Re: SIGPROF + SIGCHLD and igc

[Helmut Eller <eller.helmut@gmail.com>]

On Fri, Dec 27 2024, Helmut Eller wrote:

> Pip Cet via wrote:
>> ...forever, if the profiler thread has been suspended at this point.
>
> Hmm.  Indeed.  So back to the drawing board.

It seems that the statement

  block SIGPROF while MPS holds the lock

is logically equivalent to

  deliver SIGPROF only while MPS does not hold the lock.

This variant might be bit easier to implement.  The "while MPS does not
hold the lock" part can be implemented by claiming the lock in the
profiler thread like so:

  mps_arena_t arena = global_igc->arena;
  ArenaEnter (arena);
  ... deliver SIGPROF part goes here ...
  ArenaLeave (arena);

The functions ArenaEnter and ArenaLeave are not part of the public API
but they are external symbols, so the linker can find them.

The "deliver SIGPROF" part goes like this:

1. The profiler thread calls pthread_kill (SIGPROF, <main_thread>) and
   then waits (on a pipe or whatever).

2. The SIGPROF handler gets called and immediately notifies the profiler
   thread (without waiting for a reply).  After that, it continues as
   usual calling get_backtrace etc.

3. The profiler thread awakes and releases the lock.

Regarding deadlocks: the profiler thread holds the lock while it waits.
So MPS should not be able to stop the profiler thread there.

Helmut