Re: [PATCH 14/19] util/async: Fixed tsan warnings

["Emilio G. Cota" <cota@braap.org>]

On Fri, May 22, 2020 at 12:07:50 -0400, Robert Foley wrote:
> For example:
>   Atomic write of size 8 at 0x7b4800113c28 by main thread (mutexes: write M30):
>     #0 __tsan_atomic64_exchange <null> (qemu-system-aarch64+0x386f85)
>     #1 aio_bh_poll util/async.c:146:5 (qemu-system-aarch64+0xcd1f61)
>     #2 aio_dispatch util/aio-posix.c:380:5 (qemu-system-aarch64+0xcd8abb)
>     #3 aio_ctx_dispatch util/async.c:298:5 (qemu-system-aarch64+0xcd31b0)
>     #4 g_main_context_dispatch <null> (libglib-2.0.so.0+0x4c416)
>     #5 qemu_main_loop softmmu/vl.c:1664:9 (qemu-system-aarch64+0x5cc6d6)
>     #6 main softmmu/main.c:49:5 (qemu-system-aarch64+0xc62857)
> 
>   Previous read of size 8 at 0x7b4800113c28 by thread T3 (mutexes: write M81):
>     #0 aio_bh_enqueue util/async.c:81:9 (qemu-system-aarch64+0xcd2267)
>     #1 qemu_bh_schedule util/async.c:181:5 (qemu-system-aarch64+0xcd2267)
>     #2 worker_thread util/thread-pool.c:113:9 (qemu-system-aarch64+0xcd473c)
>     #3 qemu_thread_start util/qemu-thread-posix.c:519:9 (qemu-system-aarch64+0xcde280)
> 
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Cc: Fam Zheng <fam@euphon.net>
> Signed-off-by: Robert Foley <robert.foley@linaro.org>
> ---
>  util/async.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/util/async.c b/util/async.c
> index 1319eee3bc..51e306bf0c 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -33,6 +33,7 @@
>  #include "block/raw-aio.h"
>  #include "qemu/coroutine_int.h"
>  #include "trace.h"
> +#include "qemu/tsan.h"
>  
>  /***********************************************************/
>  /* bottom halves (can be seen as timers which expire ASAP) */
> @@ -76,10 +77,12 @@ static void aio_bh_enqueue(QEMUBH *bh, unsigned new_flags)
>       * 2. ctx is loaded before the callback has a chance to execute and bh
>       *    could be freed.
>       */
> +    TSAN_ANNOTATE_IGNORE_WRITES_BEGIN();

Why do we need these annotations here? It looks like the fix for the
race in the commit message is below (i.e. atomic_read).

In general, I'd expect annotations to come with a comment, since
they should be used sparingly. That is, the assumption is that
tsan is almost always right.

>      old_flags = atomic_fetch_or(&bh->flags, BH_PENDING | new_flags);
>      if (!(old_flags & BH_PENDING)) {
>          QSLIST_INSERT_HEAD_ATOMIC(&ctx->bh_list, bh, next);
>      }
> +    TSAN_ANNOTATE_IGNORE_WRITES_END();
>  
>      aio_notify(ctx);
>  }
> @@ -143,7 +146,9 @@ int aio_bh_poll(AioContext *ctx)
>      BHListSlice *s;
>      int ret = 0;
>  
> +    TSAN_ANNOTATE_IGNORE_WRITES_BEGIN();
>      QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
> +    TSAN_ANNOTATE_IGNORE_WRITES_END();

ditto

>      QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
>  
>      while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
> @@ -280,14 +285,16 @@ aio_ctx_check(GSource *source)
>      aio_notify_accept(ctx);
>  
>      QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {
> -        if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
> +        if ((atomic_read(&bh->flags) & (BH_SCHEDULED | BH_DELETED))
> +             == BH_SCHEDULED) {
>              return true;
>          }
>      }
>  
>      QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
>          QSLIST_FOREACH_RCU(bh, &s->bh_list, next) {
> -            if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
> +            if ((atomic_read(&bh->flags) & (BH_SCHEDULED | BH_DELETED))
> +                 == BH_SCHEDULED) {

This hunk like the real fix. Also, I'd put "fix race" in the commit
title as opposed to "fix warning" since fixing races is the goal, not
fixing warnings.

Thanks,

		Emilio