[openib-general] SDP_CONN_LOCK

[Michael S. Tsirkin]

Quoting r. Libor Michalek <libor at topspin.com>:
> Subject: Re: [openib-general] SDP_CONN_LOCK
> 
> On Thu, Feb 17, 2005 at 11:29:37PM +0200, Michael S. Tsirkin wrote:
> > Quoting r. Libor Michalek <libor at topspin.com>:
> > > Subject: Re: [openib-general] SDP_CONN_LOCK
> > > 
> > > On Thu, Feb 17, 2005 at 05:35:17PM +0200, Michael S. Tsirkin wrote:
> > > > Hi, Libor!
> > > > Could you please explain what are the SDP_CONN_LOCK
> > > > and friends doing in SDP?
> > > > 
> > > > It seems they just implement exclusive access to socket,
> > > > but if so, why is a simple semaphore or mutex not used?
> > > 
> > >   They do implement exclusive access to the socket, but they implement
> > > exclusive access from both process and irq context, which is why a
> > > semaphore was not used. In interrupt context SDP_CONN_LOCK_BH is used
> > > to lock the connection, look in sdp_cq_event_handler() for it's use,
> > > and in process context SDP_CONN_LOCK is used.
> > 
> > I dont really understand how it works.
> > When an interrupt arrives while users != 0, it seems you are
> > calling scheduler().
> > What is sdp_conn_internal_lock doing? I understand it is to be called
> > from interrupt context, but how can it call scheduler() then?
> 
>   SDP_CONN_LOCK and SDP_CONN_UNLOCK are never called from interrupt
> context, only from process context. The spinlock conn->lock.slock
> is called in interrupt context and process context before the 'users'
> variable is changed or read. If users != 0, then the connection is in
> use by process context. If the connection is in use then another lock
> request in process context will sleep until users == 0, if the connection
> is in use then a lock request in interrupt context will mark the
> connection as having had an event and return. When a process context
> lock holder releases the lock, it checks if there has been an interrupt
> event, which it then processes by draining and arming the CQ, and it
> wakes any process context waiting for the lock.
> 
> 
> -Libor
> 

Cant all this be done by a semaphore in process context, and 
down_trylock in interrupt context?
The "connection has an event" bit could be protected by a spinlock.

-- 
MST - Michael S. Tsirkin