Re: [PATCH] sys_getppid oopses on debug kernel

From: Kirill Korotaev
Date: Tue Aug 08 2006 - 11:40:48 EST

Accessing freed memory is a bug, always, not just *only* when slab
debugging is on, right? Doesn't this mean we could get junk, or that
the reader could potentially run off a bad pointer?
no, read the comment in sys_getppid.
It is a valid optimization. _safe_ and alowing to bypass taking the lock.
BUT! This optimization relies on the fact that kernel memory (DMA + normal zone)
is always mapped into virtual address space.
Which is invalid for debug kernels only.

It seems that this patch only papers over the problem in the case when
it is observed, but doesn't really even fix the normal case.

Could we use a seqlock to determine when real_parent is in flux, and
re-read real_parent until we get a consistent one? We could use in in
lieu of the existing for() loop.


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