Re: [PATCH v6 32/37] cxlflash: Fix to avoid potential deadlock on EEH

[Tomas Henzl <thenzl@redhat.com>]

On 21.10.2015 22:15, Matthew R. Ochs wrote:
> Ioctl threads that use scsi_execute() can run for an excessive amount
> of time due to the fact that they have lengthy timeouts and retry logic
> built in. Under normal operation this is not an issue. However, once EEH
> enters the picture, a long execution time coupled with the possibility
> that a timeout can trigger entry to the driver via registered reset
> callbacks becomes a liability.
>
> In particular, a deadlock can occur when an EEH event is encountered
> while in running in scsi_execute(). As part of the recovery, the EEH
> handler drains all currently running ioctls, waiting until they have
> completed before proceeding with a reset. As the scsi_execute()'s are
> situated on the ioctl path, the EEH handler will wait until they (and
> the remainder of the ioctl handler they're associated with) have
> completed. Normally this would not be much of an issue aside from the
> longer recovery period. Unfortunately, the scsi_execute() triggers a
> reset when it times out. The reset handler will see that the device is
> already being reset and wait until that reset completed. This creates
> a condition where the EEH handler becomes stuck, infinitely waiting for
> the ioctl thread to complete.
>
> To avoid this behavior, temporarily unmark the scsi_execute() threads
> as an ioctl thread by releasing the ioctl read semaphore. This allows
> the EEH handler to proceed with a recovery while the thread is still
> running. Once the scsi_execute() returns, the ioctl read semaphore is
> reacquired and the adapter state is rechecked in case it changed while
> inside of scsi_execute(). The state check will wait if the adapter is
> still being recovered or returns a failure if the recovery failed. In
> the event that the adapter reset failed, the failure is simply returned
> as the ioctl would be unable to continue.
>
> Reported-by: Brian King <brking@linux.vnet.ibm.com>
> Signed-off-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
> Signed-off-by: Manoj N. Kumar <manoj@linux.vnet.ibm.com>
> Reviewed-by: Brian King <brking@linux.vnet.ibm.com>
> Reviewed-by: Daniel Axtens <dja@axtens.net>

Reviewed-by: Tomas Henzl <thenzl@redhat.com>

Tomas