On 2017-04-25 16:16, Peter Rosin wrote:
> On 2017-04-24 16:59, Philipp Zabel wrote:
>> On Mon, 2017-04-24 at 16:36 +0200, Peter Rosin wrote:
>> [...]
>>>> How about an atomic use_count on the mux_control, a bool shared that is
>>>> only set by the first consumer, and controls whether selecting locks?
>>>
>>> That has the drawback that it is hard to restore the mux-control in a safe
>>> way so that exclusive consumers are allowed after the last shared consumer
>>> puts the mux away.
>>
>> True.
>>
>>> Agreed, it's a corner case, but I had this very similar
>>> patch going through the compiler when I got this mail. Does it work as well
>>> as what you suggested?
>>
>> Yes, this patch works just as well.
>
> Right, as expected :-) However, I don't like it much. It divides the mux
> consumers into two camps in a way that makes it difficult to select which
> camp a consumer should be in.
>
> E.g. consider the iio-mux. The current implementation only supports quick
> accesses that fit the mux_control_get_shared case. But if that mux in the
> future needs to grow continuous buffered accesses, I think there will be
> pressure to switch it over to the exclusive mode. Because that is a lot
> closer to what you are doing with the video-mux. And then what? It will be
> impossible to predict if the end user is going to use buffered accesses or
> not...
>
> So, I think the best approach is to skip the distinction between shared
> and exclusive consumers and instead implement the locking with an ordinary
> semaphore (instead of the old rwsem or the current mutex). Semaphores don't
> have the property that the same task should down/up them (mutexes require
> that for lock/unlock, and is also the reason for the lockdep complaint) and
> thus fits better for long-time use such as yours or the above iio-mux with
> buffered accesses. It should also hopefully be cheaper that an rwsem, and
> not have any downgrade_write calls thus possibly keeping Greg sufficiently
> happy...
>
> Sure, consumers can still dig themselves into a hole by not calling deselect
> as they should, but at least I think it can be made to work w/o dividing the
> consumers...
Like this (only compile-tested). Philipp, it should work the same as with
the rwsem in v13 and earlier. At least for your case...
Cheers,
peda
diff --git a/drivers/mux/mux-core.c b/drivers/mux/mux-core.c
index c02fa4dd2d09..f99b70d4e319 100644
--- a/drivers/mux/mux-core.c
+++ b/drivers/mux/mux-core.c
@@ -116,7 +116,7 @@ struct mux_chip *mux_chip_alloc(struct device *dev,
struct mux_control *mux = &mux_chip->mux[i];
mux->chip = mux_chip;
- mutex_init(&mux->lock);
+ sema_init(&mux->lock, 1);
mux->cached_state = MUX_CACHE_UNKNOWN;
mux->idle_state = MUX_IDLE_AS_IS;
}
@@ -372,12 +372,14 @@ int mux_control_select(struct mux_control *mux, unsigned int state)
{
int ret;
- mutex_lock(&mux->lock);
+ ret = down_killable(&mux->lock);
+ if (ret < 0)
+ return ret;
ret = __mux_control_select(mux, state);
if (ret < 0)
- mutex_unlock(&mux->lock);
+ up(&mux->lock);
return ret;
}
@@ -399,13 +401,13 @@ int mux_control_try_select(struct mux_control *mux, unsigned int state)
{
int ret;
- if (!mutex_trylock(&mux->lock))
+ if (down_trylock(&mux->lock))
return -EBUSY;
ret = __mux_control_select(mux, state);
if (ret < 0)
- mutex_unlock(&mux->lock);
+ up(&mux->lock);
return ret;
}
@@ -427,7 +429,7 @@ int mux_control_deselect(struct mux_control *mux)
mux->idle_state != mux->cached_state)
ret = mux_control_set(mux, mux->idle_state);
- mutex_unlock(&mux->lock);
+ up(&mux->lock);
return ret;
}
diff --git a/include/linux/mux/driver.h b/include/linux/mux/driver.h
index 95269f40670a..43f65f80c275 100644
--- a/include/linux/mux/driver.h
+++ b/include/linux/mux/driver.h
@@ -15,7 +15,6 @@
#include <dt-bindings/mux/mux.h>
#include <linux/device.h>
-#include <linux/mutex.h>
#include <linux/semaphore.h>
struct mux_chip;
@@ -44,7 +43,7 @@ struct mux_control_ops {
* mux drivers.
*/
struct mux_control {
- struct mutex lock; /* protects the state of the mux */
+ struct semaphore lock; /* protects the state of the mux */
struct mux_chip *chip;
int cached_state;
--
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