Re: [PATCH] soc: fsl: qbman: fix race condition in qman_destroy_fq

[Richard GENOUD <richard.genoud@bootlin.com>]

Le 23/12/2025 à 08:25, Richard Genoud a écrit :
> When QMAN_FQ_FLAG_DYNAMIC_FQID is set, there's a race condition between
> fq_table[fq->idx] state and freeing/allocating from the pool and
> WARN_ON(fq_table[fq->idx]) in qman_create_fq() gets triggered.
> 
> Indeed, we can have:
>           Thread A                             Thread B
>      qman_destroy_fq()                    qman_create_fq()
>        qman_release_fqid()
>          qman_shutdown_fq()
>          gen_pool_free()
>             -- At this point, the fqid is available again --
>                                             qman_alloc_fqid()
>             -- so, we can get the just-freed fqid in thread B --
>                                             fq->fqid = fqid;
>                                             fq->idx = fqid * 2;
>                                             WARN_ON(fq_table[fq->idx]);
>                                             fq_table[fq->idx] = fq;
>       fq_table[fq->idx] = NULL;
> 
> And adding some logs between qman_release_fqid() and
> fq_table[fq->idx] = NULL makes the WARN_ON() trigger a lot more.
> 
> To prevent that, ensure that fq_table[fq->idx] is set to NULL before
> gen_pool_free() is called by using smp_wmb().
> 

Tested on a LS1046A based board.
With this patch, the warning is not triggered anymore.

Tested-by: CHAMPSEIX Thomas <thomas.champseix@alstomgroup.com>

> Fixes: c535e923bb97 ("soc/fsl: Introduce DPAA 1.x QMan device driver")
> Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
> ---
>   drivers/soc/fsl/qbman/qman.c | 24 ++++++++++++++++++++++--
>   1 file changed, 22 insertions(+), 2 deletions(-)
> 
> NB: I'm not 100% sure of the need of a barrier here, since even without
> it, the WARN_ON() wasn't triggered any more.
> 
> diff --git a/drivers/soc/fsl/qbman/qman.c b/drivers/soc/fsl/qbman/qman.c
> index 6b392b3ad4b1..39a3e7aab6ff 100644
> --- a/drivers/soc/fsl/qbman/qman.c
> +++ b/drivers/soc/fsl/qbman/qman.c
> @@ -1827,6 +1827,8 @@ EXPORT_SYMBOL(qman_create_fq);
>   
>   void qman_destroy_fq(struct qman_fq *fq)
>   {
> +	int leaked;
> +
>   	/*
>   	 * We don't need to lock the FQ as it is a pre-condition that the FQ be
>   	 * quiesced. Instead, run some checks.
> @@ -1834,11 +1836,29 @@ void qman_destroy_fq(struct qman_fq *fq)
>   	switch (fq->state) {
>   	case qman_fq_state_parked:
>   	case qman_fq_state_oos:
> -		if (fq_isset(fq, QMAN_FQ_FLAG_DYNAMIC_FQID))
> -			qman_release_fqid(fq->fqid);
> +		/*
> +		 * There's a race condition here on releasing the fqid,
> +		 * setting the fq_table to NULL, and freeing the fqid.
> +		 * To prevent it, this order should be respected:
> +		 */
> +		if (fq_isset(fq, QMAN_FQ_FLAG_DYNAMIC_FQID)) {
> +			leaked = qman_shutdown_fq(fq->fqid);
> +			if (leaked)
> +				pr_debug("FQID %d leaked\n", fq->fqid);
> +		}
>   
>   		DPAA_ASSERT(fq_table[fq->idx]);
>   		fq_table[fq->idx] = NULL;
> +
> +		if (fq_isset(fq, QMAN_FQ_FLAG_DYNAMIC_FQID) && !leaked) {
> +			/*
> +			 * fq_table[fq->idx] should be set to null before
> +			 * freeing fq->fqid otherwise it could by allocated by
> +			 * qman_alloc_fqid() while still being !NULL
> +			 */
> +			smp_wmb();
> +			gen_pool_free(qm_fqalloc, fq->fqid | DPAA_GENALLOC_OFF, 1);
> +		}
>   		return;
>   	default:
>   		break;
> 
> base-commit: 9448598b22c50c8a5bb77a9103e2d49f134c9578


-- 
Richard Genoud, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com