linux-kernel - Re: [PATCH net V2 2/4] net/mlx5: Fix deadlock between devlink lock and esw->wq

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Message-ID: <20260128205622.12e1f026@kernel.org>
Date: Wed, 28 Jan 2026 20:56:22 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Tariq Toukan <tariqt@...dia.com>
Cc: Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky
 <leon@...nel.org>, Mark Bloch <mbloch@...dia.com>,
 <netdev@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, Gal Pressman <gal@...dia.com>, Moshe
 Shemesh <moshe@...dia.com>, Simon Horman <horms@...nel.org>, Cosmin Ratiu
 <cratiu@...dia.com>
Subject: Re: [PATCH net V2 2/4] net/mlx5: Fix deadlock between devlink lock
 and esw->wq

On Tue, 27 Jan 2026 10:52:39 +0200 Tariq Toukan wrote:
> esw_functions_changed_event_handler -> esw_vfs_changed_event_handler is
> called from the esw->work_queue and acquires the devlink lock.
> 
> Changing the esw mode is done via .eswitch_mode_set (acquires devlink
> lock in the devlink_nl_pre_doit call) -> mlx5_devlink_eswitch_mode_set
> -> mlx5_eswitch_disable_locked -> mlx5_eswitch_event_handler_unregister
> -> flush_workqueue.  

This is quite an ugly hack, is there no way to avoid the flush and let 
the work discover that what it was supposed to do is no longer needed?

>  	devlink = priv_to_devlink(esw->dev);
> -	devl_lock(devlink);
> +	/* Repeatedly try to grab the lock with a delay while this work is
> +	 * still relevant.
> +	 * This allows a concurrent mlx5_eswitch_event_handler_unregister
> +	 * (holding the devlink lock) to flush the wq without deadlocking.
> +	 */
> +	while (!devl_trylock(devlink)) {
> +		if (!esw->esw_funcs.notifier_enabled)

Technically READ_ONCE/WRITE_ONCE is required on this.

> +			return;
> +		schedule_timeout_interruptible(msecs_to_jiffies(10));

Why _interruptible(), you're not handling the return value.
If somehow this thread gets a signal pending we'll turn this
loop into a busy poll which doesn't seem ideal?

Ima take this patch out of the series and apply the rest.