In scsi core, __scsi_queue_insert should just put request back on
the queue and retry using the same command as before. However, for
blk-mq, scsi_mq_requeue_cmd is employed here which will unprepare
the request. To align with the semantics of __scsi_queue_insert,
use blk_mq_requeue_request with kick_requeue_list == true and put
the reference of scsi_device.
V1 -> V2:
- add put_device on scsi_device->sdev_gendev
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Jianchao Wang <[email protected]>
---
drivers/scsi/scsi_lib.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index a86df9c..6fa7b0c 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -191,7 +191,8 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
*/
cmd->result = 0;
if (q->mq_ops) {
- scsi_mq_requeue_cmd(cmd);
+ blk_mq_requeue_request(cmd->request, true);
+ put_device(&device->sdev_gendev);
return;
}
spin_lock_irqsave(q->queue_lock, flags);
--
2.7.4
On Wed, 2018-02-28 at 16:55 +0800, Jianchao Wang wrote:
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index a86df9c..6fa7b0c 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -191,7 +191,8 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
> */
> cmd->result = 0;
> if (q->mq_ops) {
> - scsi_mq_requeue_cmd(cmd);
> + blk_mq_requeue_request(cmd->request, true);
> + put_device(&device->sdev_gendev);
> return;
> }
> spin_lock_irqsave(q->queue_lock, flags);
Anyone who sees the put_device() call that follows the blk_mq_requeue_request()
call will wonder why that call occurs there. So I think we need a comment above
that call that explains where the matching get_device() call is.
For the legacy code path, there is a get_device() call in scsi_prep_fn() but no
put_device() call in scsi_unprep_fn() - the matching put_device() calls occur in
scsi_end_request() and after blk_requeue_request().
For scsi-mq however there is a get_device() call in scsi_mq_get_budget() and a
put_device() call in scsi_mq_put_budget(). So why do we need the put_device()
calls after blk_mq_requeue_request() and in the mq path for scsi_end_request()?
Thanks,
Bart.
Hi Bart
Thanks for your precious time to review this and kindly detailed response.
On 03/01/2018 01:52 AM, Bart Van Assche wrote:
> On Wed, 2018-02-28 at 16:55 +0800, Jianchao Wang wrote:
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index a86df9c..6fa7b0c 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -191,7 +191,8 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
>> */
>> cmd->result = 0;
>> if (q->mq_ops) {
>> - scsi_mq_requeue_cmd(cmd);
>> + blk_mq_requeue_request(cmd->request, true);
>> + put_device(&device->sdev_gendev);
>> return;
>> }
>> spin_lock_irqsave(q->queue_lock, flags);
>
> Anyone who sees the put_device() call that follows the blk_mq_requeue_request()
> call will wonder why that call occurs there. So I think we need a comment above
> that call that explains where the matching get_device() call is.
Yes, I will add this.
> For the legacy code path, there is a get_device() call in scsi_prep_fn() but no
> put_device() call in scsi_unprep_fn() - the matching put_device() calls occur in
> scsi_end_request() and after blk_requeue_request().
>
> For scsi-mq however there is a get_device() call in scsi_mq_get_budget() and a
> put_device() call in scsi_mq_put_budget(). So why do we need the put_device()
> calls after blk_mq_requeue_request() and in the mq path for scsi_end_request()?
>
From the source code, we know the scsi_mq_get_budget will be invoked every time when we issue a request.
But scsi_mq_put_budget is just in the fail path.
scsi_queue_rq // if any error
-> scsi_mq_put_budget
blk_mq_dispatch_rq_list // if no driver tags
-> blk_mq_put_dispatch_budget
-> scsi_mq_put_budget
blk_mq_do_dispatch_sched/blk_mq_do_dispatch_ctx // if no requests
-> blk_mq_put_dispatch_budget
-> scsi_mq_put_budget
So we have to add put_device after blk_mq_requeue_request() and in scsi_end_request() to match the
scsi_mq_get_budget.
