Re: [Qemu-block] [PATCH 16/19] block: Allow AIO_WAIT_WHILE with NULL ctx

[Paolo Bonzini]

On 11/04/2018 18:39, Kevin Wolf wrote:
> bdrv_drain_all() wants to have a single polling loop for draining the
> in-flight requests of all nodes. This means that the AIO_WAIT_WHILE()
> condition relies on activity in multiple AioContexts, which is polled
> from the mainloop context. We must therefore call AIO_WAIT_WHILE() from
> the mainloop thread and use the AioWait notification mechanism.
> 
> Just randomly picking the AioContext of any non-mainloop thread would
> work, but instead of bothering to find such a context in the caller, we
> can just as well accept NULL for ctx.
> 
> Signed-off-by: Kevin Wolf 
> ---
>  include/block/aio-wait.h | 13 +
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
> index 783d3678dd..c85a62f798 100644
> --- a/include/block/aio-wait.h
> +++ b/include/block/aio-wait.h
> @@ -57,7 +57,8 @@ typedef struct {
>  /**
>   * AIO_WAIT_WHILE:
>   * @wait: the aio wait object
> - * @ctx: the aio context
> + * @ctx: the aio context, or NULL if multiple aio contexts (for which the
> + *   caller does not hold a lock) are involved in the polling condition.
>   * @cond: wait while this conditional expression is true
>   *
>   * Wait while a condition is true.  Use this to implement synchronous
> @@ -75,7 +76,7 @@ typedef struct {
>  bool waited_ = false;  \
>  AioWait *wait_ = (wait);   \
>  AioContext *ctx_ = (ctx);  \
> -if (in_aio_context_home_thread(ctx_)) {\
> +if (ctx_ && in_aio_context_home_thread(ctx_)) {\
>  while ((cond)) {   \
>  aio_poll(ctx_, true);  \
>  waited_ = true;\
> @@ -86,9 +87,13 @@ typedef struct {
>  /* Increment wait_->num_waiters before evaluating cond. */ \
>  atomic_inc(_->num_waiters);   \
>  while ((cond)) {   \
> -aio_context_release(ctx_); \
> +if (ctx_) {\
> +aio_context_release(ctx_); \
> +}  \
>  aio_poll(qemu_get_aio_context(), true);\
> -aio_context_acquire(ctx_); \
> +if (ctx_) {\
> +aio_context_acquire(ctx_); \
> +}  \
>  waited_ = true;\
>  }  \
>  atomic_dec(_->num_waiters);   \
> 

Looks good.

Paolo

[Qemu-block] [PATCH 16/19] block: Allow AIO_WAIT_WHILE with NULL ctx

[Kevin Wolf]

bdrv_drain_all() wants to have a single polling loop for draining the
in-flight requests of all nodes. This means that the AIO_WAIT_WHILE()
condition relies on activity in multiple AioContexts, which is polled
from the mainloop context. We must therefore call AIO_WAIT_WHILE() from
the mainloop thread and use the AioWait notification mechanism.

Just randomly picking the AioContext of any non-mainloop thread would
work, but instead of bothering to find such a context in the caller, we
can just as well accept NULL for ctx.

Signed-off-by: Kevin Wolf 
---
 include/block/aio-wait.h | 13 +
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
index 783d3678dd..c85a62f798 100644
--- a/include/block/aio-wait.h
+++ b/include/block/aio-wait.h
@@ -57,7 +57,8 @@ typedef struct {
 /**
  * AIO_WAIT_WHILE:
  * @wait: the aio wait object
- * @ctx: the aio context
+ * @ctx: the aio context, or NULL if multiple aio contexts (for which the
+ *   caller does not hold a lock) are involved in the polling condition.
  * @cond: wait while this conditional expression is true
  *
  * Wait while a condition is true.  Use this to implement synchronous
@@ -75,7 +76,7 @@ typedef struct {
 bool waited_ = false;  \
 AioWait *wait_ = (wait);   \
 AioContext *ctx_ = (ctx);  \
-if (in_aio_context_home_thread(ctx_)) {\
+if (ctx_ && in_aio_context_home_thread(ctx_)) {\
 while ((cond)) {   \
 aio_poll(ctx_, true);  \
 waited_ = true;\
@@ -86,9 +87,13 @@ typedef struct {
 /* Increment wait_->num_waiters before evaluating cond. */ \
 atomic_inc(_->num_waiters);   \
 while ((cond)) {   \
-aio_context_release(ctx_); \
+if (ctx_) {\
+aio_context_release(ctx_); \
+}  \
 aio_poll(qemu_get_aio_context(), true);\
-aio_context_acquire(ctx_); \
+if (ctx_) {\
+aio_context_acquire(ctx_); \
+}  \
 waited_ = true;\
 }  \
 atomic_dec(_->num_waiters);   \
-- 
2.13.6