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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Sat, 28 Jan 2023 16:16:34 -0500
Message-ID: <CAEXW_YTK1ejOcuNyAqJ3ibKKyfR21UYHRNa7jpWS_H71xpnTpg@mail.gmail.com>
Subject: Re: [PATCH v4] srcu: Clarify comments on memory barrier "E"
To:     paulmck@kernel.org
Cc:     linux-kernel@vger.kernel.org,
        Frederic Weisbecker <frederic@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>, rcu@vger.kernel.org,
        Steven Rostedt <rostedt@goodmis.org>
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On Sat, Jan 28, 2023 at 1:24 PM Paul E. McKenney <paulmck@kernel.org> 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@joelfernandes.org>
> 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@kernel.org>
>     Co-developed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>     Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
>     Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> 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()