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From: jai@lab.ultra.nyu.edu (Benchiao Jai)
Newsgroups: comp.os.minix
Subject: Re: (PC/AT) why not include 'sync' in keyboard.c (for rebooting?)
Message-ID: <1991Apr15.210247.26781@cmcl2.nyu.edu>
Date: 15 Apr 91 21:02:47 GMT
References: <50809@nigel.ee.udel.edu>
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In article <50809@nigel.ee.udel.edu> CRAWFORD_B%PLU.BITNET@cornellc.cit.cornell.edu (brian) writes:
>C.v.W writes:
>
>"It is not allowed that drivers call FS or MM, this is against the MINIX
> hierarchy."
>
>I thought Minix was layered as such: Level 1: Kernel; Level 2: Device
>Drivers; Level 3: FS and MM; Level 4: user applications.  I believe there
>is a figure in the book to this effect.  If this is the case, then device
>drivers must communicate with either the file system or the memory manager
>since they are a level directly above the drivers (if the driver needs to
>communicate with an application, for example).

The kernel can send a message to fs telling it to execute a sync, but
this operation is not atomic, so other things may happen in between.
Picture this:	keyboard driver send sync request to fs
		fs sends a write request to disk task
		disk task executes the request and send SUSPEND to fs
		fs suspends keyboard task and fetches another request from user
		(this is possible since no higher priority tasks can be run
		 at this moment)
		fs executes the new write request by putting the new data
		  into buffer cache
		disk task replies OK
		fs revives keyboard task and send an OK reply
		keyboard proceeds to reboot
BOOM!  Buffer cache is still dirty.

Two minor points.  First, the first level is the task switcher.  The kernel
consists of this and device drivers.  Second, the applications never know
that there is such a thing called 'kernel', they just execute the system
calls, which are handled by either MM or FS.

Benchiao Jai
jai@cs.nyu.edu