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[whatwg] localStorage mutex - a solution?

From: Rob Ennals <rob.ennals@gmail.com>
Date: Thu, 5 Nov 2009 15:14:40 -0800
Message-ID: <8E82BE35-F94C-4E17-9BEE-EBF409465FEB@gmail.com>
By "API operation" I mean anything other than core ecmascript and  
localstorage itself.

Eg you can be sure that the following will not release the mutex:
* do nothing
* function call
* if/for etc
* access to a local variable
* access to a basic ecmascript object (not host objects)

But anything else outside core ecmascript is free to release the mutex  
if it wants. Eg any call to dom or any access to any object provided  
by the host. Rather than telling them what *can* release the mutex, we  
instead tell them to assume that everything can unless it is in our  
list of things that *can't*.

This is intentionally broad for several reasons:
* to avoid us having to work out the full list of all cases that could  
require any user agent to release the mutex
* to give users an easy rule that they can remember and know will  
always be safe on any user agent
* to give the user agent the freedom to release the mutex as soon as  
possible, thus improving concurrency [I am rep for Intel so I care  
about this]

Apologies for being vague earlier.


On Nov 5, 2009, at 10:16 AM, Jeremy Orlow <jorlow at chromium.org> wrote:

> This is intriguing.  But what it comes down to is what we consider  
> an "API operation".  For example, you could define "API operation"  
> to be the existing list of thing that unlock LocalStorage.  Or it  
> could be defined in a way that Darin Fisher's idea to lock whenever  
> we're about to nest locking calls would also work.  Or a variety of  
> other things.
> Does anyone have any ideas on what the exact language for what an  
> "API operation" might look like?
> I do have a couple of concerns though:
> Leaving the language open might not be terribly useful to a typical  
> web developer since they're not going to read the spec and probably  
> aren't going to have a very firm idea of whether what they're doing  
> could unlock storage or not.  Experimentation wouldn't work very  
> well since each platform could be wildly different (since a lot of  
> possible behaviors fall between the MAY and the MAY NOT in the  
> proposed spec).
> Another concern is that the worst case performance aspects of  
> LocalStorage remain.  I cringe every time I think of one event loop  
> blocking another.  But I will admit that the average time would be  
> better--especially if we're unlocking fairly aggressively.
> I'm interested to hear what others have to say on this proposal.
> J
> On Wed, Nov 4, 2009 at 3:31 PM, Rob Ennals <rob.ennals at gmail.com>  
> wrote:
> Missed out the important final qualifier. Here's take 3:
> "the user agent MUST NOT release the storage mutex between calls to  
> local storage, except that the user agent MAY release the storage  
> mutex on any API operation /other that a local storage oeration/"
> If a local storage op can release the mutex then the whole thing is  
> pointless :-)
> -Rob
> On Nov 4, 2009, at 3:15 PM, Rob Ennals <rob.ennals at gmail.com> wrote:
> I suspect my suggested spec line was insufficiently precise. How  
> about this:
> "the user agent MUST NOT release the storage mutex between calls to  
> local storage, except that the user agent MAY release the storage  
> mutex on any API operation"
> We'd still need to define what "API operation" means, and I'm sure  
> this could be worded better, but hopefully this makes the basic idea  
> clearer.
> -Rob
> On Nov 4, 2009, at 2:56 PM, Mike Shaver <mike.shaver at gmail.com> wrote:
> On Wed, Nov 4, 2009 at 5:51 PM, Rob Ennals <rob.ennals at gmail.com>  
> wrote:
> Or to put it another way: if the thread can't call an API then it  
> can't
> block waiting for another storage mutex, thus deadlock can't occur,  
> thus we
> don't need to release the storage mutex.
> Right, but the spec text there doesn't prevent the UA from releasing
> more than in that scenario, which seems like it's not an improvement
> over where we are right now: unpredictable consistency.  Existing racy
> implementations like in IE would be conformant, so developers can't
> count on the script-sequenced-storage-ops pattern providing
> transactionality.
> More likely, though, _I_'m missing something...
> Mike
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