Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

[Jeff Layton]

On Mon, 2020-03-16 at 16:06 +1100, NeilBrown wrote:

[...]

> No, we really do need fl_blocked_requests to be empty.
> After fl_blocker is cleared, the owner might check for other blockers
> and might queue behind them leaving the blocked requests in place.
> Or it might have to detach all those blocked requests and wake them up
> so they can go and fend for themselves.
> 
> I think the worse-case scenario could go something like that.
> Process A get a lock - Al
> Process B tries to get a conflicting lock and blocks Bl -> Al
> Process C tries to get a conflicting lock and blocks on B:
>    Cl -> Bl -> Al
> 
> At much the same time that C goes to attach Cl to Bl, A
> calls unlock and B get signaled.
> 
> So A is calling locks_wake_up_blocks(Al) - which takes blocked_lock_lock.
> C is calling  locks_insert_block(Bl, Cl) - which also takes the lock
> B is calling  locks_delete_block(Bl)  which might not take the lock.
> 
> Assume C gets the lock first.
> 
> Before C calls locks_insert_block, Bl->fl_blocked_requests is empty.
> After A finishes in locks_wake_up_blocks, Bl->fl_blocker is NULL
> 
> If B sees that fl_blocker is NULL, we need it to see that
> fl_blocked_requests is no longer empty, so that it takes the lock and
> cleans up fl_blocked_requests.
> 
> If the list_empty test on fl_blocked_request goes after the fl_blocker
> test, the memory barriers we have should assure that.  I had thought
> that it would need an extra barrier, but as a spinlock places the change
> to fl_blocked_requests *before* the change to fl_blocker, I no longer
> think that is needed.

Got it. I was thinking all of the waiters of a blocker would already be
awoken once fl_blocker was set to NULL, but you're correct and they
aren't. How about this?

-----------------8<------------------

From f40e865842ae84a9d465ca9edb66f0985c1587d4 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Mon, 9 Mar 2020 14:35:43 -0400
Subject: [PATCH] locks: reinstate locks_delete_block optimization

There is measurable performance impact in some synthetic tests due to
commit 6d390e4b5d48 (locks: fix a potential use-after-free problem when
wakeup a waiter). Fix the race condition instead by clearing the
fl_blocker pointer after the wake_up, using explicit acquire/release
semantics.

This does mean that we can no longer use the clearing of fl_blocker as
the wait condition, so switch the waiters over to checking whether the
fl_blocked_member list_head is empty.

Cc: yangerkun <yangerkun@xxxxxxxxxx>
Cc: NeilBrown <neilb@xxxxxxx>
Fixes: 6d390e4b5d48 (locks: fix a potential use-after-free problem when wakeup a waiter)
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
 fs/cifs/file.c |  3 ++-
 fs/locks.c     | 41 +++++++++++++++++++++++++++++++++++------
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 3b942ecdd4be..8f9d849a0012 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -1169,7 +1169,8 @@ cifs_posix_lock_set(struct file *file, struct file_lock *flock)
 	rc = posix_lock_file(file, flock, NULL);
 	up_write(&cinode->lock_sem);
 	if (rc == FILE_LOCK_DEFERRED) {
-		rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
+		rc = wait_event_interruptible(flock->fl_wait,
+					list_empty(&flock->fl_blocked_member));
 		if (!rc)
 			goto try_again;
 		locks_delete_block(flock);
diff --git a/fs/locks.c b/fs/locks.c
index 426b55d333d5..eaf754ecdaa8 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -725,7 +725,6 @@ static void __locks_delete_block(struct file_lock *waiter)
 {
 	locks_delete_global_blocked(waiter);
 	list_del_init(&waiter->fl_blocked_member);
-	waiter->fl_blocker = NULL;
 }
 
 static void __locks_wake_up_blocks(struct file_lock *blocker)
@@ -740,6 +739,12 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
 			waiter->fl_lmops->lm_notify(waiter);
 		else
 			wake_up(&waiter->fl_wait);
+
+		/*
+		 * Tell the world we're done with it - see comment at
+		 * top of locks_delete_block().
+		 */
+		smp_store_release(&waiter->fl_blocker, NULL);
 	}
 }
 
@@ -753,11 +758,30 @@ int locks_delete_block(struct file_lock *waiter)
 {
 	int status = -ENOENT;
 
+	/*
+	 * If fl_blocker is NULL, it won't be set again as this thread "owns"
+	 * the lock and is the only one that might try to claim the lock.
+	 * Because fl_blocker is explicitly set last during a delete, it's
+	 * safe to locklessly test to see if it's NULL. If it is, then we know
+	 * that no new locks can be inserted into its fl_blocked_requests list,
+	 * and we can therefore avoid doing anything further as long as that
+	 * list is empty.
+	 */
+	if (!smp_load_acquire(&waiter->fl_blocker) &&
+	    list_empty(&waiter->fl_blocked_requests))
+		return status;
+
 	spin_lock(&blocked_lock_lock);
 	if (waiter->fl_blocker)
 		status = 0;
 	__locks_wake_up_blocks(waiter);
 	__locks_delete_block(waiter);
+
+	/*
+	 * Tell the world we're done with it - see comment at top
+	 * of this function
+	 */
+	smp_store_release(&waiter->fl_blocker, NULL);
 	spin_unlock(&blocked_lock_lock);
 	return status;
 }
@@ -1350,7 +1374,8 @@ static int posix_lock_inode_wait(struct inode *inode, struct file_lock *fl)
 		error = posix_lock_inode(inode, fl, NULL);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+		error = wait_event_interruptible(fl->fl_wait,
+					list_empty(&fl->fl_blocked_member));
 		if (error)
 			break;
 	}
@@ -1435,7 +1460,8 @@ int locks_mandatory_area(struct inode *inode, struct file *filp, loff_t start,
 		error = posix_lock_inode(inode, &fl, NULL);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl.fl_wait, !fl.fl_blocker);
+		error = wait_event_interruptible(fl.fl_wait,
+					list_empty(&fl.fl_blocked_member));
 		if (!error) {
 			/*
 			 * If we've been sleeping someone might have
@@ -1638,7 +1664,8 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
 
 	locks_dispose_list(&dispose);
 	error = wait_event_interruptible_timeout(new_fl->fl_wait,
-						!new_fl->fl_blocker, break_time);
+					list_empty(&new_fl->fl_blocked_member),
+					break_time);
 
 	percpu_down_read(&file_rwsem);
 	spin_lock(&ctx->flc_lock);
@@ -2122,7 +2149,8 @@ static int flock_lock_inode_wait(struct inode *inode, struct file_lock *fl)
 		error = flock_lock_inode(inode, fl);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+		error = wait_event_interruptible(fl->fl_wait,
+				list_empty(&fl->fl_blocked_member));
 		if (error)
 			break;
 	}
@@ -2399,7 +2427,8 @@ static int do_lock_file_wait(struct file *filp, unsigned int cmd,
 		error = vfs_lock_file(filp, cmd, fl, NULL);
 		if (error != FILE_LOCK_DEFERRED)
 			break;
-		error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+		error = wait_event_interruptible(fl->fl_wait,
+					list_empty(&fl->fl_blocked_member));
 		if (error)
 			break;
 	}
-- 
2.24.1

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