reentrant RTX51 Tiny

Is it right?
Yes.

Another question...

C functions, which use only registers for parameter and automatic variables, are inherently reentrant and may be called without any restrictions from RTX51 Tiny.

http://www.keil.com/support/docs/1861.htm

http://www.keil.com/support/docs/1861.htm says:

"If we create a function that only uses registers (and no fixed memory locations for variables), then we can rely on the ISR to save and restore the registers used. And, the function will be reentrant." (my emphasis)

So the function itself is not inherently reentrant - it is only reentrant when called via an interrupt that saves the registers

Does the same apply when calling from different RTX-51 tasks?

"As long as an interrupt saves these registers, this function may be reentered."

Does this hold if the ISR specifies the 'using' keyword...?

"Register banks are useful when processing interrupts or when using a real-time operating system because the MCU can switch to a different register bank for a task or interrupt rather than saving all 8 registers on the stack."
http://www.keil.com/support/man/docs/c51/c51_le_regbankspec.htm

See also: http://www.keil.com/forum/docs/thread8128.asp

Thank you guys for the useful information.
Right now I know that if I want to make sure functions can be called by one more tasks simultaneouslly, I should declare it with reentrant keyword.
Another test is that I create 2 functions as follows:

void test1(void) {
  unsigned char tmp1, tmp2;
  tmp1 = 10;
  tmp2 = 8;
}

void test2(void) {
  unsigned char tmp1, tmp2;
  tmp1 = 10;
  tmp2 = 8;
  tmp2 = tmp1/tmp2;
}

I found that a WARNING message occur while building target with test1(), the message is MULTIPLE CALL TO SEGMENT. But I can build with test2() sucessfully.
I know that whatever test1() or test2() all have overlay data group in *.M51. But I don't know what is the difference between test1() and test2().
Could you explain why the warning message occur?

http://www.keil.com/support/docs/2042.htm