Hallo Richard,
On Wed, Oct 18, 2000 at 10:44:11AM +0100, richardj_moore@uk.ibm.com wrote:
>
>
> We've release v1.1 of DProbes - deatils and code is on the DProbes web
> page.
>
> the enhancements include:
>
> - DProbes for kernel version 2.4.0-test7 is now available.
First thanks for this nice work.
I ported the older 1.0 dprobes to 2.4 a few weeks ago for my own use.
It is very useful for kernel work. Unfortunately the user space support
had still one ugly race which I didn't fix because it required too
extensive changes for my simple port (and it didn't concern me because
I only use kernel level breakpoints)
I see the problems are still in 1.1.
The problem is the vma loop in process_recs_in_cow_pages over the vmas of an
address_space. In 2.4 the only way to do that safely is to hold the
address_space spinlock. Unfortunately you cannot take the semaphore
or execute handle_mm_fault while holding the spinlock, because they could
sleep. The only way I think to do it relatively race free without adding locks
to the core VM is to do it two pass (first collect all the mms with mmget()
and their addresses in a separate list with the spinlock and then process it
with the spinlock released)
Then dp_vaddr_to_page has another race. It cannot hold the mm semaphore
because that would deadlock with handle_mm_struct. Not holding it means
though that the page could be swapped out again after you faulted it in
before you have a change to access it. It probably can be done with an
loop that checks and locks the page atomically (e.g. using cmpexchg)
and retries the handle_mm_fault as needed.
There may be more races I missed, the 2.4 SMP MM locking hierarchy is
unfortunately not very flexible and makes things like what dprobes wants
to do relatively hard.
Another change I added and which I found useful is a printk to show
the opcode of mismatched probes (this way wrong offsets in the probe
definitions are easier to fix)
-Andi
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