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SubjectRe: [PATCH] staging: vc04_services: replace g_free_fragments_mutex with spinlock
Cc devel@driverdev.osuosl.org

On Sun, 27 Oct 2019, Bueso wrote:

>There seems 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@stgolabs.net>
>---
>This is in an effort to further reduce semaphore users in the kernel.
>
> .../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);
> pagelist->type = PAGELIST_READ_WITH_FRAGMENTS +
> (fragments - g_fragments_base) / g_fragments_size;
> }
>@@ -591,10 +591,10 @@ free_pagelist(struct vchiq_pagelist_info *pagelistinfo,
> kunmap(pages[num_pages - 1]);
> }
>
>- down(&g_free_fragments_mutex);
>+ spin_lock(&g_free_fragments_lock);
> *(char **)fragments = g_free_fragments;
> g_free_fragments = fragments;
>- up(&g_free_fragments_mutex);
>+ spin_unlock(&g_free_fragments_lock);
> up(&g_free_fragments_sema);
> }
>
>--
>2.16.4
>

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