[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 13:08 +0000, Malcolm Crossley wrote: > On 18/11/15 12:07, Ian Campbell wrote: > > 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@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@citrix. > > > > > > > > > > > > > > com> > > > > > > > > > > > > > > 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. > > > > > > 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. > > I was just concerned that people may not pick up on the subtle difference > that the > percpu read areas are used for multiple resources (of which none are > locked simultaneously > by the same CPU) where as byte locks are typically used to lock a > particular > resource and so you can safely lock multiple resource simultaneously on > the same CPU. > > > > > (Maybe I should have said "like a bit lock with 32 or 64 bits, setting > > any > > of which corresponds to acquiring the lock" ;-)) > > > Not quite, setting the per cpu read area "takes" the read lock for the > particular > resource you passed into the percpu rwlock implementation. Writers of another > resource > ($resource1) will safely ignore readers of ($resource0). Ah yes, subtle. > The global barrier will however make _all_ readers take the per-$resource > read lock. > An optimisation could be to have a barrier variable per-$resource (stored in > the > struct grant_table in this case). Yes, this global barrier was part of what took me down the wrong path above. Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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