Ocfs2 devel - Jan 2016 - [PATCH] ocfs2: dlmglue: fix false deadlock caused by clearing UPCONVERT_FINISHING too early

This problem was introduced by commit a19128260107f951d1b4c421cf98b92f8092b069.
OCFS2_LOCK_UPCONVERT_FINISHING is set just before clearing OCFS2_LOCK_BUSY. This
will prevent dc thread from downconverting immediately, and let mask-waiters in
->l_mask_waiters list whose requesting level is compatible with ->l_level
to take
the lock. But if we have two waiters in mw list, the first is to get EX lock,
and
the second is to to get PR lock. The first may fail to get lock and then clear
UPCONVERT_FINISHING. It's too early to clear the flag because this second
will be
also queued again even if ->l_level is PR. As a result, nobody would kick up
dc
thread, leaving dlmglue a deadlock until another lockres relative thread wake it
up.

More specifically, for example:
On node1, there is thread W1 keeping writing; on node2, there are thread R1 and
R2 keeping reading; sure this 3 threads make IO on the same shared file. At a
time, node2 is receiving ast(0=>3), followed immediately by a bast requesting
EX
lock on behave of node1. Then this may happen:
node2:                                          node1:
l_level==3; R1(3); R2(3)                        l_level==3
R1(unlock); R1(3=>5, update atime)              W1(3=>5)
BAST
R2(unlock); AST(3=>0)
R2(0=>3)
                                                BAST
AST(0=>3)
set OCFS2_LOCK_UPCONVERT_FINISHING
clear OCFS2_LOCK_BUSY
                                                W1(3=>5)
BAST
dc thread requeue=yes
R1(clear OCFS2_LOCK_UPCONVERT_FINISHING,wait)
R2(wait)
...
dlmglue deadlock util dc thread woken up by others

This fix is to clear OCFS2_LOCK_UPCONVERT_FINISHING util OCFS2_LOCK_BUSY has
been cleared and every waiters has been looped.

Signed-off-by: Eric Ren <zren at suse.com>
---
 fs/ocfs2/dlmglue.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index f92612e..72f8b6c 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -824,6 +824,8 @@ static void lockres_clear_flags(struct ocfs2_lock_res
*lockres,
 				unsigned long clear)
 {
 	lockres_set_flags(lockres, lockres->l_flags & ~clear);
+	if(clear & OCFS2_LOCK_BUSY)
+		lockres->l_flags &= ~OCFS2_LOCK_UPCONVERT_FINISHING;
 }
 
 static inline void ocfs2_generic_handle_downconvert_action(struct
ocfs2_lock_res *lockres)
@@ -1522,8 +1524,6 @@ update_holders:
 
 	ret = 0;
 unlock:
-	lockres_clear_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
-
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
 out:
 	/*
-- 
2.6.2

Hi,

This fix is wrong, becuase it can ensure waking up every waiter, but cannot
guarantee every waiter finish trying its "again" patch in
__ocfs2_cluster_lock().

Other solutions now on my mind are:
1. Give every waiter an ID. When clearing OCFS2_LOCK_BUSY, we can record those
IDs
in an array. Process any waiter in mask-waiter list, remove the ID from the arry
if its ID is in the array, util array is empty we can then clear
OCFS2_LOCK_UPCONVERT_FINISHING.

I think it's a bad idea. It's inefficient to handle the array and the ID
control is
another problem.

2. Split mask-waiter list into two lists: one for OCFS2_LOCK_BUSY, and another
for
OCFS2_LOCK_BLOCKED. When OCFS2_LOCK_BUSY being cleared and OCFS2_LOCK_BLOCKED
being
set, we should process waiters in BUSY list and move waiters who cannot get the
lock into
BLOCKED list again. And when OCFS2_LOCK_BLOCKED being cleared and
OCFS2_LOCK_BUSY being
set, we should do things like that.

But is any chance that both OCFS2_LOCK_BUSY and OCFS2_LOCK_BLOCKED are set at
the same time?
If not, I prefer this one.

What do you think? Any comment would be appreciated.

