[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 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@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 { >>>>>>>>> #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. > >>> 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? Yes I got this wrong. Sorry about causing confusion. Malcolm > > Ian. > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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