dcache: let the dentry count go down to zero without taking d_lock (360f5479) · Commits · EulixOS / Software / Kernel

fs/dcache.c

+114 −4

Original line number	Diff line number	Diff line
		@@ -508,7 +508,7 @@ static void __dentry_kill(struct dentry *dentry)
		* dentry_iput drops the locks, at which point nobody (except
		* transient RCU lookups) can reach this dentry.
		*/
		BUG_ON((int)dentry->d_lockref.count > 0);
		BUG_ON(dentry->d_lockref.count > 0);
		this_cpu_dec(nr_dentry);
		if (dentry->d_op && dentry->d_op->d_release)
		dentry->d_op->d_release(dentry);
		@@ -561,7 +561,7 @@ static inline struct dentry lock_parent(struct dentry dentry)
		struct dentry *parent = dentry->d_parent;
		if (IS_ROOT(dentry))
		return NULL;
		if (unlikely((int)dentry->d_lockref.count < 0))
		if (unlikely(dentry->d_lockref.count < 0))
		return NULL;
		if (likely(spin_trylock(&parent->d_lock)))
		return parent;
		@@ -590,6 +590,110 @@ static inline struct dentry lock_parent(struct dentry dentry)
		return parent;
		}

		/*
		* Try to do a lockless dput(), and return whether that was successful.
		*
		* If unsuccessful, we return false, having already taken the dentry lock.
		*
		* The caller needs to hold the RCU read lock, so that the dentry is
		* guaranteed to stay around even if the refcount goes down to zero!
		*/
		static inline bool fast_dput(struct dentry *dentry)
		{
		int ret;
		unsigned int d_flags;

		/*
		* If we have a d_op->d_delete() operation, we sould not
		* let the dentry count go to zero, so use "put__or_lock".
		*/
		if (unlikely(dentry->d_flags & DCACHE_OP_DELETE))
		return lockref_put_or_lock(&dentry->d_lockref);

		/*
		* .. otherwise, we can try to just decrement the
		* lockref optimistically.
		*/
		ret = lockref_put_return(&dentry->d_lockref);

		/*
		* If the lockref_put_return() failed due to the lock being held
		* by somebody else, the fast path has failed. We will need to
		* get the lock, and then check the count again.
		*/
		if (unlikely(ret < 0)) {
		spin_lock(&dentry->d_lock);
		if (dentry->d_lockref.count > 1) {
		dentry->d_lockref.count--;
		spin_unlock(&dentry->d_lock);
		return 1;
		}
		return 0;
		}

		/*
		* If we weren't the last ref, we're done.
		*/
		if (ret)
		return 1;

		/*
		* Careful, careful. The reference count went down
		* to zero, but we don't hold the dentry lock, so
		* somebody else could get it again, and do another
		* dput(), and we need to not race with that.
		*
		* However, there is a very special and common case
		* where we don't care, because there is nothing to
		* do: the dentry is still hashed, it does not have
		* a 'delete' op, and it's referenced and already on
		* the LRU list.
		*
		* NOTE! Since we aren't locked, these values are
		* not "stable". However, it is sufficient that at
		* some point after we dropped the reference the
		* dentry was hashed and the flags had the proper
		* value. Other dentry users may have re-gotten
		* a reference to the dentry and change that, but
		* our work is done - we can leave the dentry
		* around with a zero refcount.
		*/
		smp_rmb();
		d_flags = ACCESS_ONCE(dentry->d_flags);
		d_flags &= DCACHE_REFERENCED \| DCACHE_LRU_LIST;

		/* Nothing to do? Dropping the reference was all we needed? */
		if (d_flags == (DCACHE_REFERENCED \| DCACHE_LRU_LIST) && !d_unhashed(dentry))
		return 1;

		/*
		* Not the fast normal case? Get the lock. We've already decremented
		* the refcount, but we'll need to re-check the situation after
		* getting the lock.
		*/
		spin_lock(&dentry->d_lock);

		/*
		* Did somebody else grab a reference to it in the meantime, and
		* we're no longer the last user after all? Alternatively, somebody
		* else could have killed it and marked it dead. Either way, we
		* don't need to do anything else.
		*/
		if (dentry->d_lockref.count) {
		spin_unlock(&dentry->d_lock);
		return 1;
		}

		/*
		* Re-get the reference we optimistically dropped. We hold the
		* lock, and we just tested that it was zero, so we can just
		* set it to 1.
		*/
		dentry->d_lockref.count = 1;
		return 0;
		}


