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

[Zachary Turner]

I'm sympathetic to this complaint, but note that I'm just going by the LLVM
Developer Pollicy (
http://llvm.org/docs/DeveloperPolicy.html#incremental-development)


On Fri, Aug 29, 2014 at 11:48 AM, Todd Fiala <tfiala at google.com> wrote:

> 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.
>>> >
>>> >
>>>
>>>
>>
>> _______________________________________________
>> lldb-commits mailing list
>> lldb-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
>>
>>
>
>
> --
> Todd Fiala | Software Engineer |  tfiala at google.com |  650-943-3180
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/lldb-commits/attachments/20140829/5f558e8a/attachment.html>