On Sat, Jan 28, 2023 at 09:09:04PM -0800, Paul E. McKenney wrote: > On Sat, Jan 28, 2023 at 04:16:34PM -0500, Joel Fernandes wrote: > > 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? > > Agreed, READ_ONCE() or stronger through condition to WRITE_ONCE() > or stronger is ordered. Replace that WRITE_ONCE() with any type of > unordered read and all bets are off. > > And now that the ARM folks chimed in, this is a solid guarantee at > the hardware level. > > Not so much at the compiler level. Oddly enough, compilers do provide > ordering for plain C-language stores, but that is helpful only if no > other CPU or thread is concurrently accessing that variable. > > > 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. > > One case where a sane compiler can reasonably enable the hardware to > do the reordering is where you have the same WRITE_ONCE() in both the > then-clause and else-clause of an "if" statement. Another is if it can > somehow prove something about the value returned from that READ_ONCE(), > for example, if that value is used to index a singleton array, then the > compiler has to do the READ_ONCE(), but it can then assume that the > value returned was zero, throwing away the real value returned. > > Fun with undefined behavior! > > > [1] https://lpc.events/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf > > > > More comments below: Except that it was pointed out to me that the Co-developed-by tags also need Signed-off-by tags. If there are no objections to the update shown below, I will fix this on my next rebase. Thanx, Paul ------------------------------------------------------------------------ commit 6c135bb38c55d354527a6659cbf2f4e7e20b4360 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). 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> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> Co-developed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx> Signed-off-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(). */ 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()
