[PATCH v3 0/2] Ensure that the SCSI error handler gets woken up

[PATCH v3 0/2] Ensure that the SCSI error handler gets woken up

[Bart Van Assche]

Hello Martin,

As reported by Pavel Tikhomirov it can happen that the SCSI error handler does
not get woken up. This is very annoying because it results in a queue
stall. The two patches in this series address this issue without acquiring the
SCSI host lock in the hot path. Please consider these patches for kernel
v4.16.

Thanks,

Bart.

Changes between v2 and v3:
- Made it again safe to call scsi_eh_scmd_add() from interrupt context.

Changes between v1 and v2:
- Ensure that host_lock is held while checking host_failed.
- Moved the lockdep_assert_held() change into a separate patch.

Bart Van Assche (2):
  Ensure that the SCSI error handler gets woken up
  Convert a source code comment into a runtime check

 drivers/scsi/hosts.c      |  6 ++++++
 drivers/scsi/scsi_error.c | 21 ++++++++++++++++++---
 drivers/scsi/scsi_lib.c   | 39 ++++++++++++++++++++++++++++-----------
 include/scsi/scsi_host.h  |  2 ++
 4 files changed, 54 insertions(+), 14 deletions(-)

-- 
2.15.0

[PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Bart Van Assche]

If scsi_eh_scmd_add() is called concurrently with
scsi_host_queue_ready() while shost->host_blocked > 0 then it can
happen that neither function wakes up the SCSI error handler. Fix
this by making every function that decreases the host_busy counter
wake up the error handler if necessary and by protecting the
host_failed checks with the SCSI host lock.

Reported-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
References: https://marc.info/?l=linux-kernel&m=150461610630736
Fixes: commit 746650160866 ("scsi: convert host_busy to atomic_t")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Konstantin Khorenko <khorenko@virtuozzo.com>
Cc: Stuart Hayes <stuart.w.hayes@gmail.com>
Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: <stable@vger.kernel.org>
---
 drivers/scsi/hosts.c      |  6 ++++++
 drivers/scsi/scsi_error.c | 18 ++++++++++++++++--
 drivers/scsi/scsi_lib.c   | 39 ++++++++++++++++++++++++++++-----------
 include/scsi/scsi_host.h  |  2 ++
 4 files changed, 52 insertions(+), 13 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index a306af6a5ea7..a0a7e4ff255c 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -324,6 +324,9 @@ static void scsi_host_dev_release(struct device *dev)
 
 	scsi_proc_hostdir_rm(shost->hostt);
 
+	/* Wait for functions invoked through call_rcu(&shost->rcu, ...) */
+	rcu_barrier();
+
 	if (shost->tmf_work_q)
 		destroy_workqueue(shost->tmf_work_q);
 	if (shost->ehandler)
@@ -331,6 +334,8 @@ static void scsi_host_dev_release(struct device *dev)
 	if (shost->work_q)
 		destroy_workqueue(shost->work_q);
 
+	destroy_rcu_head(&shost->rcu);
+
 	if (shost->shost_state == SHOST_CREATED) {
 		/*
 		 * Free the shost_dev device name here if scsi_host_alloc()
@@ -399,6 +404,7 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
 	INIT_LIST_HEAD(&shost->starved_list);
 	init_waitqueue_head(&shost->host_wait);
 	mutex_init(&shost->scan_mutex);
+	init_rcu_head(&shost->rcu);
 
 	index = ida_simple_get(&host_index_ida, 0, 0, GFP_KERNEL);
 	if (index < 0)
diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 5e89049e9b4e..258b8a741992 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -226,6 +226,17 @@ static void scsi_eh_reset(struct scsi_cmnd *scmd)
 	}
 }
 
