On Sat, 4 Sep 1999, Nate Eldredge wrote:
> * Does "lock the kernel" mean "prevent anyone else from executing any
> kernel code"?
No.
When you have the kernel lock you can guarantee that accesses to
anything else protected by the kernel lock can't happen.
So, generally you:
1. Acquire the kernel lock
You can now guarantee that no other access to code/data protected
by the kernel lock can happen.
2. Mess with shared lists / page tables / etc
3. Let go of the lock
The big kernel lock used to protect lots of stuff. Ie, acquiring the
lock used to stop a whole lot of stuff running.
These days, the big lock is being replaced with more localised locks,
to improve the granularity of access. Hence, you can have 1 CPU doing
one kernel task, and another CPU doing something unrelated, provided
they are protected by different locks.
> * If so, what happens if some other CPU is already inside the kernel?
> Is there some mechanism to stop the other CPU??
The call to acquire the kernel lock blocks until the other CPU drops
the kernel lock. Therefore you need to be careful that you don't
produce interactions that can cause deadlocks.
Eg,
CPU 1 CPU 2
acquire lock 1
acquire lock2
get to a point where we need lock 2,
so wait for lock 2 to be released.
get to a point needing
lock 1, so wait for
lock 1 to be released.
Result: Waiting for lock 1 Waiting for lock 2 which is
which is held on CPU 2 held on CPU 1
Mark
+-------------------------------------------------------------------------+
Mark Cooke The views expressed above are mine and are not
Systems Programmer necessarily representative of university policy
University Of Birmingham URL: http://www.sr.bham.ac.uk/~mpc/
+-------------------------------------------------------------------------+
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