linux-kernel - Re: [RFC PATCH v4 4/4] hazptr: Migrate per-CPU slots to backup slot on context switch

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Message-ID: <37ce5ca2-ba64-4bb9-b752-3239c57ff7aa@efficios.com>
Date: Thu, 18 Dec 2025 11:20:01 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Boqun Feng <boqun.feng@...il.com>, Joel Fernandes
 <joel@...lfernandes.org>, "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
 Michael Ellerman <mpe@...erman.id.au>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Alan Stern <stern@...land.harvard.edu>, John Stultz <jstultz@...gle.com>,
 Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Frederic Weisbecker <frederic@...nel.org>,
 Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, Lai Jiangshan
 <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
 Mateusz Guzik <mjguzik@...il.com>,
 Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, rcu@...r.kernel.org,
 linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH v4 4/4] hazptr: Migrate per-CPU slots to backup slot
 on context switch

On 2025-12-17 20:45, Mathieu Desnoyers wrote:
[...]
> +static inline
> +void hazptr_note_context_switch(void)
> +{
> +	struct hazptr_ctx *ctx;
> +
> +	list_for_each_entry(ctx, &current->hazptr_ctx_list, preempt_node) {
> +		struct hazptr_slot *slot;
> +
> +		if (hazptr_slot_is_backup(ctx, ctx->slot))
> +			continue;
> +		slot = hazptr_chain_backup_slot(ctx);
> +		/*
> +		 * Move hazard pointer from per-CPU slot to backup slot.
> +		 * This requires hazard pointer synchronize to iterate
> +		 * on per-CPU slots with load-acquire before iterating
> +		 * on the overflow list.
> +		 */
> +		WRITE_ONCE(slot->addr, ctx->slot->addr);
> +		/*
> +		 * store-release orders store to backup slot addr before
> +		 * store to per-CPU slot addr.
> +		 */
> +		smp_store_release(&ctx->slot->addr, NULL);

I missed this:

+               /* Use the backup slot for context. */
+               ctx->slot = slot;

This triggered the hazptr_release WARN_ON_ONCE() within modified
refscale tests.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com