+static void scsi_eh_inc_host_failed(struct rcu_head *head)
+{
+	struct Scsi_Host *shost = container_of(head, typeof(*shost), rcu);
+	unsigned long flags;
+
+	spin_lock_irqsave(shost->host_lock, flags);
+	shost->host_failed++;
+	scsi_eh_wakeup(shost);
+	spin_unlock_irqrestore(shost->host_lock, flags);
+}
+
 /**
  * scsi_eh_scmd_add - add scsi cmd to error handling.
  * @scmd:	scmd to run eh on.
@@ -248,9 +259,12 @@ void scsi_eh_scmd_add(struct scsi_cmnd *scmd)
 
 	scsi_eh_reset(scmd);
 	list_add_tail(&scmd->eh_entry, &shost->eh_cmd_q);
-	shost->host_failed++;
-	scsi_eh_wakeup(shost);
 	spin_unlock_irqrestore(shost->host_lock, flags);
+	/*
+	 * Ensure that all tasks observe the host state change before the
+	 * host_failed change.
+	 */
+	call_rcu(&shost->rcu, scsi_eh_inc_host_failed);
 }
 
 /**
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index b6d3842b6809..5cbc69b2b1ae 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -318,22 +318,39 @@ static void scsi_init_cmd_errh(struct scsi_cmnd *cmd)
 		cmd->cmd_len = scsi_command_size(cmd->cmnd);
 }
 
-void scsi_device_unbusy(struct scsi_device *sdev)
+/*
+ * Decrement the host_busy counter and wake up the error handler if necessary.
+ * Avoid as follows that the error handler is not woken up if shost->host_busy
+ * == shost->host_failed: use call_rcu() in scsi_eh_scmd_add() in combination
+ * with an RCU read lock in this function to ensure that this function in its
+ * entirety either finishes before scsi_eh_scmd_add() increases the
+ * host_failed counter or that it notices the shost state change made by
+ * scsi_eh_scmd_add().
+ */
+static void scsi_dec_host_busy(struct Scsi_Host *shost)
 {
-	struct Scsi_Host *shost = sdev->host;
-	struct scsi_target *starget = scsi_target(sdev);
 	unsigned long flags;
 
+	rcu_read_lock();
 	atomic_dec(&shost->host_busy);
-	if (starget->can_queue > 0)
-		atomic_dec(&starget->target_busy);
-
-	if (unlikely(scsi_host_in_recovery(shost) &&
-		     (shost->host_failed || shost->host_eh_scheduled))) {
+	if (unlikely(scsi_host_in_recovery(shost))) {
 		spin_lock_irqsave(shost->host_lock, flags);
-		scsi_eh_wakeup(shost);
+		if (shost->host_failed || shost->host_eh_scheduled)
+			scsi_eh_wakeup(shost);
 		spin_unlock_irqrestore(shost->host_lock, flags);
 	}
+	rcu_read_unlock();
+}
+
+void scsi_device_unbusy(struct scsi_device *sdev)
+{
+	struct Scsi_Host *shost = sdev->host;
+	struct scsi_target *starget = scsi_target(sdev);
+
+	scsi_dec_host_busy(shost);
+
+	if (starget->can_queue > 0)
+		atomic_dec(&starget->target_busy);
 
 	atomic_dec(&sdev->device_busy);
 }
@@ -1531,7 +1548,7 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
 		list_add_tail(&sdev->starved_entry, &shost->starved_list);
 	spin_unlock_irq(shost->host_lock);
 out_dec:
-	atomic_dec(&shost->host_busy);
+	scsi_dec_host_busy(shost);
 	return 0;
 }
 
@@ -2017,7 +2034,7 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
 	return BLK_STS_OK;
 
 out_dec_host_busy:
-       atomic_dec(&shost->host_busy);
+	scsi_dec_host_busy(shost);
 out_dec_target_busy:
 	if (scsi_target(sdev)->can_queue > 0)
 		atomic_dec(&scsi_target(sdev)->target_busy);
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index a8b7bf879ced..1a1df0d21ee3 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -571,6 +571,8 @@ struct Scsi_Host {
 		struct blk_mq_tag_set	tag_set;
 	};
 
