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 11:56 +0000, Malcolm Crossley wrote:
> On 18/11/15 11:50, Ian Campbell wrote:
> > 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.
> 
> It's not quite a byte lock because it stores a full pointer to the per-$resource
> that it's using. It could be changed to be a byte lock but then you will need a
> percpu area per-$resource.

Right, I said "in essence sort of" and put scare quotes around the "byte
lock" since I realise it's not literally a byte lock.

But really all I was getting was that it has locked and unlocked states in
some form or other.

(Maybe I should have said "like a bit lock with 32 or 64 bits, setting any
of which corresponds to acquiring the lock" ;-))

iAN.

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