On Wed, Jun 03, 2020 at 01:40:16PM +0200, Peter Zijlstra wrote:
> A KCSAN build revealed we have explicit annoations through atomic_*()
> usage, switch to arch_atomic_*() for the respective functions.
>
> vmlinux.o: warning: objtool: rcu_nmi_exit()+0x4d: call to
> __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_enter()+0x25: call to
> __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_nmi_enter()+0x4f: call to
> __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_exit()+0x2a: call to
> __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: __rcu_is_watching()+0x25: call to
> __kcsan_check_access() leaves .noinstr.text section
>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> Link:
> https://lkml.kernel.org/r/20200603084818.gb2...@hirez.programming.kicks-ass.net
> ---
> kernel/rcu/tree.c | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -250,7 +250,7 @@ static noinstr void rcu_dynticks_eqs_ent
> * next idle sojourn.
> */
> rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
> - seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
> + seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
To preserve KCSAN's ability to see this, there would be something like
instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) prior
to the instrumentation_end() invoked before rcu_dynticks_eqs_enter()
in each of rcu_eqs_enter() and rcu_nmi_exit(), correct?
> // RCU is no longer watching. Better be in extended quiescent state!
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> (seq & RCU_DYNTICK_CTRL_CTR));
> @@ -274,13 +274,13 @@ static noinstr void rcu_dynticks_eqs_exi
> * and we also must force ordering with the next RCU read-side
> * critical section.
> */
> - seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
> + seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
And same here, but after the instrumentation_begin() following
rcu_dynticks_eqs_exit() in both rcu_eqs_exit() and rcu_nmi_enter(),
correct?
> // RCU is now watching. Better not be in an extended quiescent state!
> rcu_dynticks_task_trace_exit(); // After ->dynticks update!
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> !(seq & RCU_DYNTICK_CTRL_CTR));
> if (seq & RCU_DYNTICK_CTRL_MASK) {
> - atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
> + arch_atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
This one is gone in -rcu.
> smp_mb__after_atomic(); /* _exit after clearing mask. */
> }
> }
> @@ -313,7 +313,7 @@ static __always_inline bool rcu_dynticks
> {
> struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>
> - return !(atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
> + return !(arch_atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
Also instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) as
follows:
o rcu_nmi_exit(): After each following instrumentation_begin().
o In theory in rcu_irq_exit_preempt(), but as this generates code
only in lockdep builds, it might not be worth worrying about.
o Ditto for rcu_irq_exit_check_preempt().
o Ditto for __rcu_irq_enter_check_tick().
o rcu_nmi_enter(): After each following instrumentation_begin().
o __rcu_is_watching() is itself noinstr:
o idtentry_enter_cond_rcu(): After each following
instrumentation_begin().
o rcu_is_watching(): Either before or after the call to
rcu_dynticks_curr_cpu_in_eqs().
> }
>
> /*
> @@ -692,6 +692,7 @@ noinstr void rcu_nmi_exit(void)
> {
> struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>
> + instrumentation_begin();
> /*
> * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
> * (We are exiting an NMI handler, so RCU better be paying attention
> @@ -705,7 +706,6 @@ noinstr void rcu_nmi_exit(void)
> * leave it in non-RCU-idle state.
> */
> if (rdp->dynticks_nmi_nesting != 1) {
> - instrumentation_begin();
> trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting,
> rdp->dynticks_nmi_nesting - 2,
> atomic_read(&rdp->dynticks));
> WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
> @@ -714,7 +714,6 @@ noinstr void rcu_nmi_exit(void)
> return;
> }
>
> - instrumentation_begin();
> /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0,
> atomic_read(&rdp->dynticks));
> WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
This one looks to be having no effect on instrumentation of atomics, but
rather coalescing a pair of instrumentation_begin() into one.
Do I understand correctly?
Thanx, Paul
