Re: sleeping and waiting and tasklets

Peter,

I rely on Linux Device Drivers, 3rd Edition, an O'Reilly book a lot. This is
what they have to say about copy_*_user():

===========
unsigned long copy_to_user(void _ _user *to,
const void *from,
unsigned long count);
unsigned long copy_from_user(void *to,
const void _ _user *from,
unsigned long count);
Although these functions behave like normal memcpy functions, a little extra
care must be used when accessing user space from kernel code. The user pages
being addressed might not be currently present in memory, and the virtual
memory subsystem can put the process to sleep while the page is being
transferred into place. This happens, for example, when the page must be
retrieved from swap space. The net result for the driver writer is that any
function that accesses user space must be reentrant, must be able to execute
concurrently with other driver functions, and, in particular, must be in a
position where it can legally sleep. We return to this subject in Chapter 5.
The role of the two functions is not limited to copying data to and from
user-space: they also check whether the user space pointer is valid. If the
pointer is invalid, no copy is performed; if an invalid address is
encountered during the copy, on the other hand, only part of the data is
copied. In both cases, the return value is the amount of memory still to be
copied.
==========
This edition of the book has been updated to include the 2.6 kernel.
It looks liike it agrees with you in some places and disagrees in others. As
far as verifying the user-space pointers, they say there is validation but
you both agree on what happens when there is a problem. You were much more
detailed in your explaination.
As a result of the answers here, I have a much better understanding of the
"do not" issues.
Thanks to all of you!!
Bill

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