> -----Original Message----- > From: Jan Beulich <jbeul...@suse.com> > Sent: 30 September 2020 09:32 > To: p...@xen.org > Cc: xen-devel@lists.xenproject.org; 'Andrew Cooper' > <andrew.coop...@citrix.com>; 'George Dunlap' > <george.dun...@eu.citrix.com>; 'Ian Jackson' <i...@xenproject.org>; 'Julien > Grall' <jul...@xen.org>; > 'Wei Liu' <w...@xen.org>; 'Stefano Stabellini' <sstabell...@kernel.org> > Subject: Re: [PATCH 04/12] evtchn: evtchn_set_priority() needs to acquire the > per-channel lock > > On 30.09.2020 09:31, Paul Durrant wrote: > >> -----Original Message----- > >> From: Jan Beulich <jbeul...@suse.com> > >> Sent: 30 September 2020 07:42 > >> To: p...@xen.org > >> Cc: xen-devel@lists.xenproject.org; 'Andrew Cooper' > >> <andrew.coop...@citrix.com>; 'George Dunlap' > >> <george.dun...@eu.citrix.com>; 'Ian Jackson' <i...@xenproject.org>; > >> 'Julien Grall' <jul...@xen.org>; > >> 'Wei Liu' <w...@xen.org>; 'Stefano Stabellini' <sstabell...@kernel.org> > >> Subject: Re: [PATCH 04/12] evtchn: evtchn_set_priority() needs to acquire > >> the per-channel lock > >> > >> On 29.09.2020 18:31, Paul Durrant wrote: > >>>> -----Original Message----- > >>>> From: Xen-devel <xen-devel-boun...@lists.xenproject.org> On Behalf Of > >>>> Jan Beulich > >>>> Sent: 28 September 2020 11:58 > >>>> To: xen-devel@lists.xenproject.org > >>>> Cc: Andrew Cooper <andrew.coop...@citrix.com>; George Dunlap > >>>> <george.dun...@eu.citrix.com>; Ian > >>>> Jackson <i...@xenproject.org>; Julien Grall <jul...@xen.org>; Wei Liu > >>>> <w...@xen.org>; Stefano > >> Stabellini > >>>> <sstabell...@kernel.org> > >>>> Subject: [PATCH 04/12] evtchn: evtchn_set_priority() needs to acquire > >>>> the per-channel lock > >>>> > >>>> evtchn_fifo_set_pending() (invoked with the per-channel lock held) has > >>>> two uses of the channel's priority field. > >>> > >>> AFAICT it is invoked with only the sending end's lock held... > >>> > >>>> The field gets updated by > >>>> evtchn_fifo_set_priority() with only the per-domain event_lock held, > >>>> i.e. the two reads may observe two different values. While the 2nd use > >>>> could - afaict - in principle be replaced by q->priority, I think > >>>> evtchn_set_priority() should acquire the per-channel lock in any event. > >>>> > >>> > >>> ... so how is this going to help? > >> > >> I guess the reasoning needs to change here - it should focus solely > >> on using the finer grained lock here (as holding the per-domain one > >> doesn't help anyway). It would then be patch 10 which addresses the > >> (FIFO-specific) concern of possibly reading inconsistent values. > >> > > > > Yes, it looks like patch #10 should ensure consistency. > > > > Prior to ad34d0656fc at least the first layer of calls done in > > evtchn_send() didn't take the evtchn > itself as an arg. Of course, evtchn_set_pending() then looked up the evtchn > and passed it to > evtchn_port_set_pending() without any locking in the interdomain case. I > wonder whether, to make > reasoning easier, there ought to be a rule that ABI entry points are always > called with the evtchn > lock held? > > What do you mean by "ABI entry points" here? To me this would sound > like what's directly accessible to guests, but that's hardly what > you mean. Do you perhaps mean the hooks in struct evtchn_port_ops?
Yes, by ABI I mean 'fifo' or '2l'. (I guess that 'ABI' is just the name I chose to refer to them in the Windows PV driver code). > As per the comment that got added there recently, the locking > unfortunately is less consistent there. > I looked to me that most functions were entered with channel lock held so wondered whether it could be a rule. Paul > Jan
