[PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[Tomoki Sekiyama]

The soft lockup below happens at the boot time of the system using dm
multipath and the udev rules to switch scheduler.

[  356.127001] BUG: soft lockup - CPU#3 stuck for 22s! [sh:483]
[  356.127001] RIP: 0010:[<ffffffff81072a7d>]  [<ffffffff81072a7d>] lock_timer_base.isra.35+0x1d/0x50
...
[  356.127001] Call Trace:
[  356.127001]  [<ffffffff81073810>] try_to_del_timer_sync+0x20/0x70
[  356.127001]  [<ffffffff8118b08a>] ? kmem_cache_alloc_node_trace+0x20a/0x230
[  356.127001]  [<ffffffff810738b2>] del_timer_sync+0x52/0x60
[  356.127001]  [<ffffffff812ece22>] cfq_exit_queue+0x32/0xf0
[  356.127001]  [<ffffffff812c98df>] elevator_exit+0x2f/0x50
[  356.127001]  [<ffffffff812c9f21>] elevator_change+0xf1/0x1c0
[  356.127001]  [<ffffffff812caa50>] elv_iosched_store+0x20/0x50
[  356.127001]  [<ffffffff812d1d09>] queue_attr_store+0x59/0xb0
[  356.127001]  [<ffffffff812143f6>] sysfs_write_file+0xc6/0x140
[  356.127001]  [<ffffffff811a326d>] vfs_write+0xbd/0x1e0
[  356.127001]  [<ffffffff811a3ca9>] SyS_write+0x49/0xa0
[  356.127001]  [<ffffffff8164e899>] system_call_fastpath+0x16/0x1b

This is caused by a race between md device initialization by multipathd and
shell script to switch the scheduler using sysfs.

 - multipathd:
   SyS_ioctl -> do_vfs_ioctl -> dm_ctl_ioctl -> ctl_ioctl -> table_load
   -> dm_setup_md_queue -> blk_init_allocated_queue -> elevator_init
    q->elevator = elevator_alloc(q, e); // not yet initialized

 - sh -c 'echo deadline > /sys/$DEVPATH/queue/scheduler':
   elevator_switch (in the call trace above)
    struct elevator_queue *old = q->elevator;
    q->elevator = elevator_alloc(q, new_e);
    elevator_exit(old);                 // lockup! (*)

 - multipathd: (cont.)
    err = e->ops.elevator_init_fn(q);   // init fails; q->elevator is modified

(*) When del_timer_sync() is called, lock_timer_base() will loop infinitely
while timer->base == NULL. In this case, as timer will never initialized,
it results in lockup.

This patch introduces acquisition of q->sysfs_lock around elevator_init()
into blk_init_allocated_queue(), to provide mutual exclusion between
initialization of the q->scheduler and switching of the scheduler.

This should fix this bugzilla:
https://bugzilla.redhat.com/show_bug.cgi?id=902012

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
---
 block/blk-core.c |   10 +++++++++-
 block/elevator.c |    6 ++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 93a18d1..2f6275f 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -739,9 +739,17 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
 
 	q->sg_reserved_size = INT_MAX;
 
+	/* Protect q->elevator from elevator_change */
+	mutex_lock(&q->sysfs_lock);
+
 	/* init elevator */
-	if (elevator_init(q, NULL))
+	if (elevator_init(q, NULL)) {
+		mutex_unlock(&q->sysfs_lock);
 		return NULL;
+	}
+
+	mutex_unlock(&q->sysfs_lock);
+
 	return q;
 }
 EXPORT_SYMBOL(blk_init_allocated_queue);
diff --git a/block/elevator.c b/block/elevator.c
index 668394d..02d4390 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char *name)
 	struct elevator_type *e = NULL;
 	int err;
 
+	/*
+	 * q->sysfs_lock must be held to provide mutual exclusion between
+	 * elevator_switch() and here.
+	 */
+	lockdep_assert_held(&q->sysfs_lock);
+
 	if (unlikely(q->elevator))
 		return 0;
 

[PATCH v2 2/2] elevator: acquire q->sysfs_lock in elevator_change()

[Tomoki Sekiyama]

Add locking of q->sysfs_lock into elevator_change() (an exported function)
to ensure it is held to protect q->elevator from elevator_init(), even if
elevator_change() is called from non-sysfs paths.
sysfs path (elv_iosched_store) uses __elevator_change(), non-locking
version, as the lock is already taken by elv_iosched_store().

