| Message ID | 20240515125332.9306-2-frederic@kernel.org (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 6f3490bc464e1fa98a38b3e1b1d1343413ee1b80 |
| Headers | show |
| Series | rcu: Remove several redundant memory barriers | expand |
On 15/05/24 14:53, Frederic Weisbecker wrote: > When the grace period kthread checks the extended quiescent state > counter of a CPU, full ordering is necessary to ensure that either: > > * If the GP kthread observes the remote target in an extended quiescent > state, then that target must observe all accesses prior to the current > grace period, including the current grace period sequence number, once > it exits that extended quiescent state. Also the GP kthread must > observe all accesses performed by the target prior it entering in > EQS. > > or: > > * If the GP kthread observes the remote target NOT in an extended > quiescent state, then the target further entering in an extended > quiescent state must observe all accesses prior to the current > grace period, including the current grace period sequence number, once > it enters that extended quiescent state. Also the GP kthread later > observing that EQS must also observe all accesses performed by the > target prior it entering in EQS. > > This ordering is explicitly performed both on the first EQS snapshot > and on the second one as well through the combination of a preceding > full barrier followed by an acquire read. However the second snapshot's > full memory barrier is redundant and not needed to enforce the above > guarantees: > > GP kthread Remote target > ---- ----- > // Access prior GP > WRITE_ONCE(A, 1) > // first snapshot > smp_mb() > x = smp_load_acquire(EQS) > // Access prior GP > WRITE_ONCE(B, 1) > // EQS enter > // implied full barrier by atomic_add_return() > atomic_add_return(RCU_DYNTICKS_IDX, EQS) > // implied full barrier by atomic_add_return() > READ_ONCE(A) > // second snapshot > y = smp_load_acquire(EQS) > z = READ_ONCE(B) > > If the GP kthread above fails to observe the remote target in EQS > (x not in EQS), the remote target will observe A == 1 after further > entering in EQS. Then the second snapshot taken by the GP kthread only > need to be an acquire read in order to observe z == 1. > > Therefore remove the needless full memory barrier on second snapshot. > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Still looking at the rest, but at least so far I'm convinced this one makes sense. Reviewed-by: Valentin Schneider <vschneid@redhat.com> > --- > kernel/rcu/tree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 5e6828132007..58415cdc54f8 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -325,7 +325,7 @@ static bool rcu_dynticks_in_eqs(int snap) > */ > static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap) > { > - return snap != rcu_dynticks_snap(rdp->cpu); > + return snap != ct_dynticks_cpu_acquire(rdp->cpu); > } > > /* > -- > 2.44.0
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 5e6828132007..58415cdc54f8 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -325,7 +325,7 @@ static bool rcu_dynticks_in_eqs(int snap) */ static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap) { - return snap != rcu_dynticks_snap(rdp->cpu); + return snap != ct_dynticks_cpu_acquire(rdp->cpu); } /*
When the grace period kthread checks the extended quiescent state counter of a CPU, full ordering is necessary to ensure that either: * If the GP kthread observes the remote target in an extended quiescent state, then that target must observe all accesses prior to the current grace period, including the current grace period sequence number, once it exits that extended quiescent state. Also the GP kthread must observe all accesses performed by the target prior it entering in EQS. or: * If the GP kthread observes the remote target NOT in an extended quiescent state, then the target further entering in an extended quiescent state must observe all accesses prior to the current grace period, including the current grace period sequence number, once it enters that extended quiescent state. Also the GP kthread later observing that EQS must also observe all accesses performed by the target prior it entering in EQS. This ordering is explicitly performed both on the first EQS snapshot and on the second one as well through the combination of a preceding full barrier followed by an acquire read. However the second snapshot's full memory barrier is redundant and not needed to enforce the above guarantees: GP kthread Remote target ---- ----- // Access prior GP WRITE_ONCE(A, 1) // first snapshot smp_mb() x = smp_load_acquire(EQS) // Access prior GP WRITE_ONCE(B, 1) // EQS enter // implied full barrier by atomic_add_return() atomic_add_return(RCU_DYNTICKS_IDX, EQS) // implied full barrier by atomic_add_return() READ_ONCE(A) // second snapshot y = smp_load_acquire(EQS) z = READ_ONCE(B) If the GP kthread above fails to observe the remote target in EQS (x not in EQS), the remote target will observe A == 1 after further entering in EQS. Then the second snapshot taken by the GP kthread only need to be an acquire read in order to observe z == 1. Therefore remove the needless full memory barrier on second snapshot. Signed-off-by: Frederic Weisbecker <frederic@kernel.org> --- kernel/rcu/tree.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)