Update docs of 'fence' #41217

topecongiro · 2017-04-11T13:45:16Z

This PR updates the docs for std::sync::atomic::fence with an example and a diagram.
Part of #29377.
r? @steveklabnik

steveklabnik · 2017-04-11T15:58:45Z

@Amanieu, would you mind reviewing this? Or at least, someone other than me should make sure it's accurate. This isn't my area of specialty.

Amanieu · 2017-04-11T18:43:18Z

The example looks correct, and the explanation seems to be correct as well.

Amanieu · 2017-04-11T18:44:02Z

Actually, I think the unsafe impl are not necessary, they are already implemented by AtomicBool.

topecongiro · 2017-04-12T02:41:06Z

Thanks for comments! Removed unsafe impls from the example.

topecongiro · 2017-04-12T14:17:11Z

src/libcore/sync/atomic.rs

 /// A fence 'A' which has [`Release`] ordering semantics, synchronizes with a
 /// fence 'B' with (at least) [`Acquire`] semantics, if and only if there exists
 /// atomic operations X and Y, both operating on some atomic object 'M' such
 /// that A is sequenced before X, Y is synchronized before B and Y observes
 /// the change to M. This provides a happens-before dependence between A and B.
 ///
+///     Thread 1                                          Thread 2


Travis build is failing due to this line. I removed this line on my local environment and ./x.py test src/libcore completed successfully. Is this a bug, or intended?

CommonMark considers indented text to be code:

http://spec.commonmark.org/0.27/#indented-code-blocks

As a result, it attempts to run this as a Rust program:

Thread 1 Thread 2

and that's not a valid Rust program :)

The best idea I have here is to wrap this diagram in three backticks and annotate with 'ignore' as seen here: (ignore the backslashes)

/// \```ignore /// Thread 1 Thread 2 /// /// ... /// ... /// /// \```

@frewsxcv Thank you!

Unfortunately,

/// \```ignore /// ... /// \```

didn't work (the document got corrupted).
Instead, I tried ```text and it worked fine.

ghost

I have just a few minor nits...

The explanation and examples are correct.
Love the ASCII drawing and the example! It'd be good to see more of these. :)

ghost · 2017-04-17T00:29:06Z

src/libcore/sync/atomic.rs

+/// fences when the certain condition is met. Also, a fence prevents reordering
+/// of operations around a fence by CPU or a compiler. Each behavior is
+/// controlled by the value of [`Ordering`].
+///
 /// A fence 'A' which has [`Release`] ordering semantics, synchronizes with a


Perhaps this should say "A fence 'A' which has (at least) [Release]..."? Note the text in parentheses.

ghost · 2017-04-17T00:34:52Z

src/libcore/sync/atomic.rs

@@ -1550,12 +1550,30 @@ unsafe fn atomic_xor<T>(dst: *mut T, val: T, order: Ordering) -> T {

 /// An atomic fence.
 ///
+/// A fence creates happens-before edges between other atomic operations or


s/edges/dependence/ or s/edges/relationships/

A fence creates happens-before relationships between not only atomic operations and fences, but also between normal memory operations.

ghost · 2017-04-17T00:40:05Z

src/libcore/sync/atomic.rs

+/// A fence creates happens-before edges between other atomic operations or
+/// fences when the certain condition is met. Also, a fence prevents reordering
+/// of operations around a fence by CPU or a compiler. Each behavior is
+/// controlled by the value of [`Ordering`].


Nit: If you wish, you may rephrase this along the lines of: "Also, depending on the specified order, a fence prevents certain types of memory operation reorderings around it by CPU or compiler."

arielb1 · 2017-04-18T08:33:24Z

@steveklabnik are you looking at this?

steveklabnik · 2017-04-18T14:13:52Z

I am not sure if @stjepang 's nits should be addressed before merging or not, basically.

topecongiro · 2017-04-19T15:16:54Z

@stjepang Thank you for your feedbacks!

topecongiro · 2017-04-19T15:19:41Z

Is it ok to reflect the nits?

steveklabnik · 2017-04-19T17:27:06Z

Yup!