+	struct rcu_head rcu;
+
 	atomic_t host_busy;		   /* commands actually active on low-level */
 	atomic_t host_blocked;
 
-- 
2.15.0

[PATCH v3 2/2] Convert a source code comment into a runtime check

[Bart Van Assche]

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
---
 drivers/scsi/scsi_error.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 258b8a741992..9cae0194e21a 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -61,9 +61,10 @@ static int scsi_eh_try_stu(struct scsi_cmnd *scmd);
 static int scsi_try_to_abort_cmd(struct scsi_host_template *,
 				 struct scsi_cmnd *);
 
-/* called with shost->host_lock held */
 void scsi_eh_wakeup(struct Scsi_Host *shost)
 {
+	lockdep_assert_held(shost->host_lock);
+
 	if (atomic_read(&shost->host_busy) == shost->host_failed) {
 		trace_scsi_eh_wakeup(shost);
 		wake_up_process(shost->ehandler);
-- 
2.15.0

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Pavel Tikhomirov]

On 12/04/2017 09:06 PM, Bart Van Assche wrote:
> If scsi_eh_scmd_add() is called concurrently with
> scsi_host_queue_ready() while shost->host_blocked > 0 then it can
> happen that neither function wakes up the SCSI error handler. Fix
> this by making every function that decreases the host_busy counter
> wake up the error handler if necessary and by protecting the
> host_failed checks with the SCSI host lock.
> 
> Reported-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> References: https://marc.info/?l=linux-kernel&m=150461610630736
> Fixes: commit 746650160866 ("scsi: convert host_busy to atomic_t")
> Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com> > Cc: Konstantin Khorenko <khorenko@virtuozzo.com>
> Cc: Stuart Hayes <stuart.w.hayes@gmail.com>
> Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Johannes Thumshirn <jthumshirn@suse.de>
> Cc: <stable@vger.kernel.org>
> ---
>   drivers/scsi/hosts.c      |  6 ++++++
>   drivers/scsi/scsi_error.c | 18 ++++++++++++++++--
>   drivers/scsi/scsi_lib.c   | 39 ++++++++++++++++++++++++++++-----------
>   include/scsi/scsi_host.h  |  2 ++
>   4 files changed, 52 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index a306af6a5ea7..a0a7e4ff255c 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -324,6 +324,9 @@ static void scsi_host_dev_release(struct device *dev)
>   
>   	scsi_proc_hostdir_rm(shost->hostt);
>   
> +	/* Wait for functions invoked through call_rcu(&shost->rcu, ...) */
> +	rcu_barrier();
> +
>   	if (shost->tmf_work_q)
>   		destroy_workqueue(shost->tmf_work_q);
>   	if (shost->ehandler)
> @@ -331,6 +334,8 @@ static void scsi_host_dev_release(struct device *dev)
>   	if (shost->work_q)
>   		destroy_workqueue(shost->work_q);
>   
> +	destroy_rcu_head(&shost->rcu);
> +
>   	if (shost->shost_state == SHOST_CREATED) {
>   		/*
>   		 * Free the shost_dev device name here if scsi_host_alloc()
> @@ -399,6 +404,7 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
>   	INIT_LIST_HEAD(&shost->starved_list);
>   	init_waitqueue_head(&shost->host_wait);
>   	mutex_init(&shost->scan_mutex);
> +	init_rcu_head(&shost->rcu);
>   
>   	index = ida_simple_get(&host_index_ida, 0, 0, GFP_KERNEL);
>   	if (index < 0)
> diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
> index 5e89049e9b4e..258b8a741992 100644
> --- a/drivers/scsi/scsi_error.c
> +++ b/drivers/scsi/scsi_error.c
> @@ -226,6 +226,17 @@ static void scsi_eh_reset(struct scsi_cmnd *scmd)
>   	}
>   }
>   
> +static void scsi_eh_inc_host_failed(struct rcu_head *head)
> +{
> +	struct Scsi_Host *shost = container_of(head, typeof(*shost), rcu);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(shost->host_lock, flags);
> +	shost->host_failed++;

May be we need to increment host_failed before call_rcu(), so that all 
rcu protected readers already see a change at these point?