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
---
 block/elevator.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 02d4390..6d765f7 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -965,7 +965,7 @@ fail_init:
 /*
  * Switch this queue to the given IO scheduler.
  */
-int elevator_change(struct request_queue *q, const char *name)
+static int __elevator_change(struct request_queue *q, const char *name)
 {
 	char elevator_name[ELV_NAME_MAX];
 	struct elevator_type *e;
@@ -987,6 +987,18 @@ int elevator_change(struct request_queue *q, const char *name)
 
 	return elevator_switch(q, e);
 }
+
+int elevator_change(struct request_queue *q, const char *name)
+{
+	int ret;
+
+	/* Protect q->elevator from elevator_init() */
+	mutex_lock(&q->sysfs_lock);
+	ret = __elevator_change(q, name);
+	mutex_unlock(&q->sysfs_lock);
+
+	return ret;
+}
 EXPORT_SYMBOL(elevator_change);
 
 ssize_t elv_iosched_store(struct request_queue *q, const char *name,
@@ -997,7 +1009,7 @@ ssize_t elv_iosched_store(struct request_queue *q, const char *name,
 	if (!q->elevator)
 		return count;
 
-	ret = elevator_change(q, name);
+	ret = __elevator_change(q, name);
 	if (!ret)
 		return count;
 

Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[Tomoki Sekiyama]

Ping: any comments for this series?


On 8/30/13 18:47 , "Tomoki Sekiyama" <tomoki.sekiyama@hds.com> wrote:

>The soft lockup below happens at the boot time of the system using dm
>multipath and the udev rules to switch scheduler.
>
>[  356.127001] BUG: soft lockup - CPU#3 stuck for 22s! [sh:483]
>[  356.127001] RIP: 0010:[<ffffffff81072a7d>]  [<ffffffff81072a7d>]
>lock_timer_base.isra.35+0x1d/0x50
>...
>[  356.127001] Call Trace:
>[  356.127001]  [<ffffffff81073810>] try_to_del_timer_sync+0x20/0x70
>[  356.127001]  [<ffffffff8118b08a>] ?
>kmem_cache_alloc_node_trace+0x20a/0x230
>[  356.127001]  [<ffffffff810738b2>] del_timer_sync+0x52/0x60
>[  356.127001]  [<ffffffff812ece22>] cfq_exit_queue+0x32/0xf0
>[  356.127001]  [<ffffffff812c98df>] elevator_exit+0x2f/0x50
>[  356.127001]  [<ffffffff812c9f21>] elevator_change+0xf1/0x1c0
>[  356.127001]  [<ffffffff812caa50>] elv_iosched_store+0x20/0x50
>[  356.127001]  [<ffffffff812d1d09>] queue_attr_store+0x59/0xb0
>[  356.127001]  [<ffffffff812143f6>] sysfs_write_file+0xc6/0x140
>[  356.127001]  [<ffffffff811a326d>] vfs_write+0xbd/0x1e0
>[  356.127001]  [<ffffffff811a3ca9>] SyS_write+0x49/0xa0
>[  356.127001]  [<ffffffff8164e899>] system_call_fastpath+0x16/0x1b
>
>This is caused by a race between md device initialization by multipathd
>and
>shell script to switch the scheduler using sysfs.
>
> - multipathd:
>   SyS_ioctl -> do_vfs_ioctl -> dm_ctl_ioctl -> ctl_ioctl -> table_load
>   -> dm_setup_md_queue -> blk_init_allocated_queue -> elevator_init
>    q->elevator = elevator_alloc(q, e); // not yet initialized
>
> - sh -c 'echo deadline > /sys/$DEVPATH/queue/scheduler':
>   elevator_switch (in the call trace above)
>    struct elevator_queue *old = q->elevator;
>    q->elevator = elevator_alloc(q, new_e);
>    elevator_exit(old);                 // lockup! (*)
>
> - multipathd: (cont.)
>    err = e->ops.elevator_init_fn(q);   // init fails; q->elevator is
>modified
>
>(*) When del_timer_sync() is called, lock_timer_base() will loop
>infinitely
>while timer->base == NULL. In this case, as timer will never initialized,
>it results in lockup.
>
>This patch introduces acquisition of q->sysfs_lock around elevator_init()
>into blk_init_allocated_queue(), to provide mutual exclusion between
>initialization of the q->scheduler and switching of the scheduler.
>
>This should fix this bugzilla:
>https://bugzilla.redhat.com/show_bug.cgi?id=902012
>
>Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
>---
> block/blk-core.c |   10 +++++++++-
> block/elevator.c |    6 ++++++
> 2 files changed, 15 insertions(+), 1 deletion(-)
>
>diff --git a/block/blk-core.c b/block/blk-core.c
>index 93a18d1..2f6275f 100644
>--- a/block/blk-core.c
>+++ b/block/blk-core.c
>@@ -739,9 +739,17 @@ blk_init_allocated_queue(struct request_queue *q,
>request_fn_proc *rfn,
> 
> 	q->sg_reserved_size = INT_MAX;
> 
>+	/* Protect q->elevator from elevator_change */
>+	mutex_lock(&q->sysfs_lock);
>+
> 	/* init elevator */
>-	if (elevator_init(q, NULL))
>+	if (elevator_init(q, NULL)) {
>+		mutex_unlock(&q->sysfs_lock);
> 		return NULL;
>+	}
>+
>+	mutex_unlock(&q->sysfs_lock);
>+
> 	return q;
> }
> EXPORT_SYMBOL(blk_init_allocated_queue);
>diff --git a/block/elevator.c b/block/elevator.c
>index 668394d..02d4390 100644
>--- a/block/elevator.c
>+++ b/block/elevator.c
>@@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char
>*name)
> 	struct elevator_type *e = NULL;
> 	int err;
> 
>+	/*
>+	 * q->sysfs_lock must be held to provide mutual exclusion between
>+	 * elevator_switch() and here.
>+	 */
>+	lockdep_assert_held(&q->sysfs_lock);
>+
> 	if (unlikely(q->elevator))
> 		return 0;
> 
>

Re: [PATCH v2 2/2] elevator: acquire q->sysfs_lock in elevator_change()

[Tejun Heo]

On Fri, Aug 30, 2013 at 06:47:16PM -0400, Tomoki Sekiyama wrote:
> Add locking of q->sysfs_lock into elevator_change() (an exported function)
> to ensure it is held to protect q->elevator from elevator_init(), even if
> elevator_change() is called from non-sysfs paths.
> sysfs path (elv_iosched_store) uses __elevator_change(), non-locking
> version, as the lock is already taken by elv_iosched_store().
> 
> Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[Tejun Heo]

On Fri, Aug 30, 2013 at 06:47:07PM -0400, Tomoki Sekiyama wrote:
> @@ -739,9 +739,17 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
>  
>  	q->sg_reserved_size = INT_MAX;
>  
> +	/* Protect q->elevator from elevator_change */
> +	mutex_lock(&q->sysfs_lock);
> +
>  	/* init elevator */
> -	if (elevator_init(q, NULL))
> +	if (elevator_init(q, NULL)) {
> +		mutex_unlock(&q->sysfs_lock);
>  		return NULL;
> +	}
> +
> +	mutex_unlock(&q->sysfs_lock);
> +
>  	return q;
>  }
>  EXPORT_SYMBOL(blk_init_allocated_queue);
> diff --git a/block/elevator.c b/block/elevator.c
> index 668394d..02d4390 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char *name)
>  	struct elevator_type *e = NULL;
>  	int err;
>  
> +	/*
> +	 * q->sysfs_lock must be held to provide mutual exclusion between
> +	 * elevator_switch() and here.
> +	 */
> +	lockdep_assert_held(&q->sysfs_lock);
> +
>  	if (unlikely(q->elevator))
>  		return 0;

Hmm... why aren't we just changing elevator_init() to grab sysfs_lock
where necessary?  It'd be more consistent with elevator_exit() that
way.

