diff mbox series

[v3,07/33] block/nbd: simplify waking of nbd_co_establish_connection()

Message ID 20210416080911.83197-8-vsementsov@virtuozzo.com (mailing list archive)
State New
Headers show
Series block/nbd: rework client connection | expand

Commit Message

Vladimir Sementsov-Ogievskiy April 16, 2021, 8:08 a.m. UTC
Instead of connect_bh, bh_ctx and wait_connect fields we can live with
only one link to waiting coroutine, protected by mutex.

So new logic is:

nbd_co_establish_connection() sets wait_co under mutex, release the
mutex and do yield(). Note, that wait_co may be scheduled by thread
immediately after unlocking the mutex. Still, in main thread (or
iothread) we'll not reach the code for entering the coroutine until the
yield() so we are safe.

Both connect_thread_func() and nbd_co_establish_connection_cancel() do
the following to handle wait_co:

Under mutex, if thr->wait_co is not NULL, call aio_co_wake() (which
never tries to acquire aio context since previous commit, so we are
safe to do it under thr->mutex) and set thr->wait_co to NULL.
This way we protect ourselves of scheduling it twice.

Also this commit make nbd_co_establish_connection() less connected to
bs (we have generic pointer to the coroutine, not use s->connection_co
directly). So, we are on the way of splitting connection API out of
nbd.c (which is overcomplicated now).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/nbd.c | 49 +++++++++----------------------------------------
 1 file changed, 9 insertions(+), 40 deletions(-)

