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Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close()
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close() |
Date: |
Fri, 12 Jul 2019 13:01:10 +0200 |
User-agent: |
Mutt/1.11.3 (2019-02-01) |
Am 12.07.2019 um 12:47 hat Max Reitz geschrieben:
> On 12.07.19 11:24, Kevin Wolf wrote:
> > Am 11.07.2019 um 21:58 hat Max Reitz geschrieben:
> >> When nbd_close() is called from a coroutine, the connection_co never
> >> gets to run, and thus nbd_teardown_connection() hangs.
> >>
> >> This is because aio_co_enter() only puts the connection_co into the main
> >> coroutine's wake-up queue, so this main coroutine needs to yield and
> >> reschedule itself to let the connection_co run.
> >>
> >> Signed-off-by: Max Reitz <address@hidden>
> >> ---
> >> block/nbd.c | 12 +++++++++++-
> >> 1 file changed, 11 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/block/nbd.c b/block/nbd.c
> >> index 81edabbf35..b83b6cd43e 100644
> >> --- a/block/nbd.c
> >> +++ b/block/nbd.c
> >> @@ -135,7 +135,17 @@ static void nbd_teardown_connection(BlockDriverState
> >> *bs)
> >> qio_channel_shutdown(s->ioc,
> >> QIO_CHANNEL_SHUTDOWN_BOTH,
> >> NULL);
> >> - BDRV_POLL_WHILE(bs, s->connection_co);
> >> +
> >> + if (qemu_in_coroutine()) {
> >> + /* Let our caller poll and just yield until connection_co is done
> >> */
> >> + while (s->connection_co) {
> >> + aio_co_schedule(qemu_get_current_aio_context(),
> >> + qemu_coroutine_self());
> >> + qemu_coroutine_yield();
> >> + }
> >
> > Isn't this busy waiting? Why not let s->connection_co wake us up when
> > it's about to terminate instead of immediately rescheduling ourselves?
>
> Yes, it is busy waiting, but I didn’t find that bad. The connection_co
> will be invoked in basically every iteration, and once there is no
> pending data, it will quit.
>
> The answer to “why not...” of course is because it’d be more complicated.
>
> But anyway.
>
> Adding a new function qemu_coroutine_run_after(target) that adds
> qemu_coroutine_self() to the given @target coroutine’s wake-up queue and
> then using that instead of scheduling works, too, yes.
>
> I don’t really like being responsible for coroutine code, though...
>
> (And maybe it’d be better to make it qemu_coroutine_yield_for(target),
> which does the above and then yields?)
Or just do something like this, which is arguably not only a fix for the
busy wait, but also a code simplification:
diff --git a/block/nbd.c b/block/nbd.c
index b83b6cd43e..c061bd1bfc 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -61,6 +61,7 @@ typedef struct BDRVNBDState {
CoMutex send_mutex;
CoQueue free_sema;
Coroutine *connection_co;
+ Coroutine *teardown_co;
int in_flight;
NBDClientRequest requests[MAX_NBD_REQUESTS];
@@ -137,12 +138,9 @@ static void nbd_teardown_connection(BlockDriverState *bs)
NULL);
if (qemu_in_coroutine()) {
- /* Let our caller poll and just yield until connection_co is done */
- while (s->connection_co) {
- aio_co_schedule(qemu_get_current_aio_context(),
- qemu_coroutine_self());
- qemu_coroutine_yield();
- }
+ /* just yield until connection_co is done */
+ s->teardown_co = qemu_coroutine_self();
+ qemu_coroutine_yield();
} else {
BDRV_POLL_WHILE(bs, s->connection_co);
}
@@ -217,6 +215,9 @@ static coroutine_fn void nbd_connection_entry(void *opaque)
bdrv_dec_in_flight(s->bs);
s->connection_co = NULL;
+ if (s->teardown_co) {
+ aio_co_wake(s->teardown_co);
+ }
aio_wait_kick();
}
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- [Qemu-devel] [RFC 0/5] block: Generic file truncation/creation fallbacks, Max Reitz, 2019/07/11
- [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Max Reitz, 2019/07/11
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Kevin Wolf, 2019/07/12
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Max Reitz, 2019/07/12
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(),
Kevin Wolf <=
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Max Reitz, 2019/07/12
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Kevin Wolf, 2019/07/12
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Max Reitz, 2019/07/12
- Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close(), Kevin Wolf, 2019/07/12
[Qemu-devel] [RFC 3/5] block: Fall back to fallback truncate function, Max Reitz, 2019/07/11
[Qemu-devel] [RFC 2/5] block: Generic truncation fallback, Max Reitz, 2019/07/11