linux-kernel - Re: [PATCH] PCI/P2PDMA: Save a few cycles in 'pci_alloc

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Date:   Wed, 15 Dec 2021 15:58:04 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] PCI/P2PDMA: Save a few cycles in 'pci_alloc_p2pmem()'

On Wed, Nov 03, 2021 at 10:16:53PM +0100, Christophe JAILLET wrote:
> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> save a few cycles when it is known that the rcu lock is already
> taken/released.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>

Applied to pci/p2pdma for v5.17, thanks!

  PCI/P2PDMA: Use percpu_ref_tryget_live_rcu() inside RCU critical section

  Since pci_alloc_p2pmem() has already called rcu_read_lock(), we're in an
  RCU read-side critical section and don't need to take the lock again.  Use
  percpu_ref_tryget_live_rcu() instead of percpu_ref_tryget_live() to save a
  few cycles.

> ---
>  drivers/pci/p2pdma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 8d47cb7218d1..081c391690d4 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -710,7 +710,7 @@ void *pci_alloc_p2pmem(struct pci_dev *pdev, size_t size)
>  	if (!ret)
>  		goto out;
>  
> -	if (unlikely(!percpu_ref_tryget_live(ref))) {
> +	if (unlikely(!percpu_ref_tryget_live_rcu(ref))) {
>  		gen_pool_free(p2pdma->pool, (unsigned long) ret, size);
>  		ret = NULL;
>  		goto out;
> -- 
> 2.30.2
>