qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [PATCH] ide: fix double free


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] ide: fix double free
Date: Wed, 02 Jul 2014 12:16:29 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

Il 02/07/2014 11:46, Gonglei (Arei) ha scritto:
Hi, Paolo. We have tested your above patch, and it works well for us.

I'm still not sure where the fix is. I jotted the patch quickly, but I'd rather understand it better before submitting it. Here is it again:

--- a/dma-helpers.c
+++ b/dma-helpers.c
@@ -181,15 +181,15 @@ static void dma_aio_cancel(BlockDriverAIOCB *acb)
     trace_dma_aio_cancel(dbs);

+    dbs->in_cancel = true;
     if (dbs->acb) {
         BlockDriverAIOCB *acb = dbs->acb;
         dbs->acb = NULL;
-        dbs->in_cancel = true;
         bdrv_aio_cancel(acb);
-        dbs->in_cancel = false;
     }
     dbs->common.cb = NULL;
     dma_complete(dbs, 0);
+    qemu_aio_release(dbs);
 }

 static const AIOCBInfo dma_aiocb_info = {

and here is dma_complete:

static void dma_complete(DMAAIOCB *dbs, int ret)
{
    trace_dma_complete(dbs, ret, dbs->common.cb);
    dma_bdrv_unmap(dbs);
    if (dbs->common.cb) {
        dbs->common.cb(dbs->common.opaque, ret);
    }
    qemu_iovec_destroy(&dbs->iov);
    if (dbs->bh) {
        qemu_bh_delete(dbs->bh);
        dbs->bh = NULL;
    }
    if (!dbs->in_cancel) {
        /* Requests may complete while dma_aio_cancel is in progress.
         * this case, the AIOCB should not be released because it is
         * still referenced by dma_aio_cancel.  */
        qemu_aio_release(dbs);
    }
}

The qemu_aio_release call obviously happened during the second dma_complete call.

However, the second dma_complete call does not call the callback, and even if dma_bdrv_unmap resulted in a call to continue_after_map_failure, the bottom half would be cancelled immediately. So we changed from:

    dbs->in_cancel = false;

    /* dma_complete calls dma_bdrv_unmap */
    for (i = 0; i < dbs->iov.niov; ++i) {
        /* might schedule dbs->bh */
        dma_memory_unmap(dbs->sg->as, dbs->iov.iov[i].iov_base,
                         dbs->iov.iov[i].iov_len, dbs->dir,
                         dbs->iov.iov[i].iov_len);
    }
    qemu_iovec_reset(&dbs->iov);

    /* back to dma_complete, dbs->common.cb == NULL */
    qemu_iovec_destroy(&dbs->iov);
    if (dbs->bh) {
        /* dbs->bh now unscheduled, so it will never run */
        qemu_bh_delete(dbs->bh);
        dbs->bh = NULL;
    }
    /* dbs->in_cancel is false here.  */
    if (!dbs->in_cancel) {
        /* Requests may complete while dma_aio_cancel is in progress.
         * this case, the AIOCB should not be released because it is
         * still referenced by dma_aio_cancel.  */
        qemu_aio_release(dbs);
    }

to

    dbs->in_cancel = false;

    /* dma_complete calls dma_bdrv_unmap */
    for (i = 0; i < dbs->iov.niov; ++i) {
        /* might schedule dbs->bh */
        dma_memory_unmap(dbs->sg->as, dbs->iov.iov[i].iov_base,
                         dbs->iov.iov[i].iov_len, dbs->dir,
                         dbs->iov.iov[i].iov_len);
    }
    qemu_iovec_reset(&dbs->iov);

    /* back to dma_complete, dbs->common.cb == NULL */
    qemu_iovec_destroy(&dbs->iov);
    if (dbs->bh) {
        /* dbs->bh now unscheduled, so it will never run */
        qemu_bh_delete(dbs->bh);
        dbs->bh = NULL;
    }

    /* after the patch dbs->in_cancel is true, the if never triggers */
    if (!dbs->in_cancel) {
        /* Requests may complete while dma_aio_cancel is in progress.
         * this case, the AIOCB should not be released because it is
         * still referenced by dma_aio_cancel.  */
        qemu_aio_release(dbs);
    }

    /* back to dma_aio_cancel */
    qemu_aio_release(dbs);

This doesn't make sense to me.  Can you find out where I'm wrong?

Paolo



reply via email to

[Prev in Thread] Current Thread [Next in Thread]