Thanks
Jianchao
On Thu, 2018-03-01 at 09:57 +0800, jianchao.wang wrote:
> On 03/01/2018 01:52 AM, Bart Van Assche wrote:
> > On Wed, 2018-02-28 at 16:55 +0800, Jianchao Wang wrote:
> > > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > > index a86df9c..6fa7b0c 100644
> > > --- a/drivers/scsi/scsi_lib.c
> > > +++ b/drivers/scsi/scsi_lib.c
> > > @@ -191,7 +191,8 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
> > > */
> > > cmd->result = 0;
> > > if (q->mq_ops) {
> > > - scsi_mq_requeue_cmd(cmd);
> > > + blk_mq_requeue_request(cmd->request, true);
> > > + put_device(&device->sdev_gendev);
> > > return;
> > > }
> > > spin_lock_irqsave(q->queue_lock, flags);
> >
> > Anyone who sees the put_device() call that follows the blk_mq_requeue_request()
> > call will wonder why that call occurs there. So I think we need a comment above
> > that call that explains where the matching get_device() call is.
>
> Yes, I will add this.
>
> > For the legacy code path, there is a get_device() call in scsi_prep_fn() but no
> > put_device() call in scsi_unprep_fn() - the matching put_device() calls occur in
> > scsi_end_request() and after blk_requeue_request().
> >
> > For scsi-mq however there is a get_device() call in scsi_mq_get_budget() and a
> > put_device() call in scsi_mq_put_budget(). So why do we need the put_device()
> > calls after blk_mq_requeue_request() and in the mq path for scsi_end_request()?
> >
>
> From the source code, we know the scsi_mq_get_budget will be invoked every time
> when we issue a request. But scsi_mq_put_budget is just in the fail path.
>
> scsi_queue_rq // if any error
> -> scsi_mq_put_budget
>
> blk_mq_dispatch_rq_list // if no driver tags
> -> blk_mq_put_dispatch_budget
> -> scsi_mq_put_budget
> blk_mq_do_dispatch_sched/blk_mq_do_dispatch_ctx // if no requests
> -> blk_mq_put_dispatch_budget
> -> scsi_mq_put_budget
>
> So we have to add put_device after blk_mq_requeue_request() and in
> scsi_end_request() to match the scsi_mq_get_budget.
Hello Jianchao,
Yes, the block layer core guarantees that scsi_mq_get_budget() will be called
before scsi_queue_rq(). I think the full picture is as follows:
* Before scsi_queue_rq() calls .queuecommand(), get_device() is called for the
SCSI device and the device, target and host busy counters are incremented.
* If the SCSI core decides to requeue a command, scsi_queue_insert() causes
__scsi_queue_insert() to call scsi_device_unbusy(). That last function
decreases the device, target and host busy counters but not put_device(sdev).
Hence the need for a separate put_device() call after requeuing.
It's unfortunate that the SCSI core became so asymmetric. Anyway, since I am
now convinced that this patch is correct, feel free to add:
Reviewed-by: Bart Van Assche <[email protected]>
Jianchao,
> In scsi core, __scsi_queue_insert should just put request back on the
> queue and retry using the same command as before. However, for blk-mq,
> scsi_mq_requeue_cmd is employed here which will unprepare the
> request. To align with the semantics of __scsi_queue_insert, use
> blk_mq_requeue_request with kick_requeue_list == true and put the
> reference of scsi_device.
>
> V1 -> V2:
> - add put_device on scsi_device->sdev_gendev
Also, please put changelog after the --- delimiter.
