Commit 50417d22 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Al Viro
Browse files

fs/dcache: Move wakeup out of i_seq_dir write held region. 



__d_add() and __d_move() wake up waiters on dentry::d_wait from within
the i_seq_dir write held region.  This violates the PREEMPT_RT
constraints as the wake up acquires wait_queue_head::lock which is a
"sleeping" spinlock on RT.

There is no requirement to do so. __d_lookup_unhash() has cleared
DCACHE_PAR_LOOKUP and dentry::d_wait and returned the now unreachable wait
queue head pointer to the caller, so the actual wake up can be postponed
until the i_dir_seq write side critical section is left. The only
requirement is that dentry::lock is held across the whole sequence
including the wake up. The previous commit includes an analysis why this
is considered safe.

Move the wake up past end_dir_add() which leaves the i_dir_seq write side
critical section and enables preemption.

For non RT kernels there is no difference because preemption is still
disabled due to dentry::lock being held, but it shortens the time between
wake up and unlocking dentry::lock, which reduces the contention for the
woken up waiter.

Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 45f78b0a

fs/dcache.c

+5 −5
Original line number Diff line number Diff line
@@ -2581,11 +2581,13 @@ static inline unsigned start_dir_add(struct inode *dir) 
	}

}



static inline void end_dir_add(struct inode *dir, unsigned n)

static inline void end_dir_add(struct inode *dir, unsigned int n,

			       wait_queue_head_t *d_wait)

{

	smp_store_release(&dir->i_dir_seq, n + 2);

	if (IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_enable();

	wake_up_all(d_wait);

}



static void d_wait_lookup(struct dentry *dentry)
@@ -2756,7 +2758,6 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode) 
		dir = dentry->d_parent->d_inode;

		n = start_dir_add(dir);

		d_wait = __d_lookup_unhash(dentry);

		wake_up_all(d_wait);

	}

	if (inode) {

		unsigned add_flags = d_flags_for_inode(inode);
@@ -2768,7 +2769,7 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode) 
	}

	__d_rehash(dentry);

	if (dir)

		end_dir_add(dir, n);

		end_dir_add(dir, n, d_wait);

	spin_unlock(&dentry->d_lock);

	if (inode)

		spin_unlock(&inode->i_lock);
@@ -2947,7 +2948,6 @@ static void __d_move(struct dentry *dentry, struct dentry *target, 
		dir = target->d_parent->d_inode;

		n = start_dir_add(dir);

		d_wait = __d_lookup_unhash(target);

		wake_up_all(d_wait);

	}



	write_seqcount_begin(&dentry->d_seq);
@@ -2983,7 +2983,7 @@ static void __d_move(struct dentry *dentry, struct dentry *target, 
	write_seqcount_end(&dentry->d_seq);



	if (dir)

		end_dir_add(dir, n);

		end_dir_add(dir, n, d_wait);



	if (dentry->d_parent != old_parent)

		spin_unlock(&dentry->d_parent->d_lock);