Comments

Roman Kagan April 27, 2021, 9:44 p.m. UTC | #1
On Fri, Apr 16, 2021 at 11:08:45AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Instead of connect_bh, bh_ctx and wait_connect fields we can live with
> only one link to waiting coroutine, protected by mutex.
> 
> So new logic is:
> 
> nbd_co_establish_connection() sets wait_co under mutex, release the
> mutex and do yield(). Note, that wait_co may be scheduled by thread
> immediately after unlocking the mutex. Still, in main thread (or
> iothread) we'll not reach the code for entering the coroutine until the
> yield() so we are safe.
> 
> Both connect_thread_func() and nbd_co_establish_connection_cancel() do
> the following to handle wait_co:
> 
> Under mutex, if thr->wait_co is not NULL, call aio_co_wake() (which
> never tries to acquire aio context since previous commit, so we are
> safe to do it under thr->mutex) and set thr->wait_co to NULL.
> This way we protect ourselves of scheduling it twice.
> 
> Also this commit make nbd_co_establish_connection() less connected to
> bs (we have generic pointer to the coroutine, not use s->connection_co
> directly). So, we are on the way of splitting connection API out of
> nbd.c (which is overcomplicated now).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/nbd.c | 49 +++++++++----------------------------------------
>  1 file changed, 9 insertions(+), 40 deletions(-)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index d67556c7ee..e1f39eda6c 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -87,12 +87,6 @@ typedef enum NBDConnectThreadState {
>  typedef struct NBDConnectThread {
>      /* Initialization constants */
>      SocketAddress *saddr; /* address to connect to */
> -    /*
> -     * Bottom half to schedule on completion. Scheduled only if bh_ctx is not
> -     * NULL
> -     */
> -    QEMUBHFunc *bh_func;
> -    void *bh_opaque;
>  
>      /*
>       * Result of last attempt. Valid in FAIL and SUCCESS states.
> @@ -101,10 +95,10 @@ typedef struct NBDConnectThread {
>      QIOChannelSocket *sioc;
>      Error *err;
>  
> -    /* state and bh_ctx are protected by mutex */
>      QemuMutex mutex;
> +    /* All further fields are protected by mutex */
>      NBDConnectThreadState state; /* current state of the thread */
> -    AioContext *bh_ctx; /* where to schedule bh (NULL means don't schedule) */
> +    Coroutine *wait_co; /* nbd_co_establish_connection() wait in yield() */
>  } NBDConnectThread;
>  
>  typedef struct BDRVNBDState {
> @@ -138,7 +132,6 @@ typedef struct BDRVNBDState {
>      char *x_dirty_bitmap;
>      bool alloc_depth;
>  
> -    bool wait_connect;
>      NBDConnectThread *connect_thread;
>  } BDRVNBDState;
>  
> @@ -374,15 +367,6 @@ static bool nbd_client_connecting_wait(BDRVNBDState *s)
>      return qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTING_WAIT;
>  }
>  
> -static void connect_bh(void *opaque)
> -{
> -    BDRVNBDState *state = opaque;
> -
> -    assert(state->wait_connect);
> -    state->wait_connect = false;
> -    aio_co_wake(state->connection_co);
> -}
> -
>  static void nbd_init_connect_thread(BDRVNBDState *s)
>  {
>      s->connect_thread = g_new(NBDConnectThread, 1);
> @@ -390,8 +374,6 @@ static void nbd_init_connect_thread(BDRVNBDState *s)
>      *s->connect_thread = (NBDConnectThread) {
>          .saddr = QAPI_CLONE(SocketAddress, s->saddr),
>          .state = CONNECT_THREAD_NONE,
> -        .bh_func = connect_bh,
> -        .bh_opaque = s,
>      };
>  
>      qemu_mutex_init(&s->connect_thread->mutex);
> @@ -429,11 +411,9 @@ static void *connect_thread_func(void *opaque)
>      switch (thr->state) {
>      case CONNECT_THREAD_RUNNING:
>          thr->state = ret < 0 ? CONNECT_THREAD_FAIL : CONNECT_THREAD_SUCCESS;
> -        if (thr->bh_ctx) {
> -            aio_bh_schedule_oneshot(thr->bh_ctx, thr->bh_func, thr->bh_opaque);
> -
> -            /* play safe, don't reuse bh_ctx on further connection attempts */
> -            thr->bh_ctx = NULL;
> +        if (thr->wait_co) {
> +            aio_co_schedule(NULL, thr->wait_co);
> +            thr->wait_co = NULL;
>          }
>          break;
>      case CONNECT_THREAD_RUNNING_DETACHED:
> @@ -487,20 +467,14 @@ nbd_co_establish_connection(BlockDriverState *bs, Error **errp)
>          abort();
>      }
>  
> -    thr->bh_ctx = qemu_get_current_aio_context();
> +    thr->wait_co = qemu_coroutine_self();
>  
>      qemu_mutex_unlock(&thr->mutex);
>  
> -
>      /*
>       * We are going to wait for connect-thread finish, but
>       * nbd_client_co_drain_begin() can interrupt.
> -     *
> -     * Note that wait_connect variable is not visible for connect-thread. It
> -     * doesn't need mutex protection, it used only inside home aio context of
> -     * bs.
>       */
> -    s->wait_connect = true;
>      qemu_coroutine_yield();
>  
>      qemu_mutex_lock(&thr->mutex);
> @@ -555,24 +529,19 @@ static void nbd_co_establish_connection_cancel(BlockDriverState *bs)
>  {
>      BDRVNBDState *s = bs->opaque;
>      NBDConnectThread *thr = s->connect_thread;
> -    bool wake = false;
>  
>      qemu_mutex_lock(&thr->mutex);
>  
>      if (thr->state == CONNECT_THREAD_RUNNING) {

This check looks redundant and can probably go.  This patch may be quite
appropriate for this: the logic becomes even more straightforward.

>          /* We can cancel only in running state, when bh is not yet scheduled */
> -        thr->bh_ctx = NULL;
> -        if (s->wait_connect) {
> -            s->wait_connect = false;
> -            wake = true;
> +        if (thr->wait_co) {
> +            aio_co_schedule(NULL, thr->wait_co);

This will probably be replaced by a new function per our discussion of
the previous patch.

Note however that if that one doesn't fly for whatever reason you can
retain the aio_context on NBDConnectThread and pass it explicitly into
aio_co_schedule here.

Roman.

> +            thr->wait_co = NULL;
>          }
>      }
>  
>      qemu_mutex_unlock(&thr->mutex);
>  
> -    if (wake) {
> -        aio_co_wake(s->connection_co);
> -    }
>  }
>  
>  static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
diff mbox series

Patch

diff --git a/block/nbd.c b/block/nbd.c
index d67556c7ee..e1f39eda6c 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -87,12 +87,6 @@  typedef enum NBDConnectThreadState {
 typedef struct NBDConnectThread {
     /* Initialization constants */
     SocketAddress *saddr; /* address to connect to */
-    /*
-     * Bottom half to schedule on completion. Scheduled only if bh_ctx is not
-     * NULL
-     */
-    QEMUBHFunc *bh_func;
-    void *bh_opaque;
 