> Cc: Christoph Hellwig <[email protected]>
> Signed-off-by: Jianchao Wang <[email protected]>
> ---
> drivers/scsi/scsi_lib.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index a86df9c..6fa7b0c 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -191,7 +191,8 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
> */
> cmd->result = 0;
> if (q->mq_ops) {
> - scsi_mq_requeue_cmd(cmd);
> + blk_mq_requeue_request(cmd->request, true);
> + put_device(&device->sdev_gendev);
> return;
> }
> spin_lock_irqsave(q->queue_lock, flags);
--
Martin K. Petersen Oracle Linux Engineering
Jianchao,
> Yes, the block layer core guarantees that scsi_mq_get_budget() will be
> called before scsi_queue_rq(). I think the full picture is as follows:
> * Before scsi_queue_rq() calls .queuecommand(), get_device() is called for the
> SCSI device and the device, target and host busy counters are incremented.
> * If the SCSI core decides to requeue a command, scsi_queue_insert() causes
> __scsi_queue_insert() to call scsi_device_unbusy(). That last function
> decreases the device, target and host busy counters but not put_device(sdev).
> Hence the need for a separate put_device() call after requeuing.
>
> It's unfortunate that the SCSI core became so asymmetric. Anyway,
> since I am now convinced that this patch is correct, feel free to add:
Please add something akin to Bart's explanation as a comment and repost.
Thanks!
--
Martin K. Petersen Oracle Linux Engineering
Hi martin
Thanks for your kindly response.
On 03/02/2018 09:43 AM, Martin K. Petersen wrote:
>
> Jianchao,
>
>> Yes, the block layer core guarantees that scsi_mq_get_budget() will be
>> called before scsi_queue_rq(). I think the full picture is as follows:
>
>> * Before scsi_queue_rq() calls .queuecommand(), get_device() is called for the
>> SCSI device and the device, target and host busy counters are incremented.
>> * If the SCSI core decides to requeue a command, scsi_queue_insert() causes
>> __scsi_queue_insert() to call scsi_device_unbusy(). That last function
>> decreases the device, target and host busy counters but not put_device(sdev).
>> Hence the need for a separate put_device() call after requeuing.
>>
>> It's unfortunate that the SCSI core became so asymmetric. Anyway,
>> since I am now convinced that this patch is correct, feel free to add:
>
> Please add something akin to Bart's explanation as a comment and repost.
yes, sure.
Thanks
Jianchao
Hi martin
On 03/02/2018 09:44 AM, Martin K. Petersen wrote:
>> In scsi core, __scsi_queue_insert should just put request back on the
>> queue and retry using the same command as before. However, for blk-mq,
>> scsi_mq_requeue_cmd is employed here which will unprepare the
>> request. To align with the semantics of __scsi_queue_insert, use
>> blk_mq_requeue_request with kick_requeue_list == true and put the
>> reference of scsi_device.
>>
>> V1 -> V2:
>> - add put_device on scsi_device->sdev_gendev
> Also, please put changelog after the --- delimiter.
>
Yes, I will modify this next version.
Thanks
Jianchao
Hi Bart
Thanks for your precious time and detailed summary.
On 03/02/2018 01:43 AM, Bart Van Assche wrote:
> Yes, the block layer core guarantees that scsi_mq_get_budget() will be called
> before scsi_queue_rq(). I think the full picture is as follows:
> * Before scsi_queue_rq() calls .queuecommand(), get_device() is called for the
> SCSI device and the device, target and host busy counters are incremented.
Supply some details here:
scsi_mq_get_budget before calling .queuecommand get_device and increase device_busy.
scsi_queue_rq increases target_busy and host_busy.
> * If the SCSI core decides to requeue a command, scsi_queue_insert() causes
> __scsi_queue_insert() to call scsi_device_unbusy(). That last function
> decreases the device, target and host busy counters but not put_device(sdev).
> Hence the need for a separate put_device() call after requeuing.
>
> It's unfortunate that the SCSI core became so asymmetric. Anyway, since I am
> now convinced that this patch is correct, feel free to add:
>
> Reviewed-by: Bart Van Assche <[email protected]>
Sincerely
Jianchao