[9fans] Race condition in /sys/src/9/pc/trap.c?

[Matthew J Jones <elly1@andrew.cmu.edu>]

My familiarity with the kernel source code is superficial to say the
least, but it seems to me that this code (from /sys/src/9/pc/trap.c)
contains a race condition:

  702         if(sp<(USTKTOP-BY2PG) || sp>(USTKTOP-sizeof(Sargs)-BY2WD))
  703             validaddr(sp, sizeof(Sargs)+BY2WD, 0);
  704
  705         up->s = *((Sargs*)(sp+BY2WD));

We verify that the address is correct; is there any reason another thread
in the same address space cannot start running after line 703 completes
and unmap that memory, causing us to access unmapped memory at line 705?
The system call entry is itself an interrupt gate, but line 689 is
spllo(), and we appear to hold no locks at this point.

(Please forgive me if my terminology is not in line: as I said, my
experience is very limited and I am just beginning to explore the source).

-- Elly