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