> +	scsi_eh_wakeup(shost);
> +	spin_unlock_irqrestore(shost->host_lock, flags);
> +}
> +
>   /**
>    * scsi_eh_scmd_add - add scsi cmd to error handling.
>    * @scmd:	scmd to run eh on.
> @@ -248,9 +259,12 @@ void scsi_eh_scmd_add(struct scsi_cmnd *scmd)
>   
>   	scsi_eh_reset(scmd);
>   	list_add_tail(&scmd->eh_entry, &shost->eh_cmd_q);
> -	shost->host_failed++;
> -	scsi_eh_wakeup(shost);
>   	spin_unlock_irqrestore(shost->host_lock, flags);
> +	/*
> +	 * Ensure that all tasks observe the host state change before the
> +	 * host_failed change.
> +	 */
> +	call_rcu(&shost->rcu, scsi_eh_inc_host_failed);
>   }
>   
>   /**
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index b6d3842b6809..5cbc69b2b1ae 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -318,22 +318,39 @@ static void scsi_init_cmd_errh(struct scsi_cmnd *cmd)
>   		cmd->cmd_len = scsi_command_size(cmd->cmnd);
>   }
>   
> -void scsi_device_unbusy(struct scsi_device *sdev)
> +/*
> + * Decrement the host_busy counter and wake up the error handler if necessary.
> + * Avoid as follows that the error handler is not woken up if shost->host_busy
> + * == shost->host_failed: use call_rcu() in scsi_eh_scmd_add() in combination
> + * with an RCU read lock in this function to ensure that this function in its
> + * entirety either finishes before scsi_eh_scmd_add() increases the
> + * host_failed counter or that it notices the shost state change made by
> + * scsi_eh_scmd_add().
> + */
> +static void scsi_dec_host_busy(struct Scsi_Host *shost)
>   {
> -	struct Scsi_Host *shost = sdev->host;
> -	struct scsi_target *starget = scsi_target(sdev);
>   	unsigned long flags;
>   
> +	rcu_read_lock();
>   	atomic_dec(&shost->host_busy);
> -	if (starget->can_queue > 0)
> -		atomic_dec(&starget->target_busy);
> -
> -	if (unlikely(scsi_host_in_recovery(shost) &&
> -		     (shost->host_failed || shost->host_eh_scheduled))) {
> +	if (unlikely(scsi_host_in_recovery(shost))) {
>   		spin_lock_irqsave(shost->host_lock, flags);
> -		scsi_eh_wakeup(shost);
> +		if (shost->host_failed || shost->host_eh_scheduled)
> +			scsi_eh_wakeup(shost);
>   		spin_unlock_irqrestore(shost->host_lock, flags);
>   	}
> +	rcu_read_unlock();
> +}
> +
> +void scsi_device_unbusy(struct scsi_device *sdev)
> +{
> +	struct Scsi_Host *shost = sdev->host;
> +	struct scsi_target *starget = scsi_target(sdev);
> +
> +	scsi_dec_host_busy(shost);
> +
> +	if (starget->can_queue > 0)
> +		atomic_dec(&starget->target_busy);
>   
>   	atomic_dec(&sdev->device_busy);
>   }
> @@ -1531,7 +1548,7 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
>   		list_add_tail(&sdev->starved_entry, &shost->starved_list);
>   	spin_unlock_irq(shost->host_lock);
>   out_dec:
> -	atomic_dec(&shost->host_busy);
> +	scsi_dec_host_busy(shost);
>   	return 0;
>   }
>   
> @@ -2017,7 +2034,7 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
>   	return BLK_STS_OK;
>   
>   out_dec_host_busy:
> -       atomic_dec(&shost->host_busy);
> +	scsi_dec_host_busy(shost);
>   out_dec_target_busy:
>   	if (scsi_target(sdev)->can_queue > 0)
>   		atomic_dec(&scsi_target(sdev)->target_busy);
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index a8b7bf879ced..1a1df0d21ee3 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -571,6 +571,8 @@ struct Scsi_Host {
>   		struct blk_mq_tag_set	tag_set;
>   	};
>   
> +	struct rcu_head rcu;
> +
>   	atomic_t host_busy;		   /* commands actually active on low-level */
>   	atomic_t host_blocked;
>   
> 

