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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 916
SEC. 11.3
SYSTEM STRUCTURE
885
response to a timer interrupt.
To avoid blocking threads, timer events which need
to run for an extended time should queue requests to the pool of worker threads the
kernel maintains for background activities.
Asynchronous Procedure Calls
The other special kernel control object is the
APC
(
Asynchronous Procedure
Call
) object. APCs are like DPCs in that they defer processing of a system rou-
tine, but unlike DPCs, which operate in the context of particular CPUs, APCs ex-
ecute in the context of a specific thread.
When processing a key press, it does not
matter which context the DPC runs in because a DPC is simply another part of in-
terrupt processing, and interrupts only need to manage the physical device and per-
form thread-independent operations such as recording the data in a buffer in kernel
space.
The DPC routine runs in the context of whatever thread happened to be run-
ning when the original interrupt occurred.
It calls into the I/O system to report that
the I/O operation has been completed, and the I/O system queues an APC to run in
the context of the thread making the original I/O request, where it can access the
user-mode address space of the thread that will process the input.
At the next convenient time the kernel layer delivers the APC to the thread and
schedules the thread to run.
An APC is designed to look like an unexpected proce-
dure call, somewhat similar to signal handlers in UNIX.
The kernel-mode APC for
completing I/O executes in the context of the thread that initiated the I/O, but in
kernel mode.
This gives the APC access to both the kernel-mode buffer as well as
all of the user-mode address space belonging to the process containing the thread.
When
an APC is delivered depends on what the thread is already doing, and even
what type of system.
In a multiprocessor system the thread receiving the APC may
begin executing even before the DPC finishes running.
User-mode APCs can also be used to deliver notification of I/O completion in
user mode to the thread that initiated the I/O.
User-mode APCs invoke a user-
mode procedure designated by the application, but only when the target thread has
blocked in the kernel and is marked as willing to accept APCs.
The kernel inter-
rupts the thread from waiting and returns to user mode, but with the user-mode
stack and registers modified to run the APC dispatch routine in the
ntdll.dll
system
library. The APC dispatch routine invokes the user-mode routine that the applica-
tion has associated with the I/O operation.
Besides specifying user-mode APCs as
a means of executing code when I/Os complete, the Win32 API
QueueUserAPC
allows APCs to be used for arbitrary purposes.
The executive layer also uses APCs for operations other than I/O completion.
Because the APC mechanism is carefully designed to deliver APCs only when it is
safe to do so, it can be used to safely terminate threads.
If it is not a good time to
terminate the thread, the thread will have declared that it was entering a critical re-
gion and defer deliveries of APCs until it leaves. Kernel threads mark themselves
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