[Lldb-commits] [PATCH] Implements a HostThread class.

Todd Fiala tfiala at google.com
Fri Aug 29 11:48:41 PDT 2014 

This actually happens to be one of the challenges I find when we try to cut
up patches a little too small or introduce concepts before use.  It is easy
to lose what the real functional purpose for a change is all about.

I tend to be a bit more in favor of seeing new ideas/refactorings/changes
along with something that drove the change so that we don't lose track of
the big picture.


On Fri, Aug 29, 2014 at 11:35 AM, Zachary Turner <zturner at google.com> wrote:

> I mentioned this a few times earlier, but admittedly the thread has grown
> pretty long.
>
> HostThread is a replacement for thread_t.  Anywhere you use a thread_t,
> you can use a HostThread.  This applies obviously to threads inside the
> debugger itself, but also to any other threads on the machine.  In the case
> of debugging process P locally, for example, it might be used when viewing
> or manipulating the threads of P.  Even in remote debuggign scenarios, the
> debugserver might use a HostThread to manipulate threads in the inferior.
>  There's already a method in Host called FindProcessThreads, for example.
>  Instead of returning a list of thread ids, it could return a list of
> HostThreads.  The size of the class should not be much (if any) larger than
> an lldb::thread_t, it just has extra methods for convenience.
>
>
> On Fri, Aug 29, 2014 at 11:28 AM, <jingham at apple.com> wrote:
>
>> One thing I am unclear about, I had thought that HostThread in particular
>> would be used for threads IN lldb, but not for handling threads in a
>> program lldb is debugging when on that Host.  So for instance, Suspend &
>> Resume don't seem necessary or desirable when dealing with threads lldb is
>> using for implementing lldb, and Launch is possible but quite tricky and
>> not required for remote threads on the same Host.
>>
>> What is the actual story here?
>>
>> Jim
>>
>>
>> > On Aug 29, 2014, at 11:19 AM, Zachary Turner <zturner at google.com>
>> wrote:
>> >
>> > What do you think about this?  I implement the abstract base class.
>> I'll give it the following methods:
>> >
>> > Pure virtual:
>> > * Launch
>> > * Join
>> > * Cancel
>> > * GetState (enumerated value : invalid, running, exited, suspended,
>> cancelled)
>> > * Suspend
>> > * Resume
>> > * GetThreadResult
>> >
>> > Virtual with default implementations:
>> > * GetName
>> > * SetName
>> >
>> > Cancel will return an lldb::Error if cancellation is not supported (for
>> example because we're on windows and we don't own the thread routine).  If
>> you think of any other methods that should go here, let me know.
>> >
>> > Initial patch will go in like this, with no code yet updated to use the
>> new HostThread.  In a subsequent patch, I will change thread_t's to
>> HostThreads, updating code as necessary.  We can evaluate the decision
>> about the typedef vs. the base class at this point, when we see what (if
>> any) impact this will have on whether specific methods will be accessed
>> from generic code.  At this point, if there's still disagreement, I can
>> even make two separate patches - one that does it each way, so that we can
>> compare actual code that uses both methods.
>> >
>> >
>> > On Fri, Aug 29, 2014 at 11:01 AM, <jingham at apple.com> wrote:
>> >
>> > > On Aug 29, 2014, at 10:09 AM, Zachary Turner <zturner at google.com>
>> wrote:
>> > >
>> > > On Fri, Aug 29, 2014 at 9:54 AM, <jingham at apple.com> wrote:
>> > >
>> > > > SystemLog
>> > > > ThreadDetach
>> > > > ThreadCancel
>> > >
>> > > There must be some way on Windows to tell a thread to exit at the
>> next cancellation point?  Can you really not cancel a worker thread's
>> operation?
>> > >
>> > > Not an arbitrary thread, no.  You can forcefully terminate it (with
>> caveats, and it may not work and/or have unexpected behavior, so it is
>> strongly discouraged), but generally if you want to gracefully cancel a
>> thread, the thread has to have specific code in its run loop to make that
>> possible.  So it might work for threads that we create inside the lldb
>> process, since we control the thread routine, but it wouldn't be meaningful
>> for threads in other process.  The reason for this discrepancy is that
>> there is no concept of signals on Windows, which I assume is how thread
>> cancellation is implemented behind the scenes on posix platforms.
>> >
>> > I don't think thread cancellation requires signals for same process
>> thread cancellation for pthreads, but it does use kernel support.  The
