[Qemu-devel] bdrv_read race

[Qemu-devel] bdrv_read race

[Vladimir Sementsov-Ogievskij]

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calling bdrv_read in a loop leads to the follwing situation:

bs->drv->bdrv_aio_readv is called, and finally calls bdrv_co_io_em_complete
in other thread context.
there is a possibility of calling bdrv_co_io_em_complete before
calling qemu_coroutine_yield in bdrv_co_io_em. And qemu fails with
"co-routine re-entered recursively".

static void bdrv_co_io_em_complete(void *opaque, int ret)
{
    CoroutineIOCompletion *co = opaque;

    co->ret = ret;
    qemu_coroutine_enter(co->coroutine, NULL);
}

static int coroutine_fn bdrv_co_io_em(BlockDriverState *bs, int64_t
sector_num,
                                      int nb_sectors, QEMUIOVector *iov,
                                      bool is_write)
{
    CoroutineIOCompletion co = {
        .coroutine = qemu_coroutine_self(),
    };
    BlockDriverAIOCB *acb;

    if (is_write) {
        acb = bs->drv->bdrv_aio_writev(bs, sector_num, iov, nb_sectors,
                                       bdrv_co_io_em_complete, &co);
    } else {
        acb = bs->drv->bdrv_aio_readv(bs, sector_num, iov, nb_sectors,
                                      bdrv_co_io_em_complete, &co);
    }

    trace_bdrv_co_io_em(bs, sector_num, nb_sectors, is_write, acb);
    if (!acb) {
        return -EIO;
    }
    qemu_coroutine_yield();

    return co.ret;
}

is it a bug, or may be I don't understand something?

Best regards,
Vladimir

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Re: [Qemu-devel] bdrv_read race

[Stefan Hajnoczi]

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On Thu, Aug 28, 2014 at 03:52:38PM +0400, Vladimir Sementsov-Ogievskij wrote:
> is it a bug, or may be I don't understand something?

You also opened a bug on launchpad for this.  I've already responded to
the bug, please see launchpad.

Stefan

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