On Sat, Jan 28, 2023 at 1:24 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Sat, Jan 28, 2023 at 03:59:01AM +0000, Joel Fernandes (Google) wrote: > > During a flip, we have a full memory barrier before srcu_idx is incremented. > > > > The idea is we intend to order the first phase scan's read of lock > > counters with the flipping of the index. > > > > However, such ordering is already enforced because of the > > control-dependency between the 2 scans. We would be flipping the index > > only if lock and unlock counts matched. > > > > But such match will not happen if there was a pending reader before the flip > > in the first place (observation courtesy Mathieu Desnoyers). > > > > The litmus test below shows this: > > (test courtesy Frederic Weisbecker, Changes for ctrldep by Boqun/me): > > Much better, thank you! > > I of course did the usual wordsmithing, as shown below. Does this > version capture your intent and understanding? > It looks good to me. According to [1] , the architecture at least should not be reordering read-write control dependency. Only read-read is problematic. But I am not 100% sure, is that not true? For the compiler, you are saying that read-write control dependency can be reordered even with *ONCE() accesses? In other words, the flipping of idx can happen in ->po order before the locks and unlock counts match? That sounds sort of like a broken compiler. [1] https://lpc.events/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf More comments below: > ------------------------------------------------------------------------ > > commit 963f34624beb2af1ec08527e637d16ab6a1dacbd > Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Date: Sat Jan 28 03:59:01 2023 +0000 > > srcu: Clarify comments on memory barrier "E" > > There is an smp_mb() named "E" in srcu_flip() immediately before the > increment (flip) of the srcu_struct structure's ->srcu_idx. > > The purpose of E is to order the preceding scan's read of lock counters > against the flipping of the ->srcu_idx, in order to prevent new readers > from continuing to use the old ->srcu_idx value, which might needlessly > extend the grace period. > > However, this ordering is already enforced because of the control > dependency between the preceding scan and the ->srcu_idx flip. > This control dependency exists because atomic_long_read() is used > to scan the counts, because WRITE_ONCE() is used to flip ->srcu_idx, > and because ->srcu_idx is not flipped until the ->srcu_lock_count[] and > ->srcu_unlock_count[] counts match. And such a match cannot happen when > there is an in-flight reader that started before the flip (observation > courtesy Mathieu Desnoyers). Agreed. > The litmus test below (courtesy of Frederic Weisbecker, with changes > for ctrldep by Boqun and Joel) shows this: > > C srcu > (* > * bad condition: P0's first scan (SCAN1) saw P1's idx=0 LOCK count inc, though P1 saw flip. > * > * So basically, the ->po ordering on both P0 and P1 is enforced via ->ppo > * (control deps) on both sides, and both P0 and P1 are interconnected by ->rf > * relations. Combining the ->ppo with ->rf, a cycle is impossible. > *) > > {} > > // updater > P0(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1) > { > int lock1; > int unlock1; > int lock0; > int unlock0; > > // SCAN1 > unlock1 = READ_ONCE(*UNLOCK1); > smp_mb(); // A > lock1 = READ_ONCE(*LOCK1); > > // FLIP > if (lock1 == unlock1) { // Control dep > smp_mb(); // E // Remove E and still passes. > WRITE_ONCE(*IDX, 1); > smp_mb(); // D > > // SCAN2 > unlock0 = READ_ONCE(*UNLOCK0); > smp_mb(); // A > lock0 = READ_ONCE(*LOCK0); > } > } > > // reader > P1(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1) > { > int tmp; > int idx1; > int idx2; > > // 1st reader > idx1 = READ_ONCE(*IDX); > if (idx1 == 0) { // Control dep > tmp = READ_ONCE(*LOCK0); > WRITE_ONCE(*LOCK0, tmp + 1); > smp_mb(); /* B and C */ > tmp = READ_ONCE(*UNLOCK0); > WRITE_ONCE(*UNLOCK0, tmp + 1); > } else { > tmp = READ_ONCE(*LOCK1); > WRITE_ONCE(*LOCK1, tmp + 1); > smp_mb(); /* B and C */ > tmp = READ_ONCE(*UNLOCK1); > WRITE_ONCE(*UNLOCK1, tmp + 1); > } > } > > exists (0:lock1=1 /\ 1:idx1=1) > > More complicated litmus tests with multiple SRCU readers also show that > memory barrier E is not needed. > > This commit therefore clarifies the comment on memory barrier E. > > Why not also remove that redundant smp_mb()? > > Because control dependencies are quite fragile due to their not being > recognized by most compilers and tools. Control dependencies therefore > exact an ongoing maintenance burden, and such a burden cannot be justified > in this slowpath. Therefore, that smp_mb() stays until such time as > its overhead becomes a measurable problem in a real workload running on > a real production system, or until such time as compilers start paying > attention to this sort of control dependency. > > Co-developed-by: Frederic Weisbecker <frederic@xxxxxxxxxx> > Co-developed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index c541b82646b63..cd46fe063e50f 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -1085,16 +1085,36 @@ static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount) > static void srcu_flip(struct srcu_struct *ssp) > { > /* > - * Ensure that if this updater saw a given reader's increment > - * from __srcu_read_lock(), that reader was using an old value > - * of ->srcu_idx. Also ensure that if a given reader sees the > - * new value of ->srcu_idx, this updater's earlier scans cannot > - * have seen that reader's increments (which is OK, because this > - * grace period need not wait on that reader). > + * Because the flip of ->srcu_idx is executed only if the > + * preceding call to srcu_readers_active_idx_check() found that > + * the ->srcu_unlock_count[] and ->srcu_lock_count[] sums matched > + * and because that summing uses atomic_long_read(), there is > + * ordering due to a control dependency between that summing and > + * the WRITE_ONCE() in this call to srcu_flip(). This ordering > + * ensures that if this updater saw a given reader's increment from > + * __srcu_read_lock(), that reader was using a value of ->srcu_idx > + * from before the previous call to srcu_flip(), which should be > + * quite rare. This ordering thus helps forward progress because > + * the grace period could otherwise be delayed by additional > + * calls to __srcu_read_lock() using that old (soon to be new) > + * value of ->srcu_idx. > + * > + * This sum-equality check and ordering also ensures that if > + * a given call to __srcu_read_lock() uses the new value of > + * ->srcu_idx, this updater's earlier scans cannot have seen > + * that reader's increments, which is all to the good, because > + * this grace period need not wait on that reader. After all, > + * if those earlier scans had seen that reader, there would have > + * been a sum mismatch and this code would not be reached. > + * > + * This means that the following smp_mb() is redundant, but > + * it stays until either (1) Compilers learn about this sort of > + * control dependency or (2) Some production workload running on > + * a production system is unduly delayed by this slowpath smp_mb(). > */ I agree that a read-write control dependency reordering by the compiler can cause a reader to observe the flipped srcu_idx too soon, thus perhaps delaying the grace period from ending (because the second scan will now end up waiting on that reader..). Thanks, - Joel > smp_mb(); /* E */ /* Pairs with B and C. */ > > - WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); > + WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); // Flip the counter. > > /* > * Ensure that if the updater misses an __srcu_read_unlock()
