Re: [Qemu-devel] [PATCH for-2.11 1/4] Revert "coroutine: abort if we try to schedule or enter a pending coroutine"

[Jeff Cody <jcody@redhat.com>]

On Tue, Nov 28, 2017 at 04:43:47PM +0100, Kevin Wolf wrote:
> This reverts commit 6133b39f3c36623425a6ede9e89d93175fde15cd.
> 
> The commit checked conditions that would expose a bug, but there is no
> real reason to forbid them apart from the bug, which we'll fix in a
> minute.
> 
> In particular, reentering a coroutine during co_aio_sleep_ns() is fine;
> the function is explicitly written to allow this.
> 
> aio_co_schedule() can indeed conflict with direct coroutine invocations,
> but this is exactky what we want to fix, so remove that check again,

s/exactky/exactly/


Reviewed-by: Jeff Cody <jcody@redhat.com>

> too.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/qemu/coroutine_int.h | 13 +++----------
>  util/async.c                 | 13 -------------
>  util/qemu-coroutine-sleep.c  | 12 ------------
>  util/qemu-coroutine.c        | 14 --------------
>  4 files changed, 3 insertions(+), 49 deletions(-)
> 
> diff --git a/include/qemu/coroutine_int.h b/include/qemu/coroutine_int.h
> index 59e8406398..cb98892bba 100644
> --- a/include/qemu/coroutine_int.h
> +++ b/include/qemu/coroutine_int.h
> @@ -46,21 +46,14 @@ struct Coroutine {
>  
>      size_t locks_held;
>  
> -    /* Only used when the coroutine has yielded.  */
> -    AioContext *ctx;
> -
> -    /* Used to catch and abort on illegal co-routine entry.
> -     * Will contain the name of the function that had first
> -     * scheduled the coroutine. */
> -    const char *scheduled;
> -
> -    QSIMPLEQ_ENTRY(Coroutine) co_queue_next;
> -
>      /* Coroutines that should be woken up when we yield or terminate.
>       * Only used when the coroutine is running.
>       */
>      QSIMPLEQ_HEAD(, Coroutine) co_queue_wakeup;
>  
> +    /* Only used when the coroutine has yielded.  */
> +    AioContext *ctx;
> +    QSIMPLEQ_ENTRY(Coroutine) co_queue_next;
>      QSLIST_ENTRY(Coroutine) co_scheduled_next;
>  };
>  
> diff --git a/util/async.c b/util/async.c
> index 4dd9d95a9e..0e1bd8780a 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -388,9 +388,6 @@ static void co_schedule_bh_cb(void *opaque)
>          QSLIST_REMOVE_HEAD(&straight, co_scheduled_next);
>          trace_aio_co_schedule_bh_cb(ctx, co);
>          aio_context_acquire(ctx);
> -
> -        /* Protected by write barrier in qemu_aio_coroutine_enter */
> -        atomic_set(&co->scheduled, NULL);
>          qemu_coroutine_enter(co);
>          aio_context_release(ctx);
>      }
> @@ -441,16 +438,6 @@ fail:
>  void aio_co_schedule(AioContext *ctx, Coroutine *co)
>  {
>      trace_aio_co_schedule(ctx, co);
> -    const char *scheduled = atomic_cmpxchg(&co->scheduled, NULL,
> -                                           __func__);
> -
> -    if (scheduled) {
> -        fprintf(stderr,
> -                "%s: Co-routine was already scheduled in '%s'\n",
> -                __func__, scheduled);
> -        abort();
> -    }
> -
>      QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines,
>                                co, co_scheduled_next);
>      qemu_bh_schedule(ctx->co_schedule_bh);
> diff --git a/util/qemu-coroutine-sleep.c b/util/qemu-coroutine-sleep.c
> index 254349cdbb..9c5655041b 100644
> --- a/util/qemu-coroutine-sleep.c
> +++ b/util/qemu-coroutine-sleep.c
> @@ -13,7 +13,6 @@
>  
>  #include "qemu/osdep.h"
>  #include "qemu/coroutine.h"
> -#include "qemu/coroutine_int.h"
>  #include "qemu/timer.h"
>  #include "block/aio.h"
>  
> @@ -26,8 +25,6 @@ static void co_sleep_cb(void *opaque)
>  {
>      CoSleepCB *sleep_cb = opaque;
>  
> -    /* Write of schedule protected by barrier write in aio_co_schedule */
> -    atomic_set(&sleep_cb->co->scheduled, NULL);
>      aio_co_wake(sleep_cb->co);
>  }
>  
> @@ -37,15 +34,6 @@ void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type,
>      CoSleepCB sleep_cb = {
>          .co = qemu_coroutine_self(),
>      };
> -
> -    const char *scheduled = atomic_cmpxchg(&sleep_cb.co->scheduled, NULL,
> -                                           __func__);
> -    if (scheduled) {
> -        fprintf(stderr,
> -                "%s: Co-routine was already scheduled in '%s'\n",
> -                __func__, scheduled);
> -        abort();
> -    }
>      sleep_cb.ts = aio_timer_new(ctx, type, SCALE_NS, co_sleep_cb, &sleep_cb);
>      timer_mod(sleep_cb.ts, qemu_clock_get_ns(type) + ns);
>      qemu_coroutine_yield();
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 9eff7fd450..d6095c1d5a 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -107,22 +107,8 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co)
>      Coroutine *self = qemu_coroutine_self();
>      CoroutineAction ret;
>  
> -    /* Cannot rely on the read barrier for co in aio_co_wake(), as there are
> -     * callers outside of aio_co_wake() */
> -    const char *scheduled = atomic_mb_read(&co->scheduled);
> -
>      trace_qemu_aio_coroutine_enter(ctx, self, co, co->entry_arg);
>  
> -    /* if the Coroutine has already been scheduled, entering it again will
> -     * cause us to enter it twice, potentially even after the coroutine has
> -     * been deleted */
> -    if (scheduled) {
> -        fprintf(stderr,
> -                "%s: Co-routine was already scheduled in '%s'\n",
> -                __func__, scheduled);
> -        abort();
> -    }
> -
>      if (co->caller) {
>          fprintf(stderr, "Co-routine re-entered recursively\n");
>          abort();
> -- 
> 2.13.6
>