Message-ID: <c4739d51-9aec-6697-9c70-b888015df764@huawei.com>
Date:   Tue, 10 Mar 2020 22:18:27 +0800
From:   yangerkun <yangerkun@...wei.com>
To:     Jeff Layton <jlayton@...nel.org>, NeilBrown <neilb@...e.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
CC:     kernel test robot <rong.a.chen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, <lkp@...ts.01.org>,
        Bruce Fields <bfields@...ldses.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6%
 regression



On 2020/3/10 20:52, Jeff Layton wrote:
> On Tue, 2020-03-10 at 11:24 +0800, yangerkun wrote:
>> On 2020/3/10 6:11, Jeff Layton wrote:
>>> On Tue, 2020-03-10 at 08:42 +1100, NeilBrown wrote:
>>>> On Mon, Mar 09 2020, Jeff Layton wrote:
>>>>
>>>>> On Mon, 2020-03-09 at 13:22 -0400, Jeff Layton wrote:
>>>>>> On Mon, 2020-03-09 at 08:52 -0700, Linus Torvalds wrote:
>>>>>>> On Mon, Mar 9, 2020 at 7:36 AM Jeff Layton <jlayton@...nel.org> wrote:
>>>>>>>> On Sun, 2020-03-08 at 22:03 +0800, kernel test robot wrote:
>>>>>>>>> FYI, we noticed a -96.6% regression of will-it-scale.per_process_ops due to commit:
>>>>>>>>
>>>>>>>> This is not completely unexpected as we're banging on the global
>>>>>>>> blocked_lock_lock now for every unlock. This test just thrashes file
>>>>>>>> locks and unlocks without doing anything in between, so the workload
>>>>>>>> looks pretty artificial [1].
>>>>>>>>
>>>>>>>> It would be nice to avoid the global lock in this codepath, but it
>>>>>>>> doesn't look simple to do. I'll keep thinking about it, but for now I'm
>>>>>>>> inclined to ignore this result unless we see a problem in more realistic
>>>>>>>> workloads.
>>>>>>>
>>>>>>> That is a _huge_ regression, though.
>>>>>>>
>>>>>>> What about something like the attached? Wouldn't that work? And make
>>>>>>> the code actually match the old comment about wow "fl_blocker" being
>>>>>>> NULL being special.
>>>>>>>
>>>>>>> The old code seemed to not know about things like memory ordering either.
>>>>>>>
>>>>>>> Patch is entirely untested, but aims to have that "smp_store_release()
>>>>>>> means I'm done and not going to touch it any more", making that
>>>>>>> smp_load_acquire() test hopefully be valid as per the comment..
>>>>>>
>>>>>> Yeah, something along those lines maybe. I don't think we can use
>>>>>> fl_blocker that way though, as the wait_event_interruptible is waiting
>>>>>> on it to go to NULL, and the wake_up happens before fl_blocker is
>>>>>> cleared.
>>>>>>
>>>>>> Maybe we need to mix in some sort of FL_BLOCK_ACTIVE flag and use that
>>>>>> instead of testing for !fl_blocker to see whether we can avoid the
>>>>>> blocked_lock_lock?
>>>>>>     
>>>>>
>>>>> How about something like this instead? (untested other than for
>>>>> compilation)
>>>>>
>>>>> Basically, this just switches the waiters over to wait for
>>>>> fl_blocked_member to go empty. That still happens before the wakeup, so
>>>>> it should be ok to wait on that.
>>>>>
>>>>> I think we can also eliminate the lockless list_empty check in
>>>>> locks_delete_block, as the fl_blocker check should be sufficient now.
>>>>> -- 
>>>>> Jeff Layton <jlayton@...nel.org>
>>>>>   From c179d779c9b72838ed9996a65d686d86679d1639 Mon Sep 17 00:00:00 2001
>>>>> From: Linus Torvalds <torvalds@...ux-foundation.org>
>>>>> Date: Mon, 9 Mar 2020 14:35:43 -0400
>>>>> Subject: [PATCH] locks: reinstate locks_delete_lock optimization
>>>>>
>>>>> ...by using smp_load_acquire and smp_store_release to close the race
>>>>> window.
>>>>>
>>>>> [ jlayton: wait on the fl_blocked_requests list to go empty instead of
>>>>> 	   the fl_blocker pointer to clear. Remove the list_empty check
>>>>> 	   from locks_delete_lock shortcut. ]
>>>>
>>>> Why do you think it is OK to remove that list_empty check?  I don't
>>>> think it is.  There might be locked requests that need to be woken up.
>>>>
