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Re: [Qemu-devel] [PATCH 1/3] rcu: Add automatically released rcu_read_lo
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 1/3] rcu: Add automatically released rcu_read_lock variant |
Date: |
Wed, 11 Sep 2019 18:04:23 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
* Daniel P. Berrangé (address@hidden) wrote:
> On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's
> > g_auto infrastrcture (and thus whatever the compilers hooks are) to
> > release it on all exits of the block.
> >
> > Note this macro has a variable declaration in, and hence is not in
> > a while loop.
> >
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> > include/qemu/rcu.h | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> > index 22876d1428..6a25b27d28 100644
> > --- a/include/qemu/rcu.h
> > +++ b/include/qemu/rcu.h
> > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc
> > *func);
> > }), \
> > (RCUCBFunc *)g_free);
> >
> > +typedef char rcu_read_auto_t;
> > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r)
> > +{
> > + rcu_read_unlock();
> > +}
> > +
> > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock)
> >
> > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \
> > + _rcu_read_auto = 'x'; \
> > + rcu_read_lock();
> > +
>
> Functionally this works, but my gut feeling would be to follow
> the design of GMutexLocker as-is:
>
> https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new
>
> so you get a use pattern of
>
> g_autoptr(rcu_read_locker) locker = rcu_read_locker_new();
>
> This makes it explicit that the code is creating a variable here, which
> in turns means it is clear to force unlock early with
>
> g_clear_pointer(&locker, rcu_read_locker_free)
The difference compared to the g-mutex-locker is that I don't have
another object to use as my pointer; that uses the address of the GMutex
as the dummy pointer value. I did try an experiment with g_autoptr
and found that it did need to return a non-NULL value for it to work,
which then lead me to think what value to use - while it seems to work
if I return (void *)1 it makes me nervous.
Dave
>
>
> Regards,
> Daniel
> --
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
Re: [Qemu-devel] [PATCH 1/3] rcu: Add automatically released rcu_read_lock variant, Eric Blake, 2019/09/11
Re: [Qemu-devel] [PATCH 1/3] rcu: Add automatically released rcu_read_lock variant, Dr. David Alan Gilbert, 2019/09/11