On Tue, Apr 28, 2015 at 12:47 PM, Chris Muller <asqueaker at gmail.com> wrote:

> Wait, the newer one has a non-local return in it, but
> Mutex>>#critical: has an ensure: in it anyway, so maybe I don't see
> the problem..?
>

If one hits ctrl-period when the system is in the critical section then the
debugger can't open because it interrupts the critical section, preventing
the ensure block from running, attempts to access e.g. scroll bar
preferences when it tries to open, and the system deadlocks.  So
preferences either need to be *not* protected by a critical section, or the
Debugger needs not to access preferences.

IMO, we should try and write preferences so that they don't require a
lock.  Writing them as a lock-free data structure would be a really good
idea. First that critical section is slow and clunky.  Second, I presume it
is there only for the rare case of a write to preferences, not to protect
reads.

IMO, a simple implementation which copied and replaced the entire
preferences dictionary on write would be sufficient.  Sure there's a danger
that some client would get a stale value if it read concurrently while
there was a write, but then so what?  A preference is a preference, not a
hard-and-fast value, and code should work accessing a preference no matter
its value, so momentarily getting a stale value shouldn't matter.  So the
implementation could be as simple as

addPreference: aName categories: categoryList default: aValue balloonHelp:
helpString projectLocal: localBoolean changeInformee: informeeSymbol
changeSelector: aChangeSelector type: aType
"Add or replace a preference as indicated.  Reuses the preexisting
Preference object for this symbol, if there is one, so that UI artifacts
that interact with it will remain valid."

| aPreference aPrefSymbol |
aPrefSymbol := aName asSymbol.
aPreference := DictionaryOfPreferences
at: aPrefSymbol
ifAbsent:
[| newPreference |
 newPreference := aPreference
name:aPrefSymbol
defaultValue:aValue
helpString:helpString
localToProject:localBoolean
categoryList:categoryList
changeInformee:informeeSymbol
changeSelector:aChangeSelector
type: aType.
 AccessLock critical:
[| newDict |
 newDict := DictionaryOfPreferences copy.
 newDict at: aPrefSymbol put: newPreference].
self  compileAccessMethodForPreference:aPreference.
 newPreference]


> On Tue, Apr 28, 2015 at 2:43 PM, Chris Muller <asqueaker at gmail.com> wrote:
> >> The above change restores the old behavior of locking up the image, so
> it
> >> should be reverted. An additional comment explaininng why aBlock must
> not be
> >> evaluated inside the argument of #accessDictionaryOfPreferencesIn:
> would be
> >> helpful.
> >
> > Ahh, because aBlock might have a non-local return in it, leaving the
> > Mutex unsignaled (and critical unenterable), is that right?
> >
> > Took me a minute to see that problem.
> >
> > Okay, I'll revert that method if no one else does by my next commit..
> >
> >> It would be even better to finally get rid of DictionaryOfPreferences.
> >>
> >>
> >> Levente
> >>
>
>


-- 
best,
Eliot
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