[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 2/2] grant_table: convert grant table rwlock to percpu rwlock
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@xxxxxxxxxx> 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@xxxxxxxxxx> wrote: > > > > > > On 17/11/15 17:04, Jan Beulich wrote: > > > > > > > > > > On 03.11.15 at 18:58, <malcolm.crossley@xxxxxxxxxx> > > > > > > > > > > wrote: > > > > > > > > --- a/xen/common/grant_table.c > > > > > > > > +++ b/xen/common/grant_table.c > > > > > > > > @@ -178,6 +178,10 @@ struct active_grant_entry { > > > > > > > > Â#define _active_entry(t, e) \ > > > > > > > > ÂÂÂÂÂ((t)->active[(e)/ACGNT_PER_PAGE][(e)%ACGNT_PER_PAGE]) > > > > > > > > Â > > > > > > > > +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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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