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nbd: Use CoQueue for free_sema instead of CoMutex 

NBD is using the CoMutex in a way that wasn't anticipated. For example, if there are
N(N=26, MAX_NBD_REQUESTS=16) nbd write requests, so we will invoke nbd_client_co_pwritev
N times.
----------------------------------------------------------------------------------------
time request Actions
1    1       in_flight=1, Coroutine=C1
2    2       in_flight=2, Coroutine=C2
...
15   15      in_flight=15, Coroutine=C15
16   16      in_flight=16, Coroutine=C16, free_sema->holder=C16, mutex->locked=true
17   17      in_flight=16, Coroutine=C17, queue C17 into free_sema->queue
18   18      in_flight=16, Coroutine=C18, queue C18 into free_sema->queue
...
26   N       in_flight=16, Coroutine=C26, queue C26 into free_sema->queue
----------------------------------------------------------------------------------------

Once nbd client recieves request No.16' reply, we will re-enter C16. It's ok, because
it's equal to 'free_sema->holder'.
----------------------------------------------------------------------------------------
time request Actions
27   16      in_flight=15, Coroutine=C16, free_sema->holder=C16, mutex->locked=false
----------------------------------------------------------------------------------------

Then nbd_coroutine_end invokes qemu_co_mutex_unlock what will pop coroutines from
free_sema->queue's head and enter C17. More free_sema->holder is C17 now.
----------------------------------------------------------------------------------------
time request Actions
28   17      in_flight=16, Coroutine=C17, free_sema->holder=C17, mutex->locked=true
----------------------------------------------------------------------------------------

In above scenario, we only recieves request No.16' reply. As time goes by, nbd client will
almostly recieves replies from requests 1 to 15 rather than request 17 who owns C17. In this
case, we will encounter assert "mutex->holder == self" failed since Kevin's commit 0e438cdc
"coroutine: Let CoMutex remember who holds it". For example, if nbd client recieves request
No.15' reply, qemu will stop unexpectedly:
----------------------------------------------------------------------------------------
time request       Actions
29   15(most case) in_flight=15, Coroutine=C15, free_sema->holder=C17, mutex->locked=false
----------------------------------------------------------------------------------------

Per Paolo's suggestion "The simplest fix is to change it to CoQueue, which is like a condition
variable", this patch replaces CoMutex with CoQueue.

Cc: Wen Congyang <wency@cn.fujitsu.com>
Reported-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Message-Id: <1476267508-19499-1-git-send-email-xiecl.fnst@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Changlong Xie 2016-10-12 18:18:28 +08:00 committed by Paolo Bonzini
parent e20e718cde
commit 9bc9732fae
2 changed files with 5 additions and 5 deletions
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8
block/nbd-client.c
View File

@ -199,8 +199,8 @@ static void nbd_coroutine_start(NbdClientSession *s,
{
/* Poor man semaphore. The free_sema is locked when no other request
* can be accepted, and unlocked after receiving one reply. */
if (s->in_flight >= MAX_NBD_REQUESTS - 1) {
qemu_co_mutex_lock(&s->free_sema);
if (s->in_flight == MAX_NBD_REQUESTS) {
qemu_co_queue_wait(&s->free_sema);
assert(s->in_flight < MAX_NBD_REQUESTS);
}
s->in_flight++;
@ -214,7 +214,7 @@ static void nbd_coroutine_end(NbdClientSession *s,
int i = HANDLE_TO_INDEX(s, request->handle);
s->recv_coroutine[i] = NULL;
if (s->in_flight-- == MAX_NBD_REQUESTS) {
qemu_co_mutex_unlock(&s->free_sema);
qemu_co_queue_next(&s->free_sema);
}
}
@ -386,7 +386,7 @@ int nbd_client_init(BlockDriverState *bs,
}
qemu_co_mutex_init(&client->send_mutex);
qemu_co_mutex_init(&client->free_sema);
qemu_co_queue_init(&client->free_sema);
client->sioc = sioc;
object_ref(OBJECT(client->sioc));

2
block/nbd-client.h
View File

@ -24,7 +24,7 @@ typedef struct NbdClientSession {
off_t size;
CoMutex send_mutex;
CoMutex free_sema;
CoQueue free_sema;
Coroutine *send_coroutine;
int in_flight;