linux-kernel - Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

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Message-ID: <CAJhGHyD8MzUssfuKSGnu1arnayNOyBnUA03vYB0WWwbE3WzoZg@mail.gmail.com>
Date: Thu, 19 Sep 2024 14:39:13 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, linux-mm@...ck.org, 
	lkmm@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>, 
	Frederic Weisbecker <frederic@...nel.org>, Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, 
	Joel Fernandes <joel@...lfernandes.org>, Josh Triplett <josh@...htriplett.org>, 
	Uladzislau Rezki <urezki@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Zqiang <qiang.zhang1211@...il.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Kent Overstreet <kent.overstreet@...il.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com, 
	Neeraj Upadhyay <neeraj.upadhyay@....com>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

On Tue, Sep 17, 2024 at 10:34 PM Boqun Feng <boqun.feng@...il.com> wrote:

> +static void hazptr_context_snap_readers_locked(struct hazptr_reader_tree *tree,
> +                                              struct hazptr_context *hzcp)
> +{
> +       lockdep_assert_held(hzcp->lock);
> +
> +       for (int i = 0; i < HAZPTR_SLOT_PER_CTX; i++) {
> +               /*
> +                * Pairs with smp_store_release() in hazptr_{clear,free}().
> +                *
> +                * Ensure
> +                *
> +                * <reader>             <updater>
> +                *
> +                * [access protected pointers]
> +                * hazptr_clear();
> +                *   smp_store_release()
> +                *                      // in reader scan.
> +                *                      smp_load_acquire(); // is null or unused.
> +                *                      [run callbacks] // all accesses from
> +                *                                      // reader must be
> +                *                                      // observed.
> +                */
> +               hazptr_t val = smp_load_acquire(&hzcp->slots[i]);
> +
> +               if (!is_null_or_unused(val)) {
> +                       struct hazptr_slot_snap *snap = &hzcp->snaps[i];
> +
> +                       // Already in the tree, need to remove first.
> +                       if (!is_null_or_unused(snap->slot)) {
> +                               reader_del(tree, snap);
> +                       }
> +                       snap->slot = val;
> +                       reader_add(tree, snap);
> +               }
> +       }
> +}

Hello

I'm curious about whether there are any possible memory leaks here.

It seems that call_hazptr() never frees the memory until the slot is
set to another valid value.

In the code here, the snap is not deleted when hzcp->snaps[i] is null/unused
and snap->slot is not which I think it should be.

And it can cause unneeded deletion and addition of the snap if the slot
value is unchanged.

I'm not so sure...

Thanks
Lai