Do mojo sync functions synchronize multiple calls?

[Koji Ishii]

Hello chromium-mojo experts,

Could any experts here help us to figure out if/how mojo synchronizes multiple simultaneous calls?

Renderer calls Windows DirectWrite functions using [sync] calls in dwrite_font_proxy.mojom. Its implementation, DWriteFontProxyImpl, has DirectWrite objects, and nobody should call them simultaneously.

When multiple renderer processes call these mojo functions simultaneously, does the [sync] attribute serializes the requests and prevents calling DWriteFontProxyImpl functions simultaneously, or does DWriteFontProxyImpl need to synchronize by itself?

[Dave Tapuska]

I don't believe there is any special handling really on the receiver side. They are essentially handled like an async message. If you require coordination on your service between the time you receive a message and when you call the callback (presumably you are doing something else while waiting on something) you'll have to build that yourself. 

Dave

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[Ken Rockot]

On Sat, Oct 9, 2021 at 4:36 AM Dave Tapuska <dtap...@chromium.org> wrote:

I don't believe there is any special handling really on the receiver side. They are essentially handled like an async message. If you require coordination on your service between the time you receive a message and when you call the callback (presumably you are doing something else while waiting on something) you'll have to build that yourself. 

By the same token, if there's only one DWriteFontProxyImpl or all instances are bound to the same sequence, you don't need to do anything to ensure mutual exclusion of calls. All calls will be sequenced.

 

Dave

On Sat., Oct. 9, 2021, 3:46 a.m. Koji Ishii, <ko...@chromium.org> wrote:

Hello chromium-mojo experts,

Could any experts here help us to figure out if/how mojo synchronizes multiple simultaneous calls?

Renderer calls Windows DirectWrite functions using [sync] calls in dwrite_font_proxy.mojom. Its implementation, DWriteFontProxyImpl, has DirectWrite objects, and nobody should call them simultaneously.

When multiple renderer processes call these mojo functions simultaneously, does the [sync] attribute serializes the requests and prevents calling DWriteFontProxyImpl functions simultaneously, or does DWriteFontProxyImpl need to synchronize by itself?

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[Koji Ishii]

Thanks Dave and Ken, this is great info.

A little more advice is appreciated, my knowledge can't determine if the current code suffices the "if" conditions.

From the code search, I can find two codes. One in RenderProcessHostImpl::RegisterMojoInterfaces:

  registry->AddInterface(

      base::BindRepeating(&DWriteFontProxyImpl::Create),

      base::ThreadPool::CreateSequencedTaskRunner(

          {base::TaskPriority::USER_BLOCKING, base::MayBlock()}));

I guess the use of a SequencedTaskRunner suffices the "if" condition, is that correct?

Another one in BrowserChildProcessHostImpl::BindHostReceiver.

  if (auto r = receiver.As<blink::mojom::DWriteFontProxy>()) {

    base::ThreadPool::CreateSequencedTaskRunner(

        {base::TaskPriority::USER_BLOCKING, base::MayBlock()})

        ->PostTask(FROM_HERE,

                   base::BindOnce(&DWriteFontProxyImpl::Create, std::move(r)));

    return;

  }

I'm not sure what this is doing.

Can you see whether this is sequenced or not from these code snippets?

[Daniel Cheng]

Unfortunately, the current code will bind a DWriteFontProxyImpl for each RenderProcessHost to a different sequence, since RegisterMojoInterfaces() would be called for each RenderProcessHost. Each call to CreateSequencedTaskRunner() will create a distinct sequence.

If we need this to be synchronized, then we should create a sequenced task runner once and pass the same task runner to all the sites that create DWriteFontProxyImpl.

Daniel

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[Daniel Cheng]

To be clear though, this creates distinct DWriteFontProxyImpl instances for each RPH. Is it also not allowed to simultaneously handle calls for different instances?

Daniel

[Koji Ishii]

Thanks, very helpful. Understood what this code means better.

And good point, I don't know if it's safe for multiple threads to call different instances of DWriteFactory and system font collections. I'll check with Microsoft engineers.