qemu-kvm/SOURCES/kvm-ide-Increment-BB-in-flight-counter-for-TRIM-BH.patch

From eaade87072e903cf550dfdb8ed1480dddc6bb0e3 Mon Sep 17 00:00:00 2001
From: Hanna Reitz <hreitz@redhat.com>
Date: Thu, 20 Jan 2022 15:22:59 +0100
Subject: [PATCH 21/24] ide: Increment BB in-flight counter for TRIM BH
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RH-Author: Hanna Reitz <hreitz@redhat.com>
RH-MergeRequest: 188: ide: Increment BB in-flight counter for TRIM BH
RH-Commit: [1/1] 1e702e735ff63f2b8b69c20cac1b309dd085cd62
RH-Bugzilla: 2029980
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

When we still have an AIOCB registered for DMA operations, we try to
settle the respective operation by draining the BlockBackend associated
with the IDE device.

However, this assumes that every DMA operation is associated with an
increment of the BlockBackend’s in-flight counter (e.g. through some
ongoing I/O operation), so that draining the BB until its in-flight
counter reaches 0 will settle all DMA operations.  That is not the case:
For TRIM, the guest can issue a zero-length operation that will not
result in any I/O operation forwarded to the BlockBackend, and also not
increment the in-flight counter in any other way.  In such a case,
blk_drain() will be a no-op if no other operations are in flight.

It is clear that if blk_drain() is a no-op, the value of
s->bus->dma->aiocb will not change between checking it in the `if`
condition and asserting that it is NULL after blk_drain().

The particular problem is that ide_issue_trim() creates a BH
(ide_trim_bh_cb()) to settle the TRIM request: iocb->common.cb() is
ide_dma_cb(), which will either create a new request, or find the
transfer to be done and call ide_set_inactive(), which clears
s->bus->dma->aiocb.  Therefore, the blk_drain() must wait for
ide_trim_bh_cb() to run, which currently it will not always do.

To fix this issue, we increment the BlockBackend's in-flight counter
when the TRIM operation begins (in ide_issue_trim(), when the
ide_trim_bh_cb() BH is created) and decrement it when ide_trim_bh_cb()
is done.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2029980
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220120142259.120189-1-hreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Tested-by: John Snow <jsnow@redhat.com>
(cherry picked from commit 7e5cdb345f77d76cb4877fe6230c4e17a7d0d0ca)
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
 hw/ide/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/hw/ide/core.c b/hw/ide/core.c
index e28f8aad61..15138225be 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -433,12 +433,16 @@ static const AIOCBInfo trim_aiocb_info = {
 static void ide_trim_bh_cb(void *opaque)
 {
     TrimAIOCB *iocb = opaque;
+    BlockBackend *blk = iocb->s->blk;
 
     iocb->common.cb(iocb->common.opaque, iocb->ret);
 
     qemu_bh_delete(iocb->bh);
     iocb->bh = NULL;
     qemu_aio_unref(iocb);
+
+    /* Paired with an increment in ide_issue_trim() */
+    blk_dec_in_flight(blk);
 }
 
 static void ide_issue_trim_cb(void *opaque, int ret)
@@ -508,6 +512,9 @@ BlockAIOCB *ide_issue_trim(
     IDEState *s = opaque;
     TrimAIOCB *iocb;
 
+    /* Paired with a decrement in ide_trim_bh_cb() */
+    blk_inc_in_flight(s->blk);
+
     iocb = blk_aio_get(&trim_aiocb_info, s->blk, cb, cb_opaque);
     iocb->s = s;
     iocb->bh = qemu_bh_new(ide_trim_bh_cb, iocb);
-- 
2.35.3