[PATCH] RFC: mmc: block: replace semaphore with freezing

Linus Walleij
2016-11-16T10:51:22Z

The MMC/SD block core request processing thread is taking a
semaphore in the request processing section and the same
semaphore is taken around suspend/resume operations.

The purpose of the semaphore is to block any request from being
issued to the MMC/SD host controllers when the system is
suspended. A semaphore is used in place of a mutex since the
calls are coming from different threads.

This construction predates the kernel thread freezer mechanism:
we can easily replace the semaphore by instead activating the
freezer with set_freezable() and call try_to_freeze() instead
of the up(); schedule(); down(); construction that is devised
to let the suspend/resume calls get in and grab the semaphore.

Tested with a few suspend/resume to memory cycles on the Ux500
when doing intense dd operations in the background: the
thread thaws and resumes operation after resume.

Cc: linux-pm@vger.kernel.org
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
I haven't seen any need to preserve the call to schedule()
in the request processing thread, but I want advice on whether
that has a point. I would guess the thread will just anyway
be preempted if needed anyway as it is marked interruptible?
---
drivers/mmc/card/queue.c | 13 ++-----------
drivers/mmc/card/queue.h | 1 -
2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/drivers/mmc/card/queue.c b/drivers/mmc/card/queue.c
index 8037f73a109a..09a932ffe46e 100644
--- a/drivers/mmc/card/queue.c
+++ b/drivers/mmc/card/queue.c
@@ -55,8 +55,8 @@ static int mmc_queue_thread(void *d)
struct request_queue *q = mq->queue;

current->flags |= PF_MEMALLOC;
+ set_freezable();

- down(&mq->thread_sem);
do {
struct request *req = NULL;

@@ -95,12 +95,9 @@ static int mmc_queue_thread(void *d)
set_current_state(TASK_RUNNING);
break;
}
- up(&mq->thread_sem);
- schedule();
- down(&mq->thread_sem);
+ try_to_freeze();
}
} while (1);
- up(&mq->thread_sem);

return 0;
}
@@ -289,8 +286,6 @@ int mmc_init_queue(struct mmc_queue *mq, struct mmc_card *card,
goto cleanup_queue;
}

- sema_init(&mq->thread_sem, 1);
-
mq->thread = kthread_run(mmc_queue_thread, mq, "mmcqd/%d%s",
host->index, subname ? subname : "");

@@ -424,8 +419,6 @@ void mmc_queue_suspend(struct mmc_queue *mq)
spin_lock_irqsave(q->queue_lock, flags);
blk_stop_queue(q);
spin_unlock_irqrestore(q->queue_lock, flags);
-
- down(&mq->thread_sem);
}
}

@@ -441,8 +434,6 @@ void mmc_queue_resume(struct mmc_queue *mq)
if (mq->flags & MMC_QUEUE_SUSPENDED) {
mq->flags &= ~MMC_QUEUE_SUSPENDED;

- up(&mq->thread_sem);
-
spin_lock_irqsave(q->queue_lock, flags);
blk_start_queue(q);
spin_unlock_irqrestore(q->queue_lock, flags);
diff --git a/drivers/mmc/card/queue.h b/drivers/mmc/card/queue.h
index 3c15a75bae86..fe10f94795de 100644
--- a/drivers/mmc/card/queue.h
+++ b/drivers/mmc/card/queue.h
@@ -53,7 +53,6 @@ struct mmc_queue_req {
struct mmc_queue {
struct mmc_card *card;
struct task_struct *thread;
- struct semaphore thread_sem;
unsigned int flags;
#define MMC_QUEUE_SUSPENDED (1 << 0)
#define MMC_QUEUE_NEW_REQUEST (1 << 1)
--
2.7.4