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Bart Van Assche]

On Tue, 2017-12-05 at 13:18 +0300, Pavel Tikhomirov wrote:
> On 12/04/2017 09:06 PM, Bart Van Assche wrote:
> > +static void scsi_eh_inc_host_failed(struct rcu_head *head)
> > +{
> > +	struct Scsi_Host *shost = container_of(head, typeof(*shost), rcu);
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(shost->host_lock, flags);
> > +	shost->host_failed++;
> 
> May be we need to increment host_failed before call_rcu(), so that all 
> rcu protected readers already see a change at these point?

Sorry but I don't think that would work. If host_failed would be changed first
then it can happen that scsi_dec_host_busy() encounters that the host state is
"running" and hence that it skips the host_failed check. That can result in a
missed error handler wakeup, which is what we want to avoid.

Bart.

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Bart Van Assche]

On Wed, 2017-12-06 at 00:17 +0300, ptikhomirov wrote:
> I mean threads in scsi_dec_host_busy() the part under rcu_read_lock are
> divided into two groups: a) finished before call_rcu, b) beginning rcu
> section after call_rcu. So first, in scsi_eh_inc_host_failed() we will
> see all changes to host busy from group (a), second, all threads in group
> (b) will see our change to host_failed. Either there is nobody in (b) and
> we will start EH, or the thread from (b) which entered spin_lock last will
> start EH.
> 
> In your case tasks from b does not see host_failed was incremented, and
> will not start EH.

Hello Pavel,

What does "your case" mean? In my previous e-mail I explained a scenario that
cannot happen so it's not clear to me what "your case" refers to?

Additionally, it seems like you are assuming that RCU guarantees ordering of
RCU read-locked sections against call_rcu()? That's not how RCU works. RCU
guarantees serialization of read-locked sections against grace periods. The
function scsi_eh_inc_host_failed() is invoked through call_rcu() and hence
will be called during a grace period.

Anyway, the different scenarios I see are as follows:
(a) scsi_dec_host_busy() finishes before scsi_eh_inc_host_failed() starts.
(b) scsi_dec_host_busy() starts after scsi_eh_inc_host_failed() has
    finished.

In case (a) scsi_eh_inc_host_failed() will wake up the error handler. And in
case (b) scsi_dec_host_busy() will wake up the error handler. So it's not
clear to me why you think that there is a scenario in which the EH won't be
woken up?

Bart.

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Pavel Tikhomirov]

Hello, Bart!

On 12/06/2017 12:46 AM, Bart Van Assche wrote:
> On Wed, 2017-12-06 at 00:17 +0300, ptikhomirov wrote:
>> I mean threads in scsi_dec_host_busy() the part under rcu_read_lock are
>> divided into two groups: a) finished before call_rcu, b) beginning rcu
>> section after call_rcu. So first, in scsi_eh_inc_host_failed() we will
>> see all changes to host busy from group (a), second, all threads in group
>> (b) will see our change to host_failed. Either there is nobody in (b) and
>> we will start EH, or the thread from (b) which entered spin_lock last will
>> start EH.
>>
>> In your case tasks from b does not see host_failed was incremented, and
>> will not start EH.
>
> Hello Pavel,
>
> What does "your case" mean? In my previous e-mail I explained a scenario that
> cannot happen so it's not clear to me what "your case" refers to?

By "your case" I meant how your code works, especially that host_failed 
increment is inside scsi_eh_inc_host_failed() in your patch.

>
> Additionally, it seems like you are assuming that RCU guarantees ordering of
> RCU read-locked sections against call_rcu()?

