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

[yangerkun <yangerkun@huawei.com>]

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@kernel.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@kernel.org>
>>>  From c179d779c9b72838ed9996a65d686d86679d1639 Mon Sep 17 00:00:00 2001
>>> From: Linus Torvalds <torvalds@linux-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.



 From 40a0604199e9810d0380f90c403bbd4300075cad Mon Sep 17 00:00:00 2001
From: yangerkun <yangerkun@huawei.com>
Date: Tue, 10 Mar 2020 10:12:57 +0800
Subject: [PATCH] fs/locks: fix the regression in flocks

'6d390e4b5d48 ("locks: fix a potential use-after-free problem when
wakeup a waiter")' introduce a regression since we will acquire
blocked_lock_lock everytime we lock or unlock. Actually, what patch
'16306a61d3b7 ("fs/locks: always delete_block after waiting.")' want to
do is that we should wakeup waiter not only for error equals to
-ERESTARTSYS, some other error code like -ENOMEM return from
flock_lock_inode need be treated the same as the file_lock may block other
flock too(flock a -> conflict with others and begin to wait -> flock b
conflict with a and wait for a -> someone wakeup flock a then
flock_lock_inode return -ENOMEM). Fix this regression by check error.

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@huawei.com>
---
  fs/locks.c | 14 ++++++++++----
  1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/locks.c b/fs/locks.c
index 426b55d333d5..403ed2230dd4 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1354,7 +1354,9 @@ 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);
+
  	return error;
  }

@@ -1447,7 +1449,8 @@ int locks_mandatory_area(struct inode *inode, 
struct file *filp, loff_t start,

  		break;
  	}
-	locks_delete_block(&fl);
+	if (error)
+		locks_delete_block(&fl);

  	return error;
  }
@@ -2126,7 +2129,9 @@ 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);
+
  	return error;
  }

@@ -2403,7 +2408,8 @@ 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);

  	return error;
  }
-- 
2.17.2