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SubjectRe: [PATCH] scsi/eh: fix hang adding ehandler wakeups after decrementing host_busy
Date
Hi. Please tell if there is something I can do to help the patch get 
processed? It is on the list without reply for almost a month.

On 09/05/2017 03:54 PM, Pavel Tikhomirov wrote:
> We have a problem on several our nodes with scsi EH. Imagine such an
> order of execution of two threads:
>
> CPU1 scsi_eh_scmd_add CPU2 scsi_host_queue_ready
> /* shost->host_busy == 1 initialy */
>
> if (shost->shost_state == SHOST_RECOVERY)
> /* does not get here */
> return 0;
>
> lock(shost->host_lock);
> shost->shost_state = SHOST_RECOVERY;
>
> busy = shost->host_busy++;
> /* host->can_queue == 1 initialy, busy == 1
> * - go to starved label */
> lock(shost->host_lock) /* wait */
>
> shost->host_failed++;
> /* shost->host_busy == 2, shost->host_failed == 1 */
> call scsi_eh_wakeup(shost) {
> if (host_busy == host_failed) {
> /* does not get here */
> wake_up_process(shost->ehandler)
> }
> }
> unlock(shost->host_lock)
>
> /* acquire lock */
> shost->host_busy--;
>
> Finaly we do not wakeup scsi_error_handler and all other commands
> coming will hang as we are in never ending recovery state as there
> is no one left to wakeup handler.
>
> So scsi disc in these host becomes unresponsive and all bio on node
> hangs. (We trigger these problem when scsi cmnds to DVD drive timeout.)
>
> Main idea of the fix is to try to do wake up every time we decrement
> host_busy or increment host_failed(the latter is already OK).
>
> Now the very *last* one of busy threads getting host_lock after
> decrementing host_busy will see all write operations on host's
> shost_state, host_busy and host_failed completed thanks to implied
> memory barriers on spin_lock/unlock, so at the time of busy==failed
> we will trigger wakeup in at least one thread. (Thats why putting
> recovery and failed checks under lock)
>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> ---
> drivers/scsi/scsi_lib.c | 21 +++++++++++++++++----
> 1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index f6097b89d5d3..6c99221d60aa 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -320,12 +320,11 @@ void scsi_device_unbusy(struct scsi_device *sdev)
> if (starget->can_queue > 0)
> atomic_dec(&starget->target_busy);
>
> + spin_lock_irqsave(shost->host_lock, flags);
> if (unlikely(scsi_host_in_recovery(shost) &&
> - (shost->host_failed || shost->host_eh_scheduled))) {
> - spin_lock_irqsave(shost->host_lock, flags);
> + (shost->host_failed || shost->host_eh_scheduled)))
> scsi_eh_wakeup(shost);
> - spin_unlock_irqrestore(shost->host_lock, flags);
> - }
> + spin_unlock_irqrestore(shost->host_lock, flags);
>
> atomic_dec(&sdev->device_busy);
> }
> @@ -1503,6 +1502,13 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
> spin_unlock_irq(shost->host_lock);
> out_dec:
> atomic_dec(&shost->host_busy);
> +
> + spin_lock_irq(shost->host_lock);
> + if (unlikely(scsi_host_in_recovery(shost) &&
> + (shost->host_failed || shost->host_eh_scheduled)))
> + scsi_eh_wakeup(shost);
> + spin_unlock_irq(shost->host_lock);
> +
> return 0;
> }
>
> @@ -1964,6 +1970,13 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
>
> out_dec_host_busy:
> atomic_dec(&shost->host_busy);
> +
> + spin_lock_irq(shost->host_lock);
> + if (unlikely(scsi_host_in_recovery(shost) &&
> + (shost->host_failed || shost->host_eh_scheduled)))
> + scsi_eh_wakeup(shost);
> + spin_unlock_irq(shost->host_lock);
> +
> out_dec_target_busy:
> if (scsi_target(sdev)->can_queue > 0)
> atomic_dec(&scsi_target(sdev)->target_busy);
>

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

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