>>>> As the problem here is a use-after-free due to a race, one option would
>>>> be to use rcu_free() on the file_lock, and hold rcu_read_lock() around
>>>> test/use.
>>>>
>>>> Another option is to use a different lock.  The fl_wait contains a
>>>> spinlock, and we have wake_up_locked() which is provided for exactly
>>>> these sorts of situations where the wake_up call can race with a thread
>>>> waking up.
>>>>
>>>> So my compile-tested-only proposal is below.
>>>> I can probably a proper change-log entry if you think the patch is a
>>>> good way to go.
>>>>
>>>> NeilBrown
>>>>
>>>>
>>>> diff --git a/fs/locks.c b/fs/locks.c
>>>> index 426b55d333d5..8aa04d5ac8b3 100644
>>>> --- a/fs/locks.c
>>>> +++ b/fs/locks.c
>>>> @@ -735,11 +735,13 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
>>>>    
>>>>    		waiter = list_first_entry(&blocker->fl_blocked_requests,
>>>>    					  struct file_lock, fl_blocked_member);
>>>> +		spin_lock(&waiter->fl_wait.lock);
>>>>    		__locks_delete_block(waiter);
>>>>    		if (waiter->fl_lmops && waiter->fl_lmops->lm_notify)
>>>>    			waiter->fl_lmops->lm_notify(waiter);
>>>>    		else
>>>> -			wake_up(&waiter->fl_wait);
>>>> +			wake_up_locked(&waiter->fl_wait);
>>>> +		spin_unlock(&waiter->fl_wait.lock);
>>>>    	}
>>>>    }
>>>>    
>>>> @@ -753,6 +755,31 @@ 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.  So it is safe to test fl_blocker locklessly.
>>>> +	 * Also if fl_blocker is NULL, this waiter is not listed on
>>>> +	 * fl_blocked_requests for some lock, so no other request can
>>>> +	 * be added to the list of fl_blocked_requests for this
>>>> +	 * request.  So if fl_blocker is NULL, it is safe to
>>>> +	 * locklessly check if fl_blocked_requests is empty.  If both
>>>> +	 * of these checks succeed, there is no need to take the lock.
>>>> +	 * However, some other thread might have only *just* set
>>>> +	 * fl_blocker to NULL and it about to send a wakeup on
>>>> +	 * fl_wait, so we mustn't return too soon or we might free waiter
>>>> +	 * before that wakeup can be sent.  So take the fl_wait.lock
>>>> +	 * to serialize with the wakeup in __locks_wake_up_blocks().
>>>> +	 */
>>>> +	if (waiter->fl_blocker == NULL) {
>>>> +		spin_lock(&waiter->fl_wait.lock);
>>>> +		if (waiter->fl_blocker == NULL &&
>>>> +		    list_empty(&waiter->fl_blocked_requests)) {
>>>> +			spin_unlock(&waiter->fl_wait.lock);
>>>> +			return status;
>>>> +		}
>>>> +		spin_unlock(&waiter->fl_wait.lock);
>>>> +	}
>>>>    	spin_lock(&blocked_lock_lock);
>>>>    	if (waiter->fl_blocker)
>>>>    		status = 0;
>>>>
>>>
>>> Looks good on a cursory check, and I'm inclined to go with this since
>>> it's less fiddly for people to backport.
>>>
>>> One other difference to note -- we are holding the fl_wait lock when
>>> calling lm_notify, but I don't think it will matter to any of the
>>> existing lm_notify functions.
>>>
>>> If you want to clean up the changelog and resend that would be great.
>>>
>>> Thanks,
>>>
>> Something others. I think there is no need to call locks_delete_block
>> for all case in function like flock_lock_inode_wait. What we should do
>> as the patch '16306a61d3b7 ("fs/locks: always delete_block after
>> waiting.")' describes is that we need call locks_delete_block not only
>> for error equal to -ERESTARTSYS(please point out if I am wrong). And
>> this patch may fix the regression too since simple lock that success or
>> unlock will not try to acquire blocked_lock_lock.
>>
>>
> 
> Nice! This looks like it would work too, and it's a simpler fix.
> 
> I'd be inclined to add a WARN_ON_ONCE(fl->fl_blocker) after the if
> statements to make sure we never exit with one still queued. Also, I
> think we can do a similar optimization in __break_lease.
> 
> There are some other callers of locks_delete_block:
> 
> cifs_posix_lock_set: already only calls it in these cases