Sorry.. Not "call_rcu" itself, In my previous reply I meant the delayed 
callback which actually triggers. (s/call_rcu/call_rcu's callback start/)

> That's not how RCU works. RCU
> guarantees serialization of read-locked sections against grace periods. The
> function scsi_eh_inc_host_failed() is invoked through call_rcu() and hence
> will be called during a grace period.

May be I missunderstand something, but I think that callback from 
call_rcu is guaranteed to _start_ after a grace period, but not to end 
before a next grace period. Other threads can enter rcu_read_lock 
protected critical regions while we are still in a callback and will run 
concurrently with callback.

>
> Anyway, the different scenarios I see are as follows:
> (a) scsi_dec_host_busy() finishes before scsi_eh_inc_host_failed() starts.
> (b) scsi_dec_host_busy() starts after scsi_eh_inc_host_failed() has
>      finished.

So I think in (b) scsi_dec_host_busy starts after 
scsi_eh_inc_host_failed has _started_.

>
> In case (a) scsi_eh_inc_host_failed() will wake up the error handler. And in
> case (b) scsi_dec_host_busy() will wake up the error handler. So it's not
> clear to me why you think that there is a scenario in which the EH won't be
> woken up?

So in case (b), in my understanding, scsi_dec_host_busy can skip wakeups 
as it does not see host_failed change yet.

>
> Bart.
>

To prove my point, some parts of kernel docs:

"This function invokes func(head) after a grace period has elapsed." 
Documentation/RCU/whatisRCU.txt

"
15.     The whole point of call_rcu(), synchronize_rcu(), and friends
         is to wait until all pre-existing readers have finished before
         carrying out some otherwise-destructive operation.
...
         Because these primitives only wait for pre-existing readers, it
         is the caller's responsibility to guarantee that any subsequent
         readers will execute safely.
"
Documentation/RCU/checklist.txt

There is nothing about "callback ends before next grace period".

Sorry, if I'm missing something.

Pavel.

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Bart Van Assche]

On Wed, 2017-12-06 at 01:49 +0300, Pavel Tikhomirov wrote:
> On 12/06/2017 12:46 AM, Bart Van Assche wrote:
> > Anyway, the different scenarios I see are as follows:
> > (a) scsi_dec_host_busy() finishes before scsi_eh_inc_host_failed() starts.
> > (b) scsi_dec_host_busy() starts after scsi_eh_inc_host_failed() has
> >      finished.
> 
> So I think in (b) scsi_dec_host_busy starts after scsi_eh_inc_host_failed
> has _started_.

Agreed, and that's fine, since in that case the SCSI host state has alread been
modified and hence both functions will obtain the SCSI host lock. The relevant
code will be serialized through the SCSI host lock.

> > In case (a) scsi_eh_inc_host_failed() will wake up the error handler. And in
> > case (b) scsi_dec_host_busy() will wake up the error handler. So it's not
> > clear to me why you think that there is a scenario in which the EH won't be
> > woken up?
> 
> So in case (b), in my understanding, scsi_dec_host_busy can skip wakeups 
> as it does not see host_failed change yet.

That's not correct. If scsi_dec_host_busy() obtains the SCSI host lock before
scsi_eh_inc_host_failed() obtains it then the latter function will trigger a
SCSI EH wakeup.

Bart.

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Pavel Tikhomirov]

>>> In case (a) scsi_eh_inc_host_failed() will wake up the error handler. And in
>>> case (b) scsi_dec_host_busy() will wake up the error handler. So it's not
>>> clear to me why you think that there is a scenario in which the EH won't be
>>> woken up?
>>
>> So in case (b), in my understanding, scsi_dec_host_busy can skip wakeups
>> as it does not see host_failed change yet.
> 
> That's not correct. If scsi_dec_host_busy() obtains the SCSI host lock before
> scsi_eh_inc_host_failed() obtains it then the latter function will trigger a
> SCSI EH wakeup.

