Re: [PATCH 2/2] grant_table: convert grant table rwlock to percpu rwlock

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-11-18 at 11:23 +0000, Malcolm Crossley wrote:
> On 18/11/15 10:54, Jan Beulich wrote:
> > > > > On 18.11.15 at 11:36, <ian.campbell@citrix.com> wrote:
> > > On Tue, 2015-11-17 at 17:53 +0000, Andrew Cooper wrote:
> > > > On 17/11/15 17:39, Jan Beulich wrote:
> > > > > > > > On 17.11.15 at 18:30, <andrew.cooper3@citrix.com> wrote:
> > > > > > On 17/11/15 17:04, Jan Beulich wrote:
> > > > > > > > > > On 03.11.15 at 18:58, <malcolm.crossley@citrix.com>
> > > > > > > > > > wrote:
> > > > > > > > --- a/xen/common/grant_table.c
> > > > > > > > +++ b/xen/common/grant_table.c
> > > > > > > > @@ -178,6 +178,10 @@ struct active_grant_entry {
> > > > > > > >  #define _active_entry(t, e) \
> > > > > > > >      ((t)->active[(e)/ACGNT_PER_PAGE][(e)%ACGNT_PER_PAGE])
> > > > > > > >  
> > > > > > > > +bool_t grant_rwlock_barrier;
> > > > > > > > +
> > > > > > > > +DEFINE_PER_CPU(rwlock_t *, grant_rwlock);
> > > > > > > Shouldn't these be per grant table? And wouldn't doing so
> > > > > > > eliminate
> > > > > > > the main limitation of the per-CPU rwlocks?
> > > > > > The grant rwlock is per grant table.
> > > > > That's understood, but I don't see why the above items aren't,
> > > > > too.
> > > > 
> > > > Ah - because there is never any circumstance where two grant tables
> > > > are
> > > > locked on the same pcpu.
> > > 
> > > So per-cpu rwlocks are really a per-pcpu read lock with a fallthrough
> > > to a
> > > per-$resource (here == granttable) rwlock when any writers are
> > > present for
> > > any instance $resource, not just the one where the write lock is
> > > desired,
> > > for the duration of any write lock?
> > 
> 
> The above description is the very good for for how the per-cpu rwlocks behave.
> The code stores a pointer to the per-$resource in the percpu area when a user is
> reading the per-$resource, this is why the lock is not safe if you take the lock
> for two different per-$resource simultaneously. The grant table code only takes
> one grant table lock at any one time so it is a safe user.

So essentially the "per-pcpu read lock" as I called it is really in essence
a sort of "byte lock" via the NULL vs non-NULL state of the per-cpu pointer
to the underlying rwlock.

> > That's not how I understood it, the rwlock isn't per-pCPU (at least not
> > in what this patch does - it remains a per-domain one). The per-pCPU
> > object is a pointer to an rwlock, which gets made point to whatever
> > domain's rwlock the pCPU wants to own.
> > 
> 
> This description is correct but it's important to note that the rwlock
> is only used by the writers and could be effectively replaced with a
> spinlock.

The rwlock is taken (briefly) by readers if *writer_activating is, isn't
it?

Ian.

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