On 6/28/2020 12:41 PM, Gregory Nutt wrote:
Would userpace_s need to have an additional function pointer for
pthread_cleanup? Similar to pthread_startup?
Is the up_pthread_start/pthread_startup functions a good example to
follow
for how to call in user mode?
If you look at Issue 1263, that is not what I am proposing:
https://github.com/apache/incubator-nuttx/issues/1263. That is
similar to the solution in the TODO list. But it is overly complex
and would not work for KERNEL mode where there is no userspace_s.
I am rather proposing dividing pthread_exit() into two pieces: A user
space function called pthread_exit() the resides in
libs/lib/pthread/lib_pthread_exit.c and a kernel space functions, say
nxthread_exit() that resides in sched/pthread/pthread_exit.c:
1. pthread_exit() in libs/lib/pthread/lib_pthread_exit.c would execute
when the thread exits. It would only do the things that need to be
done in the same context as the pthread execution environment. This
would including executing all of the pthread_cleanup functions. That
would would have to be migrated from sched/pthread/pthread_exit.c. It
would also include the new logic to execute the the new
pthread-specific data destructors. And, finally, is would all the
nxthread_exit() system function.
2. nxthread_exit() would become a system call (replacing the existing
system pthread_exit() system call in syscall/ and in
include/sys/syscall*.h.
3. nxthread_exit() would essentially be the existing pthread exit
logic sans pthread_cleanup function calls.
This sounds straight forward, but there is a complication: We also
have to catch the case were the pthread main function does not call
pthread_exit() but instead just returns. Currently that is handled in
the function pthread_start() in sched/pthread/pthread_create.c. In
FLAT mode, it just calls the pthread main function; in other modes it
calls up_pthread_start() which will switch to user mode.
I have not thought through all of the details, but this would require
changes like:
1. Instead of calling the pthread main function, pthread_start() would
call a trampoline function in user-space. This already happens in the
PROTECTED mode via the pthread_startup() function in the userspace_s
structure. But not in KERNEL mode. In KERNEL mode, it uses the
SYS_pthread_start system call to start the pthread main function
directly.
2. pthread_start would not call pthread_exit().
3. The trampoline function in user space would simply call the pthread
main function and call pthread_exit() is the pthread main function
returns.
Hmm.. The kernel gets the address of the pthread trampoline from the
userspace_s structure. How would the kernel know the address of a
process specific pthread thread trampoline function in the KERNEL
mode? I am not sure righit now.
The design that you describe is probably easier to define. But one
objective that I have is to move all of the pthread logic out of the
kernel and into libs/libs/pthread. The pthreads interfaces are not
part of the kernel in other Unix-like implementations. The kernel does
provide certain necessary hooks, but the majority of the
implementation of the pthread logic is implemented in libpthread, in
user-space and not within the kernel. This is a model that I think
that NuttX should follow as well, but things are not well structured
to do that now.
"Hmm.. The kernel gets the address of the pthread trampoline from the
userspace_s structure. How would the kernel know the address of a
process specific pthread thread trampoline function in the KERNEL mode?
I am not sure righit now." ... Yes, after sleeping on it, I know what
would need to happen:
1. Rename pthread_create() in sched/pthread/pthread_create.c to
nxthread_create(). Add on new parameter: The address of the user-space
pthread startup function. Instead of calling the pthread main entry
(directly or indirectly), pthread_start() would scall the pthread
startup function, passing it the real address of the pthread main function.
The call to pthread_exist would be removed from pthread_startup() and
move into a new function in user space.
2. Add libs/libc/pthread/lib_pthread_start.c that would contain two
trivial functions:
static void pthread_startup(pthread_startroutine_t startroutine,
pthread_addr_t arg)
{
pthread_exit(startroutine(arg));
}
int pthread_create(FAR pthread_t *thread, FAR const pthread_attr_t *attr,
pthread_startroutine_t startroutine, pthread_addr_t arg)
{
return nxthread_create(pthread_startup, thread, attr, startroutine, arg);
}
And that should handle all cases.
Greg