		/*
		* This is dput
		*
		@@ -622,8 +726,14 @@ void dput(struct dentry *dentry)
		return;

		repeat:
		if (lockref_put_or_lock(&dentry->d_lockref))
		rcu_read_lock();
		if (likely(fast_dput(dentry))) {
		rcu_read_unlock();
		return;
		}

		/* Slow case: now with the dentry lock held */
		rcu_read_unlock();

		/* Unreachable? Get rid of it */
		if (unlikely(d_unhashed(dentry)))
		@@ -810,7 +920,7 @@ static void shrink_dentry_list(struct list_head *list)
		* We found an inuse dentry which was not removed from
		* the LRU because of laziness during lookup. Do not free it.
		*/
		if ((int)dentry->d_lockref.count > 0) {
		if (dentry->d_lockref.count > 0) {
		spin_unlock(&dentry->d_lock);
		if (parent)
		spin_unlock(&parent->d_lock);

include/linux/lockref.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -28,12 +28,13 @@ struct lockref {
		#endif
		struct {
		spinlock_t lock;
		unsigned int count;
		int count;
		};
		};
		};

		extern void lockref_get(struct lockref *);
		extern int lockref_put_return(struct lockref *);
		extern int lockref_get_not_zero(struct lockref *);
		extern int lockref_get_or_lock(struct lockref *);
		extern int lockref_put_or_lock(struct lockref *);

lib/lockref.c

+28 −8

Original line number	Diff line number	Diff line
		@@ -60,7 +60,7 @@ void lockref_get(struct lockref *lockref)
		EXPORT_SYMBOL(lockref_get);

		/**
		* lockref_get_not_zero - Increments count unless the count is 0
		* lockref_get_not_zero - Increments count unless the count is 0 or dead
		* @lockref: pointer to lockref structure
		* Return: 1 if count updated successfully or 0 if count was zero
		*/
		@@ -70,7 +70,7 @@ int lockref_get_not_zero(struct lockref *lockref)

		CMPXCHG_LOOP(
		new.count++;
		if (!old.count)
		if (old.count <= 0)
		return 0;
		,
		return 1;
		@@ -78,7 +78,7 @@ int lockref_get_not_zero(struct lockref *lockref)

		spin_lock(&lockref->lock);
		retval = 0;
		if (lockref->count) {
		if (lockref->count > 0) {
		lockref->count++;
		retval = 1;
		}
		@@ -88,7 +88,7 @@ int lockref_get_not_zero(struct lockref *lockref)
		EXPORT_SYMBOL(lockref_get_not_zero);

		/**
		* lockref_get_or_lock - Increments count unless the count is 0
		* lockref_get_or_lock - Increments count unless the count is 0 or dead
		* @lockref: pointer to lockref structure
		* Return: 1 if count updated successfully or 0 if count was zero
		* and we got the lock instead.
		@@ -97,14 +97,14 @@ int lockref_get_or_lock(struct lockref *lockref)
		{
		CMPXCHG_LOOP(
		new.count++;
		if (!old.count)
		if (old.count <= 0)
		break;
		,
		return 1;
		);

		spin_lock(&lockref->lock);
		if (!lockref->count)
		if (lockref->count <= 0)
		return 0;
		lockref->count++;
		spin_unlock(&lockref->lock);
		@@ -112,6 +112,26 @@ int lockref_get_or_lock(struct lockref *lockref)
		}
		EXPORT_SYMBOL(lockref_get_or_lock);

		/**
		* lockref_put_return - Decrement reference count if possible
		* @lockref: pointer to lockref structure
		*
		* Decrement the reference count and return the new value.
		* If the lockref was dead or locked, return an error.
		*/
		int lockref_put_return(struct lockref *lockref)
		{
		CMPXCHG_LOOP(
		new.count--;
		if (old.count <= 0)
		return -1;
		,
		return new.count;
		);
		return -1;
		}
		EXPORT_SYMBOL(lockref_put_return);

		/**
		* lockref_put_or_lock - decrements count unless count <= 1 before decrement
		* @lockref: pointer to lockref structure
		@@ -158,7 +178,7 @@ int lockref_get_not_dead(struct lockref *lockref)

		CMPXCHG_LOOP(
		new.count++;
		if ((int)old.count < 0)
		if (old.count < 0)
		return 0;
		,
		return 1;
		@@ -166,7 +186,7 @@ int lockref_get_not_dead(struct lockref *lockref)

		spin_lock(&lockref->lock);
		retval = 0;
		if ((int) lockref->count >= 0) {
		if (lockref->count >= 0) {
		lockref->count++;
		retval = 1;
		}