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SubjectRe: [patch] Real-Time Preemption, -RT-2.6.9-rc4-mm1-U9
Ingo Molnar writes:
>
> * Nikita Danilov <nikita@clusterfs.com> wrote:
>
> > > condition variables are fine if you 1) already know them from userspace
> > > and 2) want to use a single locking abstraction for everything. It is
> > > thus also a kitchen-sink primitive that is inevitably slow and complex.
> > > I still have to see a locking problem where condvars are the
> > > cleanest/simplest answer, and i've yet to see a locking problem where
> > > condvars are not the slowest answer ;)
> >
> > A kernel daemon that waits for some work to do is an example.
>
> what type of work - could you be a bit more specific?

Take a loop in fs/cifs/cifsfs.c:cifs_oplock_thread() (I won't copy it
here to avoid you all going blind). It can be recoded as

while(1) {
spin_lock(&GlobalMid_Lock);
while (list_empty(&GlobalOplock_Q)) {
if (kcond_timedwait(&SomeCIFSCVAR, &GlobalMid_Lock, HZ) == -EINTR) {
spin_unlock(&GlobalMid_Lock);
complete_and_exit(&cifs_oplock_exited, 0);
}
}
oplock_item = list_entry(GlobalOplock_Q.next, struct oplock_q_entry, qhead);
/* do stuff with oplock_item ... */
spin_unlock(&GlobalMid_Lock);
....
}

Point is that this is very stylistic usage---easily recognizable.

>
> Ingo

Nikita.
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