linux-kernel - Re: [PATCH resend] staging: vc04_services: replace g_free_fragments

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Date:   Sat, 2 Nov 2019 11:33:01 +0100
From:   Stefan Wahren <wahrenst@....net>
To:     Davidlohr Bueso <dave@...olabs.net>, eric@...olt.net
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org
Subject: Re: [PATCH resend] staging: vc04_services: replace
 g_free_fragments_mutex with spinlock

Hi Davidlohr,

Am 01.11.19 um 19:29 schrieb Davidlohr Bueso:
> There is no need to be using a semaphore, or a sleeping lock
> in the first place: critical region is extremely short, does not
> call into any blocking calls and furthermore lock and unlocking
> operations occur in the same context.
>
> Get rid of another semaphore user by replacing it with a spinlock.
>
> Signed-off-by: Davidlohr Bueso <dave@...olabs.net>
> ---
> This is in an effort to further reduce semaphore users in the kernel.
>
> This is a resend, which just seems simpler given the confusions.
>
>  .../staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
> index 8dc730cfe7a6..710d21654128 100644
> --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
> +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
> @@ -63,7 +63,7 @@ static char *g_free_fragments;
>  static struct semaphore g_free_fragments_sema;
>  static struct device *g_dev;
>
> -static DEFINE_SEMAPHORE(g_free_fragments_mutex);
> +static DEFINE_SPINLOCK(g_free_fragments_lock);
>
>  static irqreturn_t
>  vchiq_doorbell_irq(int irq, void *dev_id);
> @@ -528,11 +528,11 @@ create_pagelist(char __user *buf, size_t count, unsigned short type)
>
>  		WARN_ON(g_free_fragments == NULL);
>
> -		down(&g_free_fragments_mutex);
> +		spin_lock(&g_free_fragments_lock);
>  		fragments = g_free_fragments;
>  		WARN_ON(fragments == NULL);
>  		g_free_fragments = *(char **) g_free_fragments;
> -		up(&g_free_fragments_mutex);
> +		spin_unlock(&g_free_fragments_lock);
>

the reason why Greg cannot apply this patch is that you are using an old
or the wrong git tree.

Please make sure you use the following one (or a mirror):

https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git

and operate on the branch staging-next

Please fix this up and send a new version.

Regards
Stefan