You are right! Thanks a lot for pointing that out! Now when I understand 
it, your patch looks good for me:

Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

By the way, I very much like your idea of using rcu for these case.

Thanks, Pavel.

> 
> Bart.
>

Re: [PATCH v3 0/2] Ensure that the SCSI error handler gets woken up

[Martin K. Petersen]

Bart,

> As reported by Pavel Tikhomirov it can happen that the SCSI error
> handler does not get woken up. This is very annoying because it
> results in a queue stall. The two patches in this series address this
> issue without acquiring the SCSI host lock in the hot path. Please
> consider these patches for kernel v4.16.

Applied to 4.16/scsi-queue. Thank you!

-- 
Martin K. Petersen	Oracle Linux Engineering

Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Stuart Hayes]

>>>> In case (a) scsi_eh_inc_host_failed() will wake up the error handler. And in
>>>> case (b) scsi_dec_host_busy() will wake up the error handler. So it's not
>>>> clear to me why you think that there is a scenario in which the EH won't be
>>>> woken up?
>>>
>>> So in case (b), in my understanding, scsi_dec_host_busy can skip wakeups
>>> as it does not see host_failed change yet.
>>
>> That's not correct. If scsi_dec_host_busy() obtains the SCSI host lock before
>> scsi_eh_inc_host_failed() obtains it then the latter function will trigger a
>> SCSI EH wakeup.
> 
> You are right! Thanks a lot for pointing that out! Now when I understand it, your patch looks good for me:
> 
> Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> 
> By the way, I very much like your idea of using rcu for these case.
> 
> Thanks, Pavel.
> 

This patch tests ok on my system, too... it's run for over 24 hours, on a system that typically fails within ten minutes without the patch...

Tested-by: Stuart Hayes <stuart.w.hayes@gmail.com>

Thanks,
Stuart


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Re: [PATCH v3 0/2] Ensure that the SCSI error handler gets woken up

[John Garry]

On 07/12/2017 01:55, Martin K. Petersen wrote:
>
> Bart,
>
>> As reported by Pavel Tikhomirov it can happen that the SCSI error
>> handler does not get woken up. This is very annoying because it
>> results in a queue stall. The two patches in this series address this
>> issue without acquiring the SCSI host lock in the hot path. Please
>> consider these patches for kernel v4.16.
>
> Applied to 4.16/scsi-queue. Thank you!
>

Is anyone finding a build error for this patch? I built allmodconfig on 
Martin's 4.16/scsi-queue and I get this:
ERROR: "init_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
ERROR: "destroy_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!

Reverting fixes it.

 From a quick check, it seems the rcu funcitons are not exported, and 
SCSI=m means hosts.c cannot reference it.

Cheers,
John

Re: [PATCH v3 0/2] Ensure that the SCSI error handler gets woken up

[Bart Van Assche]

On Thu, 2017-12-07 at 12:02 +0000, John Garry wrote:
> On 07/12/2017 01:55, Martin K. Petersen wrote:
> > 
> > Bart,
> > 
> > > As reported by Pavel Tikhomirov it can happen that the SCSI error
> > > handler does not get woken up. This is very annoying because it
> > > results in a queue stall. The two patches in this series address this
> > > issue without acquiring the SCSI host lock in the hot path. Please
> > > consider these patches for kernel v4.16.
> > 
> > Applied to 4.16/scsi-queue. Thank you!
> > 
> 
> Is anyone finding a build error for this patch? I built allmodconfig on 
> Martin's 4.16/scsi-queue and I get this:
> ERROR: "init_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
> ERROR: "destroy_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
> 
> Reverting fixes it.
> 
>  From a quick check, it seems the rcu funcitons are not exported, and 
> SCSI=m means hosts.c cannot reference it.

Hello John,

A discussion is ongoing about this issue on the linux-next mailing list.
Paul E. McKenney proposed to export both the init_rcu_head() and
destroy_rcu_head() functions. See also https://lkml.org/lkml/2017/12/6/1150.

Bart.