Re: [Qemu-devel] [PATCH] QEMUBH: make AioContext's bh re-entrant

[Paolo Bonzini <pbonzini@redhat.com>]

Il 13/06/2013 22:55, Liu Ping Fan ha scritto:
> BH will be used outside big lock, so introduce lock to protect it.
> Note that the lock only affects the writer and bh's callback does
> not take this extra lock.
> 
> Signed-off-by: Liu Ping Fan <pingfanl@linux.vnet.ibm.com>
> ---
>  async.c             | 10 +++++++++-
>  include/block/aio.h |  2 ++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/async.c b/async.c
> index 90fe906..b1dcead 100644
> --- a/async.c
> +++ b/async.c
> @@ -47,8 +47,11 @@ QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
>      bh->ctx = ctx;
>      bh->cb = cb;
>      bh->opaque = opaque;
> +    qemu_mutex_lock(&ctx->bh_lock);
>      bh->next = ctx->first_bh;
> +    smp_wmb();
>      ctx->first_bh = bh;
> +    qemu_mutex_unlock(&ctx->bh_lock);
>      return bh;
>  }
>  
> @@ -60,7 +63,9 @@ int aio_bh_poll(AioContext *ctx)
>      ctx->walking_bh++;
>  
>      ret = 0;
> -    for (bh = ctx->first_bh; bh; bh = next) {
> +    bh = ctx->first_bh;
> +    smp_rmb();
> +    for (; bh; bh = next) {
>          next = bh->next;

The read barrier in theory should go inside the loop.  On the other
hand, it only needs to be a smp_read_barrier_depends().

Paolo

>          if (!bh->deleted && bh->scheduled) {
>              bh->scheduled = 0;
> @@ -75,6 +80,7 @@ int aio_bh_poll(AioContext *ctx)
>  
>      /* remove deleted bhs */
>      if (!ctx->walking_bh) {
> +        qemu_mutex_lock(&ctx->bh_lock);
>          bhp = &ctx->first_bh;
>          while (*bhp) {
>              bh = *bhp;
> @@ -85,6 +91,7 @@ int aio_bh_poll(AioContext *ctx)
>                  bhp = &bh->next;
>              }
>          }
> +        qemu_mutex_unlock(&ctx->bh_lock);
>      }
>  
>      return ret;
> @@ -211,6 +218,7 @@ AioContext *aio_context_new(void)
>      ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
>      ctx->pollfds = g_array_new(FALSE, FALSE, sizeof(GPollFD));
>      ctx->thread_pool = NULL;
> +    qemu_mutex_init(&ctx->bh_lock);
>      event_notifier_init(&ctx->notifier, false);
>      aio_set_event_notifier(ctx, &ctx->notifier, 
>                             (EventNotifierHandler *)
> diff --git a/include/block/aio.h b/include/block/aio.h
> index 1836793..a65d16f 100644
> --- a/include/block/aio.h
> +++ b/include/block/aio.h
> @@ -17,6 +17,7 @@
>  #include "qemu-common.h"
>  #include "qemu/queue.h"
>  #include "qemu/event_notifier.h"
> +#include "qemu/thread.h"
>  
>  typedef struct BlockDriverAIOCB BlockDriverAIOCB;
>  typedef void BlockDriverCompletionFunc(void *opaque, int ret);
> @@ -53,6 +54,7 @@ typedef struct AioContext {
>       */
>      int walking_handlers;
>  
> +    QemuMutex bh_lock;
>      /* Anchor of the list of Bottom Halves belonging to the context */
>      struct QEMUBH *first_bh;
>  
>