debian/patches/extra/0003-ide-avoid-potential-deadlock-when-draining-during-tr.patch

   1 From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
   2 From: Fiona Ebner <f.ebner@proxmox.com>
   3 Date: Tue, 7 Mar 2023 15:03:02 +0100
   4 Subject: [PATCH] ide: avoid potential deadlock when draining during trim
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   8
   9 The deadlock can happen as follows:
  10 1. ide_issue_trim is called, and increments the in_flight counter.
  11 2. ide_issue_trim_cb calls blk_aio_pdiscard.
  12 3. Somebody else starts draining (e.g. backup to insert the cbw node).
  13 4. ide_issue_trim_cb is called as the completion callback for
  14    blk_aio_pdiscard.
  15 5. ide_issue_trim_cb issues yet another blk_aio_pdiscard request.
  16 6. The request is added to the wait queue via blk_wait_while_drained,
  17    because draining has been started.
  18 7. Nobody ever decrements the in_flight counter and draining can't
  19    finish. This would be done by ide_trim_bh_cb, which is called after
  20    ide_issue_trim_cb has issued its last request, but
  21    ide_issue_trim_cb is not called anymore, because it's the
  22    completion callback of blk_aio_pdiscard, which waits on draining.
  23
  24 Quoting Hanna Czenczek:
  25 > The point of 7e5cdb345f was that we need any in-flight count to
  26 > accompany a set s->bus->dma->aiocb. While blk_aio_pdiscard() is
  27 > happening, we don’t necessarily need another count. But we do need
  28 > it while there is no blk_aio_pdiscard().
  29 > ide_issue_trim_cb() returns in two cases (and, recursively through
  30 > its callers, leaves s->bus->dma->aiocb set):
  31 > 1. After calling blk_aio_pdiscard(), which will keep an in-flight
  32 >    count,
  33 > 2. After calling replay_bh_schedule_event() (i.e.
  34 >    qemu_bh_schedule()), which does not keep an in-flight count.
  35
  36 Thus, even after moving the blk_inc_in_flight to above the
  37 replay_bh_schedule_event call, the invariant "ide_issue_trim_cb
  38 returns with an accompanying in-flight count" is still satisfied.
  39
  40 However, the issue 7e5cdb345f fixed for canceling resurfaces, because
  41 ide_cancel_dma_sync assumes that it just needs to drain once. But now
  42 the in_flight count is not consistently > 0 during the trim operation.
  43 So, change it to drain until !s->bus->dma->aiocb, which means that the
  44 operation finished (s->bus->dma->aiocb is cleared by ide_set_inactive
  45 via the ide_dma_cb when the end of the transfer is reached).
  46
  47 Discussion here:
  48 https://lists.nongnu.org/archive/html/qemu-devel/2023-03/msg02506.html
  49
  50 Fixes: 7e5cdb345f ("ide: Increment BB in-flight counter for TRIM BH")
  51 Suggested-by: Hanna Czenczek <hreitz@redhat.com>
  52 Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
  53 ---
  54  hw/ide/core.c | 12 ++++++------
  55  1 file changed, 6 insertions(+), 6 deletions(-)
  56
  57 diff --git a/hw/ide/core.c b/hw/ide/core.c
  58 index 07971c0218..6a74afe564 100644
  59 --- a/hw/ide/core.c
  60 +++ b/hw/ide/core.c
  61 @@ -444,7 +444,7 @@ static void ide_trim_bh_cb(void *opaque)
  62      iocb->bh = NULL;
  63      qemu_aio_unref(iocb);
  64
  65 -    /* Paired with an increment in ide_issue_trim() */
  66 +    /* Paired with an increment in ide_issue_trim_cb() */
  67      blk_dec_in_flight(blk);
  68  }
  69
  70 @@ -504,6 +504,8 @@ static void ide_issue_trim_cb(void *opaque, int ret)
  71  done:
  72      iocb->aiocb = NULL;
  73      if (iocb->bh) {
  74 +        /* Paired with a decrement in ide_trim_bh_cb() */
  75 +        blk_inc_in_flight(s->blk);
  76          replay_bh_schedule_event(iocb->bh);
  77      }
  78  }
  79 @@ -516,9 +518,6 @@ BlockAIOCB *ide_issue_trim(
  80      IDEDevice *dev = s->unit ? s->bus->slave : s->bus->master;
  81      TrimAIOCB *iocb;
  82
  83 -    /* Paired with a decrement in ide_trim_bh_cb() */
  84 -    blk_inc_in_flight(s->blk);
  85 -
  86      iocb = blk_aio_get(&trim_aiocb_info, s->blk, cb, cb_opaque);
  87      iocb->s = s;
  88      iocb->bh = qemu_bh_new_guarded(ide_trim_bh_cb, iocb,
  89 @@ -742,8 +741,9 @@ void ide_cancel_dma_sync(IDEState *s)
  90       */
  91      if (s->bus->dma->aiocb) {
  92          trace_ide_cancel_dma_sync_remaining();
  93 -        blk_drain(s->blk);
  94 -        assert(s->bus->dma->aiocb == NULL);
  95 +        while (s->bus->dma->aiocb) {
  96 +            blk_drain(s->blk);
  97 +        }
  98      }
  99  }
 100