| Message ID | 20221013172244.1099010-2-frederic@kernel.org (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 6558b914fc4ee221891c9f32753626ebd4acb86c |
| Headers | show |
| Series | srcu: A few NMI-safe debugging updates | expand |
On Thu, Oct 13, 2022 at 07:22:42PM +0200, Frederic Weisbecker wrote: > Using the NMI-unsafe reader API from within NMIs is very likely to be > buggy for three reasons: > > 1) NMIs aren't strictly re-entrant (a pending nested NMI will execute > at the end of the current one) so it should be fine to use a > non-atomic increment here. However breakpoints can still interrupt > NMIs and if a breakpoint callback has a reader on that same ssp, a > racy increment can happen. > > 2) If the only reader site for a given ssp is in an NMI, RCU is definetly definitely > a better choice over SRCU. Just checking - because NMI are by definition not-preemptibe, so SRCU over RCU doesn't make much sense right? Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org> thanks, - Joel > > 3) Because of the previous reason (2), an ssp having an SRCU read side > critical section in an NMI is likely to have another one from a task > context. > > For all these reasons, warn if an nmi unsafe reader API is used from an > NMI. > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > --- > kernel/rcu/srcutree.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index c54142374793..8b7ef1031d89 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe) > > if (!IS_ENABLED(CONFIG_PROVE_RCU)) > return; > + /* NMI-unsafe use in NMI is a bad sign */ > + WARN_ON_ONCE(!nmi_safe && in_nmi()); > sdp = raw_cpu_ptr(ssp->sda); > old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety); > if (!old_nmi_safe_mask) { > -- > 2.25.1 >
On Fri, Oct 14, 2022 at 10:45:04PM +0000, Joel Fernandes wrote: > On Thu, Oct 13, 2022 at 07:22:42PM +0200, Frederic Weisbecker wrote: > > Using the NMI-unsafe reader API from within NMIs is very likely to be > > buggy for three reasons: > > > > 1) NMIs aren't strictly re-entrant (a pending nested NMI will execute > > at the end of the current one) so it should be fine to use a > > non-atomic increment here. However breakpoints can still interrupt > > NMIs and if a breakpoint callback has a reader on that same ssp, a > > racy increment can happen. > > > > 2) If the only reader site for a given ssp is in an NMI, RCU is definetly > definitely > > a better choice over SRCU. > > Just checking - because NMI are by definition not-preemptibe, so SRCU over > RCU doesn't make much sense right? Agreed. But you never know... > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org> I will apply on the next rebase (after today's rebase), thank you! Thanx, Paul > thanks, > > - Joel > > > > > 3) Because of the previous reason (2), an ssp having an SRCU read side > > critical section in an NMI is likely to have another one from a task > > context. > > > > For all these reasons, warn if an nmi unsafe reader API is used from an > > NMI. > > > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > > --- > > kernel/rcu/srcutree.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > index c54142374793..8b7ef1031d89 100644 > > --- a/kernel/rcu/srcutree.c > > +++ b/kernel/rcu/srcutree.c > > @@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe) > > > > if (!IS_ENABLED(CONFIG_PROVE_RCU)) > > return; > > + /* NMI-unsafe use in NMI is a bad sign */ > > + WARN_ON_ONCE(!nmi_safe && in_nmi()); > > sdp = raw_cpu_ptr(ssp->sda); > > old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety); > > if (!old_nmi_safe_mask) { > > -- > > 2.25.1 > >
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index c54142374793..8b7ef1031d89 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe) if (!IS_ENABLED(CONFIG_PROVE_RCU)) return; + /* NMI-unsafe use in NMI is a bad sign */ + WARN_ON_ONCE(!nmi_safe && in_nmi()); sdp = raw_cpu_ptr(ssp->sda); old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety); if (!old_nmi_safe_mask) {
Using the NMI-unsafe reader API from within NMIs is very likely to be buggy for three reasons: 1) NMIs aren't strictly re-entrant (a pending nested NMI will execute at the end of the current one) so it should be fine to use a non-atomic increment here. However breakpoints can still interrupt NMIs and if a breakpoint callback has a reader on that same ssp, a racy increment can happen. 2) If the only reader site for a given ssp is in an NMI, RCU is definetly a better choice over SRCU. 3) Because of the previous reason (2), an ssp having an SRCU read side critical section in an NMI is likely to have another one from a task context. For all these reasons, warn if an nmi unsafe reader API is used from an NMI. Signed-off-by: Frederic Weisbecker <frederic@kernel.org> --- kernel/rcu/srcutree.c | 2 ++ 1 file changed, 2 insertions(+)