give an example where Classic RCU's read-side
>> - overhead is -negative-.
>> + overhead is **negative**.
>>
>> Answer:
>> Imagine a single-CPU system with a non-CONFIG_PREEMPT
>> @@ -1103,6 +1115,8 @@ Answer:
>> even the theoretical possibility of negative overhead for
>> a synchronization primitive is a bit unexpected. ;-)
>>
>> +:ref:`Back to Quick Quiz #2 <quiz_2>`
>> +
>> Quick Quiz #3:
>> If it is illegal to block in an RCU read-side
>> critical section, what the heck do you do in
>> @@ -1128,6 +1142,7 @@ Answer:
>> Besides, how does the computer know what pizza parlor
>> the human being went to???
>>
>> +:ref:`Back to Quick Quiz #3 <quiz_3>`
>>
>> ACKNOWLEDGEMENTS
>>
>> --
>> 2.20.1
>>
> _______________________________________________
> Linux-kernel-mentees mailing list
> Linux-kernel-mentees@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/linux-kernel-mentees
>