Maybe cifs_posix_lock_set should to be treated the same as 
posix_lock_inode_wait since cifs_posix_lock_set can call 
locks_delete_block only when rc equals to -ERESTARTSYS.

--------------------------------------------

[PATCH] cifs: call locks_delete_block for all error case in
  cifs_posix_lock_set

'16306a61d3b7 ("fs/locks: always delete_block after waiting.")' fix the
problem that we should call locks_delete_block for all error case.
However, cifs_posix_lock_set has been leaved alone which bug may still
exists. Fix it and reorder the code to make in simple.

Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
Signed-off-by: yangerkun <yangerkun@...wei.com>
---
  fs/cifs/file.c | 28 ++++++++++++++++------------
  1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 3b942ecdd4be..e20fc252c0a9 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -1159,21 +1159,25 @@ cifs_posix_lock_set(struct file *file, struct 
file_lock *flock)
  	if ((flock->fl_flags & FL_POSIX) == 0)
  		return rc;

-try_again:
-	cifs_down_write(&cinode->lock_sem);
-	if (!cinode->can_cache_brlcks) {
-		up_write(&cinode->lock_sem);
-		return rc;
-	}
+	for (;;) {
+		cifs_down_write(&cinode->lock_sem);
+		if (!cinode->can_cache_brlcks) {
+			up_write(&cinode->lock_sem);
+			return rc;
+		}

-	rc = posix_lock_file(file, flock, NULL);
-	up_write(&cinode->lock_sem);
-	if (rc == FILE_LOCK_DEFERRED) {
+		rc = posix_lock_file(file, flock, NULL);
+		up_write(&cinode->lock_sem);
+		if (rc != FILE_LOCK_DEFERRED)
+			break;
  		rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
-		if (!rc)
-			goto try_again;
-		locks_delete_block(flock);
+		if (rc)
+			break;
  	}
+	if (rc)
+		locks_delete_block(flock);
+	WARN_ON_ONCE(flock->fl_blocker);
+
  	return rc;
  }

-- 
2.17.2



> 
> nlmsvc_unlink_block: I think we need to call this in most cases, and
> they're not going to be high-performance codepaths in general
> 
> nfsd4 callback handling: Several calls here, most need to always be
> called. find_blocked_lock could be reworked to take the
> blocked_lock_lock only once (I'll do that in a separate patch).
> 
> How about something like this (

Thanks for this, I prefer this patch!

> 
> ----------------------8<---------------------
> 
> From: yangerkun <yangerkun@...wei.com>
> 
> [PATCH] filelock: fix regression in unlock performance
> 
> '6d390e4b5d48 ("locks: fix a potential use-after-free problem when
> wakeup a waiter")' introduces a regression since we will acquire
> blocked_lock_lock every time locks_delete_block is called.
> 
> In many cases we can just avoid calling locks_delete_block at all,
> when we know that the wait was awoken by the condition becoming true.
> Change several callers of locks_delete_block to only call it when
> waking up due to signal or other error condition.
> 
> [ jlayton: add similar optimization to __break_lease, reword changelog,
> 	   add WARN_ON_ONCE calls ]
> 
> Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
> Fixes: 6d390e4b5d48 ("locks: fix a potential use-after-free problem when wakeup a waiter")
> Signed-off-by: yangerkun <yangerkun@...wei.com>
> Signed-off-by: Jeff Layton <jlayton@...nel.org>
> ---
>   fs/locks.c | 29 ++++++++++++++++++++++-------
>   1 file changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 426b55d333d5..b88a5b11c464 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1354,7 +1354,10 @@ static int posix_lock_inode_wait(struct inode *inode, struct file_lock *fl)
>   		if (error)
>   			break;
>   	}
> -	locks_delete_block(fl);
> +	if (error)
> +		locks_delete_block(fl);
> +	WARN_ON_ONCE(fl->fl_blocker);
> +
>   	return error;
>   }
>   
> @@ -1447,7 +1450,9 @@ int locks_mandatory_area(struct inode *inode, struct file *filp, loff_t start,
>   
>   		break;
>   	}
> -	locks_delete_block(&fl);
> +	if (error)
> +		locks_delete_block(&fl);
> +	WARN_ON_ONCE(fl.fl_blocker);
>   
>   	return error;
>   }
> @@ -1638,23 +1643,28 @@ 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);
> +						 !new_fl->fl_blocker,
> +						 break_time);
>   
>   	percpu_down_read(&file_rwsem);
>   	spin_lock(&ctx->flc_lock);
>   	trace_break_lease_unblock(inode, new_fl);
> -	locks_delete_block(new_fl);
>   	if (error >= 0) {
>   		/*
>   		 * Wait for the next conflicting lease that has not been
>   		 * broken yet
>   		 */
> -		if (error == 0)
> +		if (error == 0) {
> +			locks_delete_block(new_fl);
>   			time_out_leases(inode, &dispose);
> +		}
>   		if (any_leases_conflict(inode, new_fl))
>   			goto restart;
>   		error = 0;
> +	} else {
> +		locks_delete_block(new_fl);
>   	}
> +	WARN_ON_ONCE(fl->fl_blocker);
>   out:
>   	spin_unlock(&ctx->flc_lock);
>   	percpu_up_read(&file_rwsem);
> @@ -2126,7 +2136,10 @@ static int flock_lock_inode_wait(struct inode *inode, struct file_lock *fl)
>   		if (error)
>   			break;
>   	}
> -	locks_delete_block(fl);
> +	if (error)
> +		locks_delete_block(fl);
> +	WARN_ON_ONCE(fl->fl_blocker);
> +
>   	return error;
>   }
>   
> @@ -2403,7 +2416,9 @@ static int do_lock_file_wait(struct file *filp, unsigned int cmd,
>   		if (error)
>   			break;
>   	}
> -	locks_delete_block(fl);
> +	if (error)
> +		locks_delete_block(fl);
> +	WARN_ON_ONCE(fl->fl_blocker);
>   
>   	return error;
>   }
> 

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