Re: [PATCH] counter: drop chrdev_lock
From: David Lechner
Date: Tue Oct 19 2021 - 10:44:09 EST
On 10/19/21 2:18 AM, William Breathitt Gray wrote:
On Tue, Oct 19, 2021 at 09:07:48AM +0200, Greg KH wrote:
On Tue, Oct 19, 2021 at 03:53:08PM +0900, William Breathitt Gray wrote:
On Mon, Oct 18, 2021 at 11:03:49AM -0500, David Lechner wrote:
On 10/18/21 4:14 AM, William Breathitt Gray wrote:
On Sun, Oct 17, 2021 at 01:55:21PM -0500, David Lechner wrote:
diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
index 1ccd771da25f..7bf8882ff54d 100644
--- a/drivers/counter/counter-sysfs.c
+++ b/drivers/counter/counter-sysfs.c
@@ -796,25 +796,18 @@ static int counter_events_queue_size_write(struct counter_device *counter,
u64 val)
{
DECLARE_KFIFO_PTR(events, struct counter_event);
- int err = 0;
-
- /* Ensure chrdev is not opened more than 1 at a time */
- if (!atomic_add_unless(&counter->chrdev_lock, 1, 1))
- return -EBUSY;
+ int err;
/* Allocate new events queue */
err = kfifo_alloc(&events, val, GFP_KERNEL);
if (err)
- goto exit_early;
+ return err;
/* Swap in new events queue */
kfifo_free(&counter->events);
counter->events.kfifo = events.kfifo;
Do we need to hold the events_lock mutex here for this swap in case
counter_chrdev_read() is in the middle of reading the kfifo to
userspace, or do the kfifo macros already protect us from a race
condition here?
Another possibility might be to disallow changing the size while
events are enabled. Otherwise, we also need to protect against
write after free.
I considered this:
swap(counter->events.kfifo, events.kfifo);
kfifo_free(&events);
But I'm not sure that would be safe enough.
I think it depends on whether it's safe to call kfifo_free() while other
kfifo_*() calls are executing. I suspect it is not safe because I don't
think kfifo_free() waits until all kfifo read/write operations are
finished before freeing -- but if I'm wrong here please let me know.
Because of that, will need to hold the counter->events_lock afterall so
that we don't modify the events fifo while a kfifo read/write is going
on, lest we suffer an address fault. This can happen regardless of
whether you swap before or after the kfifo_free() because the old fifo
address could still be in use within those uncompleted kfifo_*() calls
if they were called before the swap but don't complete before the
kfifo_free().
So we have a problem now that I think you have already noticed: the
kfifo_in() call in counter_push_events() also needs protection, but it's
executing within an interrupt context so we can't try to lock a mutex
lest we end up sleeping.
One option we have is as you suggested: we disallow changing size while
events are enabled. However, that will require us to keep track of when
events are disabled and implement a spinlock to ensure that we don't
disable events in the middle of a kfifo_in().
Alternatively, we could change events_lock to a spinlock and use it to
protect all these operations on the counter->events fifo. Would this
alternative be a better option so that we avoid creating another
separate lock?
I would recommend just having a single lock here if at all possible,
until you determine that there a performance problem that can be
measured that would require it to be split up.
thanks,
greg k-h
All right let's go with a single events_lock spinlock then. David, if
you make those changes and submit a v2, I'll be okay with this patch and
can provide my ack for it.
We can't use a spin lock for everything since there are operations
that can sleep that need to be in the critical sections. Likewise,
we can't use a mutex for everything since some critical sections
are in interrupt handlers. But, I suppose we can try combining
the existing mutexes. Since the kfifo is accessed from both
contexts, it seems like it still needs more consideration than
just a mutex or a spin lock, e.g. if events are enabled, don't
allow swapping out the kfifo buffer.