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;        ret

; Three following procedures are used to guarantee that

; some part of kernel code will not be terminated from outside

; while it is running.

; Note: they do not protect a thread from terminating due to errors inside

; the thread; accessing a nonexisting memory would still terminate it.

; First two procedures must be used in pair by thread-to-be-protected

; to signal the beginning and the end of an important part.

; It is OK to have nested areas.

; The last procedure must be used by outside wanna-be-terminators;

; if it is safe to terminate the given thread immediately, it returns eax=1;

; otherwise, it returns eax=0 and notifies the target thread that it should

; terminate itself when leaving a critical area (the last critical area if

; they are nested).

; Implementation. Those procedures use one dword in APPDATA for the thread,

; APPDATA.terminate_protection.

; * The upper bit is 1 during normal operations and 0 when terminate is requested.

; * Other bits form a number = depth of critical regions,

;   plus 1 if the upper bit is 1.

; * When this dword goes to zero, the thread should be destructed,

;   and the procedure in which it happened becomes responsible for destruction.

; Enter critical area. Called by thread which wants to be protected.

proc protect_from_terminate

        mov     edx, [current_slot]

; Atomically increment depth of critical areas and get the old value.

        mov     eax, 1

        lock xadd [edx+APPDATA.terminate_protection], eax

; If the old value was zero, somebody has started to terminate us,

; so we are destructing and cannot do anything protected.

; Otherwise, return to the caller.

        test    eax, eax

        jz      @f

        ret

@@:

; Wait for somebody to finish us.

        call    change_task

        jmp     @b

endp

; Leave critical area. Called by thread which wants to be protected.

proc unprotect_from_terminate

        mov     edx, [current_slot]

; Atomically decrement depth of critical areas.

        lock dec [edx+APPDATA.terminate_protection]

; If the result of decrement is zero, somebody has requested termination,

; but at that moment we were inside a critical area; terminate now.

        jz      sys_end

; Otherwise, return to the caller.

        ret

endp

; Request termination of thread identified by edx = SLOT_BASE + slot*256.

; Called by anyone.

proc request_terminate

        xor     eax, eax        ; set return value

; Atomically clear the upper bit. If it was already zero, then

; somebody has requested termination before us, so just exit.

        lock btr [edx+APPDATA.terminate_protection], 31

        jnc     .unsafe

; Atomically decrement depth of critical areas.

        lock dec [edx+APPDATA.terminate_protection]

; If the result of decrement is nonzero, the target thread is inside a

; critical area; leave termination to leaving that area.

        jnz     .unsafe

; Otherwise, it is safe to kill the target now and the caller is responsible

; for this. Return eax=1.

        inc     eax

.unsafe:

        ret

endp