| Message ID | 1371482036-15958-8-git-send-email-jlayton@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, 17 Jun 2013 11:13:50 -0400 Jeff Layton <jlayton@redhat.com> wrote: > Since we always hold the i_lock when inserting a new waiter onto the > fl_block list, we can avoid taking the global lock at all if we find > that it's empty when we go to wake up blocked waiters. > > Signed-off-by: Jeff Layton <jlayton@redhat.com> > --- > fs/locks.c | 17 ++++++++++++++--- > 1 files changed, 14 insertions(+), 3 deletions(-) > > diff --git a/fs/locks.c b/fs/locks.c > index 8f56651..a8f3b33 100644 > --- a/fs/locks.c > +++ b/fs/locks.c > @@ -532,7 +532,10 @@ static void locks_delete_block(struct file_lock *waiter) > * the order they blocked. The documentation doesn't require this but > * it seems like the reasonable thing to do. > * > - * Must be called with file_lock_lock held! > + * Must be called with both the i_lock and file_lock_lock held. The fl_block > + * list itself is protected by the file_lock_list, but by ensuring that the > + * i_lock is also held on insertions we can avoid taking the file_lock_lock > + * in some cases when we see that the fl_block list is empty. > */ > static void __locks_insert_block(struct file_lock *blocker, > struct file_lock *waiter) > @@ -560,8 +563,16 @@ static void locks_insert_block(struct file_lock *blocker, > */ > static void locks_wake_up_blocks(struct file_lock *blocker) > { > + /* > + * Avoid taking global lock if list is empty. This is safe since new > + * blocked requests are only added to the list under the i_lock, and > + * the i_lock is always held here. > + */ > + if (list_empty(&blocker->fl_block)) > + return; > + Ok, potential race here. We hold the i_lock when we check list_empty() above, but it's possible for the fl_block list to become empty between that check and when we take the spinlock below. locks_delete_block does not require that you hold the i_lock, and some callers don't hold it. This is trivially fixable by just keeping this as a while() loop. We'll do the list_empty() check twice in that case, but that shouldn't change the performance here much. I'll fix that in my tree and it'll be in the next resend. Sorry for the noise... > spin_lock(&file_lock_lock); > - while (!list_empty(&blocker->fl_block)) { > + do { > struct file_lock *waiter; > > waiter = list_first_entry(&blocker->fl_block, > @@ -571,7 +582,7 @@ static void locks_wake_up_blocks(struct file_lock *blocker) > waiter->fl_lmops->lm_notify(waiter); > else > wake_up(&waiter->fl_wait); > - } > + } while (!list_empty(&blocker->fl_block)); > spin_unlock(&file_lock_lock); > } >
diff --git a/fs/locks.c b/fs/locks.c index 8f56651..a8f3b33 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -532,7 +532,10 @@ static void locks_delete_block(struct file_lock *waiter) * the order they blocked. The documentation doesn't require this but * it seems like the reasonable thing to do. * - * Must be called with file_lock_lock held! + * Must be called with both the i_lock and file_lock_lock held. The fl_block + * list itself is protected by the file_lock_list, but by ensuring that the + * i_lock is also held on insertions we can avoid taking the file_lock_lock + * in some cases when we see that the fl_block list is empty. */ static void __locks_insert_block(struct file_lock *blocker, struct file_lock *waiter) @@ -560,8 +563,16 @@ static void locks_insert_block(struct file_lock *blocker, */ static void locks_wake_up_blocks(struct file_lock *blocker) { + /* + * Avoid taking global lock if list is empty. This is safe since new + * blocked requests are only added to the list under the i_lock, and + * the i_lock is always held here. + */ + if (list_empty(&blocker->fl_block)) + return; + spin_lock(&file_lock_lock); - while (!list_empty(&blocker->fl_block)) { + do { struct file_lock *waiter; waiter = list_first_entry(&blocker->fl_block, @@ -571,7 +582,7 @@ static void locks_wake_up_blocks(struct file_lock *blocker) waiter->fl_lmops->lm_notify(waiter); else wake_up(&waiter->fl_wait); - } + } while (!list_empty(&blocker->fl_block)); spin_unlock(&file_lock_lock); }
Since we always hold the i_lock when inserting a new waiter onto the fl_block list, we can avoid taking the global lock at all if we find that it's empty when we go to wake up blocked waiters. Signed-off-by: Jeff Layton <jlayton@redhat.com> --- fs/locks.c | 17 ++++++++++++++--- 1 files changed, 14 insertions(+), 3 deletions(-)