On 29.10.25 15:27, Kevin Wolf wrote:
Am 28.10.2025 um 17:33 hat Hanna Czenczek geschrieben:
For rbd (and others), as described in “rbd: Run co BH CB in the
coroutine’s AioContext”, the pattern of setting a completion flag and
waking a coroutine that yields while the flag is not set can only work
when both run in the same thread.
iscsi has the same pattern, but the details are a bit different:
iscsi_co_generic_cb() can (as far as I understand) only run through
iscsi_service(), not just from a random thread at a random time.
iscsi_service() in turn can only be run after iscsi_set_events() set up
an FD event handler, which is done in iscsi_co_wait_for_task().
As a result, iscsi_co_wait_for_task() will always yield exactly once,
because iscsi_co_generic_cb() can only run after iscsi_set_events(),
after the completion flag has already been checked, and the yielding
coroutine will then be woken only once the completion flag was set to
true. So as far as I can tell, iscsi has no bug and already works fine.
Still, we don’t need the completion flag because we know we have to
yield exactly once, so we can drop it. This simplifies the code and
makes it more obvious that the “rbd bug” isn’t present here.
This makes iscsi_co_generic_bh_cb() and iscsi_retry_timer_expired() a
bit boring, and actually, for the former, we could drop it and run
aio_co_wake() directly from scsi_co_generic_cb() to the same effect; but
that would remove the replay_bh_schedule_oneshot_event(), and I assume
we shouldn’t do that. At least schedule both the BH and the timer in
the coroutine’s AioContext to make them simple wrappers around
qemu_coroutine_enter(), without a further BH indirection.
I don't think we have to keep the BH. Is your concern about replay? I
doubt that this works across different QEMU versions anyway, and if it
does, it's pure luck.
It is solely about replay, yes. I assumed the
replay_bh_schedule_oneshot_event() would be a replay point, so removing
it would, well, remove a replay point. I suppose we’re going to have
one replay point per request anyway (when going through the blkreplay
driver), so maybe it doesn’t matter much?
Anyway, it seemed safer to keep it. But apart from replay, I don’t have
any concern about dropping the BH.
Finally, remove the iTask->co != NULL checks: This field is set by
iscsi_co_init_iscsitask(), which all users of IscsiTask run before even
setting up iscsi_co_generic_cb() as the callback, and it is never set or
cleared elsewhere, so it is impossible to not be set in
iscsi_co_generic_cb().
Signed-off-by: Hanna Czenczek <[email protected]>
Kevin
