Re: SIGPROF + SIGCHLD and igc

[Helmut Eller]

On Sun, Dec 29 2024, Eli Zaretskii wrote:

>> Here another observation: Let's assume that we can emulate SuspendThread
>> and ResumeThread on POSIX and MacOS (much like MPS does it).  In the
>> profiler thread we can then simply do:
>> 
>>    SuspendThread (<main-thread>)
>>    get_backtrace ()
>>    ResumeThread (<main-thread>)
>> 
>> without invoking any SIGPROF handler or the need for
>> ArenaEnter/ArenaLeave.
>
> Yes, we can.  But that means changing what the SIGPROF handler does
> will have no effect, which is bad for maintenance.  By contrast, the
> current MS-Windows implementation does basically the same, but it
> calls the handler, not its body.
>
> Also, I don't quite see why you thing calling get_backtrace directly
> will have any effect on the need to lock the arena.  I'm probably
> missing something.

Ok. The arena lock is a problem.  Is that also a problem for the Windows
implementation?  How does it avoid deadlocks when the thread holds the
arena lock?

>> The only complication seems to be, that we must avoid deadlocks when MPS
>> tries to suspend a thread is already suspended by our emulation.
>
> If MPS always runs from the main thread, it doesn't (and cannot)
> suspend the thread.  Or maybe I misunderstand which thread do you mean
> here?

I may interpret "MPS runs in a thread" differently than you do.  Maybe I
should say a "thread is inside MPS" whenever a thread holds the arena
lock.  With this meaning, every thread can be inside MPS.

Helmut