Re: [PATCH 1/3] mailbox: always wait in mbox_send_message for blocking Tx mode

[Sudeep Holla]

On 28/03/17 19:20, Jassi Brar wrote:
> Hi Sudeep,
> 
> On Tue, Mar 21, 2017 at 5:00 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
>> There exists a race when msg_submit return immediately as there was an
>> active request being processed which may have completed just before it's
>> checked again in mbox_send_message. This will result in return to the
>> caller without waiting in mbox_send_message even when it's blocking Tx.
>>
>> This patch fixes the issue by waiting for the completion always if Tx
>> is in blocking mode.
>>
>> Fixes: 2b6d83e2b8b7 ("mailbox: Introduce framework for mailbox")
>> Cc: Jassi Brar <jassisinghbrar@xxxxxxxxx>
>> Reported-by: Alexey Klimov <alexey.klimov@xxxxxxx>
>> Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
>> ---
>>  drivers/mailbox/mailbox.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Hi Jassi,
>>
>> Here are fixes for few issues we encountered when dealing with multiple
>> requests on multiple channels simultaneously.
>>
> Thanks for finding the bug.
> 
> I see patch-1 tries to fix the bug.  Patch-2,3 try to fix the
> ramifications of the bug but they may change behaviour for some users.
> Do you face any issue even after applying patch-1 ?
> 

Unfortunately yes. Are you concerned with the change in return value on
timeout ? I understand and then I chose -ETIME vs -ETIMEDOUT as hardware
can still use it and we can distinguish the software timer expiry from
that. Even -EIO seems incorrect for s/w timeout as it exists today, but
I agree it has some impact on existing users.

Also Patch 3 seems independent for me just to avoid complete call if it
was empty message.

Alexey also brought up another issue which is relating to ordering and
may require completion flags per message instead of per channel. Today
we can't guarantee that first blocker on the wait queue is same as the
first in the mailbox queue.

e.g.:
	Thread#1(T1)		   Thread#2(T2)
     mbox_send_message		 mbox_send_message
            |				|
	    V				|
	add_to_rbuf(M1)			V
	    |			  add_to_rbuf(M2)
	    |				|
	    |				V
	    V			   msg_submit(picks M1)
	msg_submit			|
	    |				V
	    V			wait_for_completion(on M2)
     wait_for_completion(on M1)		|  (1st in waitQ)
     	    |	(2nd in waitQ)		V
     	    V			wake_up(on completion of M1)<--incorrect

I will let him dive in as he had some thoughts on that.

-- 
Regards,
Sudeep