Thanks,
Eric

On Wed, Jan 20, 2016 at 12:46:53AM +0800, Eric Ren wrote:
> This problem was introduced by commit
a19128260107f951d1b4c421cf98b92f8092b069.
> OCFS2_LOCK_UPCONVERT_FINISHING is set just before clearing OCFS2_LOCK_BUSY.
This
> will prevent dc thread from downconverting immediately, and let
mask-waiters in
> ->l_mask_waiters list whose requesting level is compatible with
->l_level to take
> the lock. But if we have two waiters in mw list, the first is to get EX
lock, and
> the second is to to get PR lock. The first may fail to get lock and then
clear
> UPCONVERT_FINISHING. It's too early to clear the flag because this
second will be
> also queued again even if ->l_level is PR. As a result, nobody would
kick up dc
> thread, leaving dlmglue a deadlock until another lockres relative thread
wake it
> up.
> 
> More specifically, for example:
> On node1, there is thread W1 keeping writing; on node2, there are thread R1
and
> R2 keeping reading; sure this 3 threads make IO on the same shared file. At
a
> time, node2 is receiving ast(0=>3), followed immediately by a bast
requesting EX
> lock on behave of node1. Then this may happen:
> node2:                                          node1:
> l_level==3; R1(3); R2(3)                        l_level==3
> R1(unlock); R1(3=>5, update atime)              W1(3=>5)
> BAST
> R2(unlock); AST(3=>0)
> R2(0=>3)
>                                                 BAST
> AST(0=>3)
> set OCFS2_LOCK_UPCONVERT_FINISHING
> clear OCFS2_LOCK_BUSY
>                                                 W1(3=>5)
> BAST
> dc thread requeue=yes
> R1(clear OCFS2_LOCK_UPCONVERT_FINISHING,wait)
> R2(wait)
> ...
> dlmglue deadlock util dc thread woken up by others
> 
> This fix is to clear OCFS2_LOCK_UPCONVERT_FINISHING util OCFS2_LOCK_BUSY
has
> been cleared and every waiters has been looped.
> 
> Signed-off-by: Eric Ren <zren at suse.com>
> ---
>  fs/ocfs2/dlmglue.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index f92612e..72f8b6c 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -824,6 +824,8 @@ static void lockres_clear_flags(struct ocfs2_lock_res
*lockres,
>  				unsigned long clear)
>  {
>  	lockres_set_flags(lockres, lockres->l_flags & ~clear);
> +	if(clear & OCFS2_LOCK_BUSY)
> +		lockres->l_flags &= ~OCFS2_LOCK_UPCONVERT_FINISHING;
>  }
>  
>  static inline void ocfs2_generic_handle_downconvert_action(struct
ocfs2_lock_res *lockres)
> @@ -1522,8 +1524,6 @@ update_holders:
>  
>  	ret = 0;
>  unlock:
> -	lockres_clear_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
> -
>  	spin_unlock_irqrestore(&lockres->l_lock, flags);
>  out:
>  	/*
> -- 
> 2.6.2
>

Hi,

Very sorry, this fix is wrong, becuase it can ensure waking up every waiter, but
cannot
guarantee every waiter finish trying its "again" patch in
__ocfs2_cluster_lock().

Other solutions now on my mind are:
1. Give every waiter an ID. When clearing OCFS2_LOCK_BUSY, we can record those
IDs
in an array. Process any waiter in mask-waiter list, remove the ID from the arry
if its ID is in the array, util array is empty we can then clear
OCFS2_LOCK_UPCONVERT_FINISHING.

I think it's a bad idea. It's inefficient to handle the array and the ID
control is
another problem.

2. Split mask-waiter list into two lists: one for OCFS2_LOCK_BUSY, and another
for
OCFS2_LOCK_BLOCKED. When OCFS2_LOCK_BUSY being cleared and OCFS2_LOCK_BLOCKED
being
set, we should process waiters in BUSY list and move waiters who cannot get the
lock into
BLOCKED list again. And when OCFS2_LOCK_BLOCKED being cleared and
OCFS2_LOCK_BUSY being
set, we should do things like that.

But is any chance that both OCFS2_LOCK_BUSY and OCFS2_LOCK_BLOCKED are set at
the same time?
If not, I prefer this one.

What do you think? Any comment would be appreciated.

