On Fri, Mar 18, 2005 at 10:53:27AM +0100, Ingo Molnar wrote:
>
> * Ingo Molnar <[EMAIL PROTECTED]> wrote:
>
> > there's one detail on PREEMPT_RT though (which i think you noticed too).
> >
> > Priority inheritance handling can be done in a pretty straightforward
> > way as long as no true read-side nesting is allowed for rwsems and
> > rwlocks - i.e. there's only one owner of a lock at a time. So
> > PREEMPT_RT restricts rwsem and rwlock concurrency: readers are
> > writers, with the only exception that they are allowed to 'self-nest'.
> > [...]
>
> this does not affect read-side RCU, because read-side RCU can never
> block a higher-prio thread. (as long as callback processing is pushed
> into a separate kernel thread.)
>
> so RCU will be pretty much the only mechanism (besides lock-free code)
> that allows reader concurrency on PREEMPT_RT.
This is a relief! I was wondering how on earth I was going to solve
the multi-task priority-inheritance problem!
But... How do we handle the following scenario?
0. A bunch of low-priority threads are preempted in the
middle of various RCU read-side critical sections.
1. High-priority thread does kmalloc(), but there is no
memory, so it blocks.
2. OOM handling notices, and decides to clean up the outstanding
RCU callbacks. It therefore invokes _synchronize_kernel()
(in implementation #5).
3. The _synchronize_kernel() function tries to acquire all of
the read-side locks, which are held by numerous preempted
low-priority threads.
4. What now???
Or does the current patch do priority inheritance across the memory
allocator? In other words, can we get away with ignoring this one? ;-)
Thanx, Paul
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