On Wed, Apr 14, 2021 at 3:45 AM Yunsheng Lin <linyunsh...@huawei.com> wrote:
>
> On 2021/4/14 16:08, Lijun Pan wrote:
> > There are chances that napi_disable can be called twice by NIC driver.
> > This could generate deadlock. For example,
> > the first napi_disable will spin until NAPI_STATE_SCHED is cleared
> > by napi_complete_done, then set it again.
> > When napi_disable is called the second time, it will loop infinitely
> > because no dev->poll will be running to clear NAPI_STATE_SCHED.
> >
> > Though it is driver writer's responsibility to make sure it being
> > called only once, making napi_disable more robust does not hurt, not
> > to say it can prevent a buggy driver from crashing a system.
> > So, we check the napi state bit to make sure that if napi is already
> > disabled, we exit the call early enough to avoid spinning infinitely.
> >
> > Fixes: bea3348eef27 ("[NET]: Make NAPI polling independent of struct
> > net_device objects.")
> > Signed-off-by: Lijun Pan <lijunp...@gmail.com>
> > ---
> > v2: justify that this patch makes napi_disable more robust.
> >
> > net/core/dev.c | 18 ++++++++++++++++++
> > 1 file changed, 18 insertions(+)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 1f79b9aa9a3f..fa0aa212b7bb 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -6830,6 +6830,24 @@ EXPORT_SYMBOL(netif_napi_add);
> > void napi_disable(struct napi_struct *n)
> > {
> > might_sleep();
> > +
> > + /* make sure napi_disable() runs only once,
> > + * When napi is disabled, the state bits are like:
> > + * NAPI_STATE_SCHED (set by previous napi_disable)
> > + * NAPI_STATE_NPSVC (set by previous napi_disable)
> > + * NAPI_STATE_DISABLE (cleared by previous napi_disable)
> > + * NAPI_STATE_PREFER_BUSY_POLL (cleared by previous
> > napi_complete_done)
> > + * NAPI_STATE_MISSED (cleared by previous napi_complete_done)
> > + */
> > +
> > + if (napi_disable_pending(n))
> > + return;
> > + if (test_bit(NAPI_STATE_SCHED, &n->state) &&
> > + test_bit(NAPI_STATE_NPSVC, &n->state) &&
> > + !test_bit(NAPI_STATE_MISSED, &n->state) &&
> > + !test_bit(NAPI_STATE_PREFER_BUSY_POLL, &n->state))
> > + return;
>
> The NAPI_STATE_DISABLE is cleared at the end of napi_disable(),
> and if a buggy driver/hw triggers a interrupt and driver calls
> napi_schedule_irqoff(), which may set NAPI_STATE_MISSED
> if NAPI_STATE_SCHED is set(in napi_schedule_prep()), the above
> checking does not seem to handle it?
What I described in the commit message is the napi_disable() being
called from the same instance, same cpu. e.g.,
funcA {
napi_disable();
...
funcB{
if (blah)
napi_disable();
...
}
funcC;
}
The scenario you mentioned above seems to have napi already enabled
and scheduled, such that napi_schedule_prep() would set NAPI_STATE_MISSED.
The two scenarios are different per my understanding. Is there a way
that your scenario will finally call into my scenario?
Let me know if I understand you correctly.
Maybe testing NAPI_STATE_MISSED bit is not needed
because this bit is not that reliable.
Lijun