Thanks,
Eric

 >>>
> This problem was introduced by commit  
> a19128260107f951d1b4c421cf98b92f8092b069. 
> OCFS2_LOCK_UPCONVERT_FINISHING is set just before clearing OCFS2_LOCK_BUSY.
> This 
> will prevent dc thread from downconverting immediately, and let
mask-waiters
> in 
> ->l_mask_waiters list whose requesting level is compatible with
->l_level to
> take 
> the lock. But if we have two waiters in mw list, the first is to get EX  
> lock, and 
> the second is to to get PR lock. The first may fail to get lock and then  
> clear 
> UPCONVERT_FINISHING. It's too early to clear the flag because this
second
> will be 
> also queued again even if ->l_level is PR. As a result, nobody would
kick up
> dc 
> thread, leaving dlmglue a deadlock until another lockres relative thread  
> wake it 
> up. 
>  
> More specifically, for example: 
> On node1, there is thread W1 keeping writing; on node2, there are thread R1
> and 
> R2 keeping reading; sure this 3 threads make IO on the same shared file. At
> a 
> time, node2 is receiving ast(0=>3), followed immediately by a bast
requesting
> EX 
> lock on behave of node1. Then this may happen: 
> node2:                                          node1: 
> l_level==3; R1(3); R2(3)                        l_level==3 
> R1(unlock); R1(3=>5, update atime)              W1(3=>5) 
> BAST 
> R2(unlock); AST(3=>0) 
> R2(0=>3) 
>                                                 BAST 
> AST(0=>3) 
> set OCFS2_LOCK_UPCONVERT_FINISHING 
> clear OCFS2_LOCK_BUSY 
>                                                 W1(3=>5) 
> BAST 
> dc thread requeue=yes 
> R1(clear OCFS2_LOCK_UPCONVERT_FINISHING,wait) 
> R2(wait) 
> ... 
> dlmglue deadlock util dc thread woken up by others 
>  
> This fix is to clear OCFS2_LOCK_UPCONVERT_FINISHING util OCFS2_LOCK_BUSY
has
> been cleared and every waiters has been looped. 
>  
> Signed-off-by: Eric Ren <zren at suse.com> 
> --- 
>  fs/ocfs2/dlmglue.c | 4 ++-- 
>  1 file changed, 2 insertions(+), 2 deletions(-) 
>  
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c 
> index f92612e..72f8b6c 100644 
> --- a/fs/ocfs2/dlmglue.c 
> +++ b/fs/ocfs2/dlmglue.c 
> @@ -824,6 +824,8 @@ static void lockres_clear_flags(struct ocfs2_lock_res  
> *lockres, 
>  				unsigned long clear) 
>  { 
>  	lockres_set_flags(lockres, lockres->l_flags & ~clear); 
> +	if(clear & OCFS2_LOCK_BUSY) 
> +		lockres->l_flags &= ~OCFS2_LOCK_UPCONVERT_FINISHING; 
>  } 
>   
>  static inline void ocfs2_generic_handle_downconvert_action(struct  
> ocfs2_lock_res *lockres) 
> @@ -1522,8 +1524,6 @@ update_holders: 
>   
>  	ret = 0; 
>  unlock: 
> -	lockres_clear_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING); 
> - 
>  	spin_unlock_irqrestore(&lockres->l_lock, flags); 
>  out: 
>  	/*

Hi Eric,

This patch should fix your issue.
"NFS hangs in __ocfs2_cluster_lock due to race with
ocfs2_unblock_lock"

Thanks,
Junxiao.
On 01/20/2016 12:46 AM, Eric Ren wrote:
> This problem was introduced by commit
a19128260107f951d1b4c421cf98b92f8092b069.
> OCFS2_LOCK_UPCONVERT_FINISHING is set just before clearing OCFS2_LOCK_BUSY.
This
> will prevent dc thread from downconverting immediately, and let
mask-waiters in
> ->l_mask_waiters list whose requesting level is compatible with
->l_level to take
> the lock. But if we have two waiters in mw list, the first is to get EX
lock, and
> the second is to to get PR lock. The first may fail to get lock and then
clear
> UPCONVERT_FINISHING. It's too early to clear the flag because this
second will be
> also queued again even if ->l_level is PR. As a result, nobody would
kick up dc
> thread, leaving dlmglue a deadlock until another lockres relative thread
wake it
> up.
> 
> More specifically, for example:
> On node1, there is thread W1 keeping writing; on node2, there are thread R1
and
> R2 keeping reading; sure this 3 threads make IO on the same shared file. At
a
> time, node2 is receiving ast(0=>3), followed immediately by a bast
requesting EX
> lock on behave of node1. Then this may happen:
> node2:                                          node1:
> l_level==3; R1(3); R2(3)                        l_level==3
> R1(unlock); R1(3=>5, update atime)              W1(3=>5)
> BAST
> R2(unlock); AST(3=>0)
> R2(0=>3)
>                                                 BAST
> AST(0=>3)
> set OCFS2_LOCK_UPCONVERT_FINISHING
> clear OCFS2_LOCK_BUSY
>                                                 W1(3=>5)
> BAST
> dc thread requeue=yes
> R1(clear OCFS2_LOCK_UPCONVERT_FINISHING,wait)
> R2(wait)
> ...
> dlmglue deadlock util dc thread woken up by others
> 
> This fix is to clear OCFS2_LOCK_UPCONVERT_FINISHING util OCFS2_LOCK_BUSY
has
> been cleared and every waiters has been looped.
> 
> Signed-off-by: Eric Ren <zren at suse.com>
> ---
>  fs/ocfs2/dlmglue.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index f92612e..72f8b6c 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -824,6 +824,8 @@ static void lockres_clear_flags(struct ocfs2_lock_res
*lockres,
>  				unsigned long clear)
>  {
>  	lockres_set_flags(lockres, lockres->l_flags & ~clear);
> +	if(clear & OCFS2_LOCK_BUSY)
> +		lockres->l_flags &= ~OCFS2_LOCK_UPCONVERT_FINISHING;
>  }
>  
>  static inline void ocfs2_generic_handle_downconvert_action(struct
ocfs2_lock_res *lockres)
> @@ -1522,8 +1524,6 @@ update_holders:
>  
>  	ret = 0;
>  unlock:
> -	lockres_clear_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
> -
>  	spin_unlock_irqrestore(&lockres->l_lock, flags);
>  out:
>  	/*
>