Re: [PATCH v15 10/23] locking/refcount, kref: Add kref_put_ww_mutex()

[Dmitry Osipenko]

On 8/28/23 12:26, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:36 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> 
>> Introduce kref_put_ww_mutex() helper that will handle the wait-wound
>> mutex auto-locking on kref_put(). This helper is wanted by DRM drivers
>> that extensively use dma-reservation locking which in turns uses ww-mutex.
>>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>> ---
>>  include/linux/kref.h     | 12 ++++++++++++
>>  include/linux/refcount.h |  5 +++++
>>  lib/refcount.c           | 34 ++++++++++++++++++++++++++++++++++
>>  3 files changed, 51 insertions(+)
>>
>> diff --git a/include/linux/kref.h b/include/linux/kref.h
>> index d32e21a2538c..b2d8dc6e9ae0 100644
>> --- a/include/linux/kref.h
>> +++ b/include/linux/kref.h
>> @@ -90,6 +90,18 @@ static inline int kref_put_lock(struct kref *kref,
>>  	return 0;
>>  }
>>  
>> +static inline int kref_put_ww_mutex(struct kref *kref,
>> +				    void (*release)(struct kref *kref),
>> +				    struct ww_mutex *lock,
>> +				    struct ww_acquire_ctx *ctx)
>> +{
>> +	if (refcount_dec_and_ww_mutex_lock(&kref->refcount, lock, ctx)) {
>> +		release(kref);
>> +		return 1;
>> +	}
>> +	return 0;
>> +}
>> +
>>  /**
>>   * kref_get_unless_zero - Increment refcount for object unless it is zero.
>>   * @kref: object.
>> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
>> index a62fcca97486..be9ad272bc77 100644
>> --- a/include/linux/refcount.h
>> +++ b/include/linux/refcount.h
>> @@ -99,6 +99,8 @@
>>  #include <linux/spinlock_types.h>
>>  
>>  struct mutex;
>> +struct ww_mutex;
>> +struct ww_acquire_ctx;
>>  
>>  /**
>>   * typedef refcount_t - variant of atomic_t specialized for reference counts
>> @@ -366,4 +368,7 @@ extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
>>  extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
>>  						       spinlock_t *lock,
>>  						       unsigned long *flags) __cond_acquires(lock);
>> +extern __must_check bool refcount_dec_and_ww_mutex_lock(refcount_t *r,
>> +							struct ww_mutex *lock,
>> +							struct ww_acquire_ctx *ctx) __cond_acquires(&lock->base);
>>  #endif /* _LINUX_REFCOUNT_H */
>> diff --git a/lib/refcount.c b/lib/refcount.c
>> index a207a8f22b3c..3f6fd0ceed02 100644
>> --- a/lib/refcount.c
>> +++ b/lib/refcount.c
>> @@ -6,6 +6,7 @@
>>  #include <linux/mutex.h>
>>  #include <linux/refcount.h>
>>  #include <linux/spinlock.h>
>> +#include <linux/ww_mutex.h>
>>  #include <linux/bug.h>
>>  
>>  #define REFCOUNT_WARN(str)	WARN_ONCE(1, "refcount_t: " str ".\n")
>> @@ -184,3 +185,36 @@ bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock,
>>  	return true;
>>  }
>>  EXPORT_SYMBOL(refcount_dec_and_lock_irqsave);
>> +
>> +/**
>> + * refcount_dec_and_ww_mutex_lock - return holding ww-mutex if able to
>> + *                                  decrement refcount to 0
>> + * @r: the refcount
>> + * @lock: the ww-mutex to be locked
>> + * @ctx: wait-wound context
>> + *
>> + * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
>> + * decrement when saturated at REFCOUNT_SATURATED.
>> + *
>> + * Provides release memory ordering, such that prior loads and stores are done
>> + * before, and provides a control dependency such that free() must come after.
>> + * See the comment on top.
>> + *
>> + * Return: true and hold ww-mutex lock if able to decrement refcount to 0,
>> + *         false otherwise
>> + */
>> +bool refcount_dec_and_ww_mutex_lock(refcount_t *r, struct ww_mutex *lock,
>> +				    struct ww_acquire_ctx *ctx)
>> +{
>> +	if (refcount_dec_not_one(r))
>> +		return false;
>> +
>> +	ww_mutex_lock(lock, ctx);
> 
> Unless I'm wrong, ww_mutex_lock() can return -EDEADLK when ctx !=
> NULL, in which case, the lock is not held when it returns. Question is,
> do we really have a use case for ctx != NULL in that kref_put_ww_mutex()
> path. If we need to acquire other ww_locks, this lock, and the other
> locks should have been acquired beforehand, and we can simply call
> kref_put() when we want to release the ref on the resource.

Right, I completely forgot about the deadlocking

-- 
Best regards,
Dmitry