Re: [Qemu-block] [PATCH v6 5/7] qemu-coroutine-sleep: introduce qemu_co_sleep_wake

[Vladimir Sementsov-Ogievskiy]

07.06.2019 10:57, Kevin Wolf wrote:
> Am 11.04.2019 um 19:27 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> Introduce a function to gracefully wake-up a coroutine, sleeping in
>> qemu_co_sleep_ns() sleep.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> 
> You can simply reenter the coroutine while it has yielded in
> qemu_co_sleep_ns(). This is supported.

No it doesn't. qemu_aio_coroutine_enter checks for scheduled field, and aborts
if it is set.

If I just use aio_co_enter instead of my new function, I get into

#1  0x00007f5d2514f8f8 in __GI_abort () at abort.c:90
#2  0x000055e9c8145278 in qemu_aio_coroutine_enter (ctx=0x55e9c9b12300, 
co=0x55e9c9b23cb0)
     at util/qemu-coroutine.c:132
#3  0x000055e9c8124f6d in aio_co_enter (ctx=0x55e9c9b12300, co=0x55e9c9b23cb0) 
at util/async.c:494
#4  0x000055e9c8124eb1 in aio_co_wake (co=0x55e9c9b23cb0) at util/async.c:478
#5  0x000055e9c808d5d4 in nbd_teardown_connection (bs=0x55e9c9b1bc50) at 
block/nbd-client.c:88
#6  0x000055e9c8090673 in nbd_client_close (bs=0x55e9c9b1bc50) at 
block/nbd-client.c:1289
#7  0x000055e9c808ca3f in nbd_close (bs=0x55e9c9b1bc50) at block/nbd.c:486
#8  0x000055e9c8006cd6 in bdrv_close (bs=0x55e9c9b1bc50) at block.c:3841

(gdb) fr 2
#2  0x000055e9c8145278 in qemu_aio_coroutine_enter (ctx=0x55e9c9b12300, 
co=0x55e9c9b23cb0)
     at util/qemu-coroutine.c:132
132                 abort();
(gdb) list
127              * been deleted */
128             if (scheduled) {
129                 fprintf(stderr,
130                         "%s: Co-routine was already scheduled in '%s'\n",
131                         __func__, scheduled);
132                 abort();
133             }
134
135             if (to->caller) {
136                 fprintf(stderr, "Co-routine re-entered recursively\n");
(gdb) p scheduled
$1 = 0x55e9c818e990 "qemu_co_sleep_ns"


> 
> I think what you add here is just the condition that you wake up the
> coroutine only if it's currently sleeping, but not when it has yielded
> for other reasons. This suggests that you're trying to reenter a
> coroutine of which you don't know where exactly in its code it currently
> is. This is wrong.
> 
> Just knowing that it's sleeping doesn't tell you where the coroutine is.
> It could have called a function that sleeps internally and must not be
> woken up early. If you reenter a coroutine, you always must know the
> exact point where it yielded (or in exceptional cases, the exact points
> (plural)). Just reentering because it sleeps will wake it up in
> unexpected places, generally speaking.
> 
> So I don't think this function is a good idea. It's too easy to misuse,
> and if you don't misuse it, you can directly call aio_co_wake().
> 
> Kevin
> 


-- 
Best regards,
Vladimir