On 02.11.20 14:52, Jan Beulich wrote:
On 02.11.2020 14:41, Jürgen Groß wrote:
On 20.10.20 11:28, Jan Beulich wrote:
On 16.10.2020 12:58, Juergen Gross wrote:
--- a/xen/arch/x86/pv/shim.c
+++ b/xen/arch/x86/pv/shim.c
@@ -660,11 +660,12 @@ void pv_shim_inject_evtchn(unsigned int port)
if ( port_is_valid(guest, port) )
{
struct evtchn *chn = evtchn_from_port(guest, port);
- unsigned long flags;
- spin_lock_irqsave(&chn->lock, flags);
- evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ if ( evtchn_read_trylock(chn) )
+ {
+ evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
+ evtchn_read_unlock(chn);
+ }
Does this want some form of else, e.g. at least a printk_once()?
No, I don't think so.
This is just a race with the port_is_valid() test above where the
port is just being switched to invalid.
This may be such a race yes, but why do you think it _will_ be?
According to the outlined lock discipline there is no other
possibility (assuming that the lock discipline is honored).
I'll have a look whether I can add some ASSERT()s to catch any
lock discipline violation.
@@ -360,7 +352,7 @@ static long
evtchn_bind_interdomain(evtchn_bind_interdomain_t *bind)
if ( rc )
goto out;
- flags = double_evtchn_lock(lchn, rchn);
+ double_evtchn_lock(lchn, rchn);
This introduces an unfortunate conflict with my conversion of
the per-domain event lock to an rw one: It acquires rd's lock
in read mode only, while the requirements here would not allow
doing so. (Same in evtchn_close() then.)
Is it a problem to use write mode for those cases?
"Problem" can have a wide range of meanings - it's not going to
be the end of the world, but I view any use of a write lock as
a problem when a read lock would suffice. This can still harm
parallelism.
Both cases are very rare ones in the life time of an event channel. I
don't think you'll ever be able to measure any performance impact from
switching these case to a write lock for any well behaved guest.
Juergen