Thanks.

-- 
tejun

Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[Tomoki Sekiyama]

Hi Tejun,

Thank you for the review.

On 9/22/13 13:04 , "Tejun Heo" <tj@kernel.org> wrote:

>On Fri, Aug 30, 2013 at 06:47:07PM -0400, Tomoki Sekiyama wrote:
>> @@ -739,9 +739,17 @@ blk_init_allocated_queue(struct request_queue *q,
>>request_fn_proc *rfn,
>>  
>>  	q->sg_reserved_size = INT_MAX;
>>  
>> +	/* Protect q->elevator from elevator_change */
>> +	mutex_lock(&q->sysfs_lock);
>> +
>>  	/* init elevator */
>> -	if (elevator_init(q, NULL))
>> +	if (elevator_init(q, NULL)) {
>> +		mutex_unlock(&q->sysfs_lock);
>>  		return NULL;
>> +	}
>> +
>> +	mutex_unlock(&q->sysfs_lock);
>> +
>>  	return q;
>>  }
>>  EXPORT_SYMBOL(blk_init_allocated_queue);
>> diff --git a/block/elevator.c b/block/elevator.c
>> index 668394d..02d4390 100644
>> --- a/block/elevator.c
>> +++ b/block/elevator.c
>> @@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char
>>*name)
>>  	struct elevator_type *e = NULL;
>>  	int err;
>>  
>> +	/*
>> +	 * q->sysfs_lock must be held to provide mutual exclusion between
>> +	 * elevator_switch() and here.
>> +	 */
>> +	lockdep_assert_held(&q->sysfs_lock);
>> +
>>  	if (unlikely(q->elevator))
>>  		return 0;
>
>Hmm... why aren't we just changing elevator_init() to grab sysfs_lock
>where necessary?  

The locking cannot be moved into elevator_init() because it is called
from elevator_switch() path, where the request_queue's sysfs_lock is
already taken.

> It'd be more consistent with elevator_exit() that way.

What elevator_exit() locks is elevator_queue's sysfs_lock, not
request_queue's sysfs_lock. What we need here is request_queue's
sysfs_lock.

>Thanks.
>
>-- 
>Tejun


Thanks,
Tomoki Sekiyama

Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[Tejun Heo]

Hello,

On Mon, Sep 23, 2013 at 08:11:55PM +0000, Tomoki Sekiyama wrote:
> >Hmm... why aren't we just changing elevator_init() to grab sysfs_lock
> >where necessary?  
> 
> The locking cannot be moved into elevator_init() because it is called
> from elevator_switch() path, where the request_queue's sysfs_lock is
> already taken.
> 
> > It'd be more consistent with elevator_exit() that way.
> 
> What elevator_exit() locks is elevator_queue's sysfs_lock, not
> request_queue's sysfs_lock. What we need here is request_queue's
> sysfs_lock.

Ah, okay.

 Reviewed-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

[Tomoki Sekiyama]

Hi all,


Is this patchset going to be merged into 3.12?

Thanks,
-- 
Tomoki

On 9/23/13 16:14 , "Tejun Heo" <tj@kernel.org> wrote:

>Hello,
>
>On Mon, Sep 23, 2013 at 08:11:55PM +0000, Tomoki Sekiyama wrote:
>> >Hmm... why aren't we just changing elevator_init() to grab sysfs_lock
>> >where necessary?
>> 
>> The locking cannot be moved into elevator_init() because it is called
>> from elevator_switch() path, where the request_queue's sysfs_lock is
>> already taken.
>> 
>> > It'd be more consistent with elevator_exit() that way.
>> 
>> What elevator_exit() locks is elevator_queue's sysfs_lock, not
>> request_queue's sysfs_lock. What we need here is request_queue's
>> sysfs_lock.
>
>Ah, okay.
>
> Reviewed-by: Tejun Heo <tj@kernel.org>
>
>Thanks.
>
>-- 
>tejun