crypto: qat - fix deadlock in backlog processing

If a request has the flag CRYPTO_TFM_REQ_MAY_BACKLOG set, the function
qat_alg_send_message_maybacklog(), enqueues it in a backlog list if
either (1) there is already at least one request in the backlog list, or
(2) the HW ring is nearly full or (3) the enqueue to the HW ring fails.
If an interrupt occurs right before the lock in qat_alg_backlog_req() is
taken and the backlog queue is being emptied, then there is no request
in the HW queues that can trigger a subsequent interrupt that can clear
the backlog queue. In addition subsequent requests are enqueued to the
backlog list and not sent to the hardware.

Fix it by holding the lock while taking the decision if the request
needs to be included in the backlog queue or not. This synchronizes the
flow with the interrupt handler that drains the backlog queue.

For performance reasons, the logic has been changed to try to enqueue
first without holding the lock.

Fixes: 38682383 ("crypto: qat - add backlog mechanism")
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Closes: https://lore.kernel.org/all/af9581e2-58f9-cc19-428f-6f18f1f83d54@redhat.com/T/


Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>