…

On Wed, Apr 19, 2017 at 11:19 AM, Seiichi Uchida ***@***.***> wrote: Is it ok to reflect the nits? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#41217 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AABsigtufEOUWQIcKbjf7tHM0XSl1BMiks5rxiYRgaJpZM4M6HmX> .

topecongiro · 2017-04-24T10:35:53Z

Sorry for being lazy... I pushed the changes suggested by @stjepang. Thanks again!

ghost

Looking good - just a few more grammar fixes, and then this is ready to go! :)
Thanks so much for taking an effort to improve the docs!

ghost · 2017-04-24T12:34:50Z

src/libcore/sync/atomic.rs

+/// A fence creates happens-before relationships between other atomic operations or
+/// fences when the certain condition is met. Also, depending of the specified
+/// `order`, a fence prevents reordering certain types of memory operations
+/// around a fence by CPU or a compiler.


s/a fence/it/

@stejpang Thank you for your advices! One question, are you referring to the last a fence, or the second and the last ones?

I just meant "around a fence" -> "around it". :)
But now that I think more about this, how about the following (slightly simplified) paragraph?

"Depending on the specified order, a fence prevents the compiler and CPU from reordering certain types of memory operations around it. That creates synchronizes-with relationships between it and atomic operations or fences in other threads."

I think here it's more correct to talk about synchronizes-with rather than happens-before. Pretty much every two operations within a single thread already have a happens-before relationship.
Fences really just establish synchronizes-with relationships with fences/atomics in other threads.

See the following links for a clearer explanation:
http://preshing.com/20130702/the-happens-before-relation/
http://preshing.com/20130823/the-synchronizes-with-relation/

Sorry for being so pedantic :)

I agree with you, since rust does not have a signal and fence prohibits Relaxed ordering.
Thank you very much for your explanation!

ghost · 2017-04-24T12:35:28Z

src/libcore/sync/atomic.rs

-/// fence 'B' with (at least) [`Acquire`] semantics, if and only if there exists
-/// atomic operations X and Y, both operating on some atomic object 'M' such
+/// A fence creates happens-before relationships between other atomic operations or
+/// fences when the certain condition is met. Also, depending of the specified


ghost · 2017-04-24T12:42:08Z

src/libcore/sync/atomic.rs

+///
+/// A fence 'A' which has (at least) [`Release`] ordering semantics, synchronizes
+/// with a fence 'B' with (at least) [`Acquire`] semantics, if and only if there
+/// exists operations X and Y, both operating on some atomic object 'M' such


s/exists/exist/

arielb1 · 2017-05-02T09:24:27Z

@steveklabnik I think the nits were addressed. Can you look at this?

steveklabnik · 2017-05-02T14:08:10Z

@bors: r+ rollup

thank you so much, and sorry about all of the delays.

bors · 2017-05-02T14:08:10Z

📌 Commit 91a9866 has been approved by steveklabnik

@steveklabnik

…eveklabnik Update docs of 'fence' This PR updates the docs for `std::sync::atomic::fence` with an example and a diagram. Part of rust-lang#29377. r? @steveklabnik

Rollup of 7 pull requests - Successful merges: #41217, #41625, #41640, #41653, #41656, #41657, #41705 - Failed merges:

rust-highfive assigned steveklabnik Apr 11, 2017

alexcrichton added the A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools label Apr 11, 2017

topecongiro force-pushed the docs/atomic-fence branch from 7a25dac to ed754cb Compare April 12, 2017 02:40

topecongiro commented Apr 12, 2017

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frewsxcv added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 14, 2017

topecongiro force-pushed the docs/atomic-fence branch from ed754cb to f5b077a Compare April 14, 2017 13:50

ghost approved these changes Apr 17, 2017

View reviewed changes

topecongiro force-pushed the docs/atomic-fence branch from f5b077a to 3fd8c8e Compare April 24, 2017 10:34

ghost approved these changes Apr 24, 2017

View reviewed changes

Add an example for 'fence'

91a9866

topecongiro force-pushed the docs/atomic-fence branch from 3fd8c8e to 91a9866 Compare May 1, 2017 08:25

arielb1 added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 2, 2017

steveklabnik approved these changes May 2, 2017

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frewsxcv mentioned this pull request May 3, 2017

Rollup of 7 pull requests #41717

Merged

bors added a commit that referenced this pull request May 3, 2017

Auto merge of #41717 - frewsxcv:rollup, r=frewsxcv

146dc67

Rollup of 7 pull requests - Successful merges: #41217, #41625, #41640, #41653, #41656, #41657, #41705 - Failed merges:

bors merged commit 91a9866 into rust-lang:master May 3, 2017

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Update docs of 'fence' #41217

Update docs of 'fence' #41217

topecongiro commented Apr 11, 2017

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Update docs of 'fence' #41217

Update docs of 'fence' #41217

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topecongiro commented Apr 11, 2017

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