>> system defines "cancellation points" - e.g. select, read, etc. where a
>> thread is likely to sit waiting, and pthread_cancel just marks the target
>> thread so that if it is in a cancellation routine (they're pretty much all
>> kernel traps) it gets killed off, otherwise the request is postponed till
>> the thread enters a cancellation point and then it gets killed.   I don't
>> think you would need signals for this.
>> >
>> > But it is something we need to do for all our ports, since we rely on
>> this for shutting down the Process worker threads cleanly.  Since we only
>> really care about canceling threads we create, I think it would be okay for
>> ThreadCancel to be a no-op for other threads.
>> >
>> > >
>> > >
>> > > > GetAuxvData
>> > > >
>> > > > Before my original HostInfo refactor, it also had these methods:
>> > > >
>> > > > GetUserID
>> > > > GetGroupID
>> > > > GetEffectiveUserID
>> > > > GetEffectiveGroupID
>> > > >
>> > > > Those methods were just as accessible to anyone writing generic
>> code.  They are *less* accessible now, because they are on HostInfoPosix.
>> And this happened without needing to introduce any platform specific
>> pre-processor defines into generic LLDB.  There was maybe one exception,
>> which Jason Molenda pointed out earlier, which was the GetOSBuildString.
>> And that only happened because GetOSBuildString is not a generic concept!
>>  The design worked exactly as intended, exposing a place where code that
>> was intended to be generic actually wasn't as generic as it thought it
>> was.  Everywhere else, the GetUserID, GetGroupID, etc methods were only
>> being called from specific code (which is how it should work), but after my
>> change this is now enforced by the compiler.
>> > > >
>> > > >
>> > >
>> > > But again, if I'm writing code it seems like a real pain to have to
>> answer the question "is this an interface I can use in generic code" be:
>> > >
>> > > For now you can if it exists in all the HostThreadXXX.h files, but of
>> course if somebody introduces another platform and decides that they don't
>> want to implement this function then you can't use it anymore and have to
>> put
>> > >
>> > > #if defined
>> > >
>> > > guards around the usage.  Instead, we should look at the host
>> interfaces and say there are three classes of things:
>> > >
>> > > 1) Things you must implement in order to port lldb to a host
>> > >     These should be pure virtual methods in HostFeature.h
>> > > 2) Things you can optionally implement, but there's a reasonable
>> "couldn't do that" fallback
>> > >     These should be virtual methods in HostFeature.h
>> > > 3) Things that are purely host specific.
>> > >     These should be methods in HostFeatureMyHost.h, and can only be
>> used in HostOtherFeatureMyHost.h, but never in generic code.
>> > >
>> > > This would make the job of folks working in generic code clear, and
>> also make it obvious to the next porter (OpenVMS, anyone?) what they have
>> to do.
>> > >
>> > > I'm ok with doing all of this.  I'm all for having the compiler catch
>> things for you, and if one of the things it can catch for you is "you need
>> to implement this method" then that's great.  That said, I'm still rather
>> fond of the idea of typedefing HostFeature to HostFeatureMyHost and then
>> having everyone use HostFeature.  No matter if you do it or don't, it's
>> still equally easy to write specific code from generic code.  You could
>> just do this:
>> > >
>> > > #if defined(__APPLE__)
>> > > ((HostFeatureMacOSX&)feature).AppleSpecificMethod();
>> > > #endif
>> > >
>> > > as opposed to this
>> > >
>> > > #if defined(__APPLE__)
>> > > feature.AppleSpecificMethod();
>> > > #endif
>> > >
>> >
>> > My feeling about this is since you'd actually have to do:
>> >
>> > #if defined(__APPLE__)
>> > #include "lldb/Host/windows/HostFeatureMacOSX.h"
>> > (((HostFeatureMacOSX&)feature).AppleSpecificMethod();
>> > #endif
>> >
>> > I hope at that point you'd know you've gone off the reservation.
>> >
>> > Jim
>> >
>> >
>> >
>> > > In both cases though, the person has felt strongly enough about it to
>> put it in a pre-processor guard, so the decision has already been made.
>> And the second method has the benefit that when you're writing specific
>> code (which I anticipate to write alot of for Windows), you just always
>> have the type you need.
>> >
>> >
>>
>>
>
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-- 
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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