     /*
      * Result of last attempt. Valid in FAIL and SUCCESS states.
@@ -101,10 +95,10 @@  typedef struct NBDConnectThread {
     QIOChannelSocket *sioc;
     Error *err;
 
-    /* state and bh_ctx are protected by mutex */
     QemuMutex mutex;
+    /* All further fields are protected by mutex */
     NBDConnectThreadState state; /* current state of the thread */
-    AioContext *bh_ctx; /* where to schedule bh (NULL means don't schedule) */
+    Coroutine *wait_co; /* nbd_co_establish_connection() wait in yield() */
 } NBDConnectThread;
 
 typedef struct BDRVNBDState {
@@ -138,7 +132,6 @@  typedef struct BDRVNBDState {
     char *x_dirty_bitmap;
     bool alloc_depth;
 
-    bool wait_connect;
     NBDConnectThread *connect_thread;
 } BDRVNBDState;
 
@@ -374,15 +367,6 @@  static bool nbd_client_connecting_wait(BDRVNBDState *s)
     return qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTING_WAIT;
 }
 
-static void connect_bh(void *opaque)
-{
-    BDRVNBDState *state = opaque;
-
-    assert(state->wait_connect);
-    state->wait_connect = false;
-    aio_co_wake(state->connection_co);
-}
-
 static void nbd_init_connect_thread(BDRVNBDState *s)
 {
     s->connect_thread = g_new(NBDConnectThread, 1);
@@ -390,8 +374,6 @@  static void nbd_init_connect_thread(BDRVNBDState *s)
     *s->connect_thread = (NBDConnectThread) {
         .saddr = QAPI_CLONE(SocketAddress, s->saddr),
         .state = CONNECT_THREAD_NONE,
-        .bh_func = connect_bh,
-        .bh_opaque = s,
     };
 
     qemu_mutex_init(&s->connect_thread->mutex);
@@ -429,11 +411,9 @@  static void *connect_thread_func(void *opaque)
     switch (thr->state) {
     case CONNECT_THREAD_RUNNING:
         thr->state = ret < 0 ? CONNECT_THREAD_FAIL : CONNECT_THREAD_SUCCESS;
-        if (thr->bh_ctx) {
-            aio_bh_schedule_oneshot(thr->bh_ctx, thr->bh_func, thr->bh_opaque);
-
-            /* play safe, don't reuse bh_ctx on further connection attempts */
-            thr->bh_ctx = NULL;
+        if (thr->wait_co) {
+            aio_co_schedule(NULL, thr->wait_co);
+            thr->wait_co = NULL;
         }
         break;
     case CONNECT_THREAD_RUNNING_DETACHED:
@@ -487,20 +467,14 @@  nbd_co_establish_connection(BlockDriverState *bs, Error **errp)
         abort();
     }
 
-    thr->bh_ctx = qemu_get_current_aio_context();
+    thr->wait_co = qemu_coroutine_self();
 
     qemu_mutex_unlock(&thr->mutex);
 
-
     /*
      * We are going to wait for connect-thread finish, but
      * nbd_client_co_drain_begin() can interrupt.
-     *
-     * Note that wait_connect variable is not visible for connect-thread. It
-     * doesn't need mutex protection, it used only inside home aio context of
-     * bs.
      */
-    s->wait_connect = true;
     qemu_coroutine_yield();
 
     qemu_mutex_lock(&thr->mutex);
@@ -555,24 +529,19 @@  static void nbd_co_establish_connection_cancel(BlockDriverState *bs)
 {
     BDRVNBDState *s = bs->opaque;
     NBDConnectThread *thr = s->connect_thread;
-    bool wake = false;
 
     qemu_mutex_lock(&thr->mutex);
 
     if (thr->state == CONNECT_THREAD_RUNNING) {
         /* We can cancel only in running state, when bh is not yet scheduled */
-        thr->bh_ctx = NULL;
-        if (s->wait_connect) {
-            s->wait_connect = false;
-            wake = true;
+        if (thr->wait_co) {
+            aio_co_schedule(NULL, thr->wait_co);
+            thr->wait_co = NULL;
         }
     }
 
     qemu_mutex_unlock(&thr->mutex);
 
-    if (wake) {
-        aio_co_wake(s->connection_co);
-    }
 }
 
 static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)