Re: [dpdk-dev] [PATCH 1/2] eal/hotplug: allow monitor to be setup by multiple places

[Long Li <longli@microsoft.com>]

> Subject: Re: [dpdk-dev] [PATCH 1/2] eal/hotplug: allow monitor to be setup
> by multiple places
> 
> On Mon, 2020-11-30 at 23:56 -0800, Long Li wrote:
> > From: Stephen Hemminger <stephen@networkplumber.org>
> >
> > In some cases, a device or infrastructure may want to enable hotplug
> > but application may also try and start hotplug as well. Therefore
> > change the monitor_started from a boolean into a reference count.
> >
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > Signed-off-by: Long Li <longli@microsoft.com>
> > ---
> >  lib/librte_eal/linux/eal_dev.c | 15 +++++++--------
> >  1 file changed, 7 insertions(+), 8 deletions(-)
> >
> > diff --git a/lib/librte_eal/linux/eal_dev.c
> > b/lib/librte_eal/linux/eal_dev.c index 5c0e752b2d..5fa679989e 100644
> > --- a/lib/librte_eal/linux/eal_dev.c
> > +++ b/lib/librte_eal/linux/eal_dev.c
> > @@ -23,8 +23,11 @@
> >
> >  #include "eal_private.h"
> >
> > -static struct rte_intr_handle intr_handle = {.fd = -1 }; -static bool
> > monitor_started;
> > +static struct rte_intr_handle intr_handle = {
> > +	.type = RTE_INTR_HANDLE_DEV_EVENT,
> > +	.fd = -1,
> > +};
> > +static uint32_t monitor_refcount;
> >  static bool hotplug_handle;
> >
> >  #define EAL_UEV_MSG_LEN 4096
> > @@ -300,7 +303,7 @@ rte_dev_event_monitor_start(void)  {
> >  	int ret;
> >
> > -	if (monitor_started)
> > +	if (__atomic_fetch_add(&monitor_refcount, 1,
> __ATOMIC_RELAXED))
> >  		return 0;
> 
> If dev_uev_socket_fd_create or rte_intr_callback_register fail, you'll have
> incremented the refcount but there won't be anything registered, and calls
> to rte_dev_event_monitor_start will do nothing. Will that be a problem?
> 
> In other words, it seems to me the semantics is changing from call
> rte_dev_event_monitor_start until it succeeds, to call
> rte_dev_event_monitor_start, and if it fails call
> rte_dev_event_monitor_stop and then rte_dev_event_monitor_start again
> 
> >  	ret = dev_uev_socket_fd_create();
> > @@ -309,7 +312,6 @@ rte_dev_event_monitor_start(void)
> >  		return -1;
> >  	}
> >
> > -	intr_handle.type = RTE_INTR_HANDLE_DEV_EVENT;
> >  	ret = rte_intr_callback_register(&intr_handle, dev_uev_handler,
> > NULL);
> >
> >  	if (ret) {
> > @@ -317,8 +319,6 @@ rte_dev_event_monitor_start(void)
> >  		return -1;
> >  	}
> >
> > -	monitor_started = true;
> > -
> >  	return 0;
> >  }
> >
> > @@ -327,7 +327,7 @@ rte_dev_event_monitor_stop(void)  {
> >  	int ret;
> >
> > -	if (!monitor_started)
> > +	if (__atomic_sub_fetch(&monitor_refcount, 1,
> __ATOMIC_RELAXED))
> >  		return 0;
> 
> Same question in reverse - if rte_intr_callback_unregister fails, the refcount
> will have been decreased anyway, so nothing will close the file handle, right?

I'm sending v2 to address the comments.

> 
> >  	ret = rte_intr_callback_unregister(&intr_handle, dev_uev_handler,
> @@
> > -339,7 +339,6 @@ rte_dev_event_monitor_stop(void)
> >
> >  	close(intr_handle.fd);
> >  	intr_handle.fd = -1;
> > -	monitor_started = false;
> >
> >  	return 0;
> >  }