Blame SOURCES/kvm-block-Assert-serialisation-assumptions-in-pwritev.patch

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From e3fa109f29c42631c5f0f29e6f0167043fbd7a8d Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Tue, 14 Jan 2014 11:41:35 +0100
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Subject: [PATCH 26/37] block: Assert serialisation assumptions in pwritev
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Message-id: <1392117622-28812-27-git-send-email-kwolf@redhat.com>
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Patchwork-id: 57191
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O-Subject: [RHEL-7.0 qemu-kvm PATCH v2 26/37] block: Assert serialisation assumptions in pwritev
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Bugzilla: 748906
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RH-Acked-by: Laszlo Ersek <lersek@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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If a request calls wait_serialising_requests() and actually has to wait
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in this function (i.e. a coroutine yield), other requests can run and
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previously read data (like the head or tail buffer) could become
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outdated. In this case, we would have to restart from the beginning to
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read in the updated data.
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However, we're lucky and don't actually need to do that: A request can
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only wait in the first call of wait_serialising_requests() because we
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mark it as serialising before that call, so any later requests would
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wait. So as we don't wait in practice, we don't have to reload the data.
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This is an important assumption that may not be broken or data
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corruption will happen. Document it with some assertions.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Reviewed-by: Max Reitz <mreitz@redhat.com>
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(cherry picked from commit 28de2dcd88de31f50bbd43d9c2fcb046c3a727cb)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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---
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 block.c | 16 ++++++++++++----
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 1 file changed, 12 insertions(+), 4 deletions(-)
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---
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 block.c |   16 ++++++++++++----
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 1 files changed, 12 insertions(+), 4 deletions(-)
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diff --git a/block.c b/block.c
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index 9eeb072..76de7d2 100644
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--- a/block.c
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+++ b/block.c
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@@ -2153,14 +2153,15 @@ static bool tracked_request_overlaps(BdrvTrackedRequest *req,
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     return true;
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 }
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-static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
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+static bool coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
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 {
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     BlockDriverState *bs = self->bs;
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     BdrvTrackedRequest *req;
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     bool retry;
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+    bool waited = false;
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     if (!bs->serialising_in_flight) {
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-        return;
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+        return false;
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     }
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     do {
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@@ -2186,11 +2187,14 @@ static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
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                     qemu_co_queue_wait(&req->wait_queue);
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                     self->waiting_for = NULL;
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                     retry = true;
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+                    waited = true;
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                     break;
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                 }
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             }
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         }
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     } while (retry);
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+
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+    return waited;
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 }
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 /*
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@@ -3036,6 +3040,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
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     QEMUIOVector *qiov, int flags)
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 {
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     BlockDriver *drv = bs->drv;
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+    bool waited;
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     int ret;
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     int64_t sector_num = offset >> BDRV_SECTOR_BITS;
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@@ -3044,7 +3049,8 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
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     assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
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     assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
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-    wait_serialising_requests(req);
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+    waited = wait_serialising_requests(req);
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+    assert(!waited || !req->serialising);
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     if (flags & BDRV_REQ_ZERO_WRITE) {
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         ret = bdrv_co_do_write_zeroes(bs, sector_num, nb_sectors, flags);
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@@ -3142,9 +3148,11 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
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         QEMUIOVector tail_qiov;
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         struct iovec tail_iov;
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         size_t tail_bytes;
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+        bool waited;
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         mark_request_serialising(&req, align);
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-        wait_serialising_requests(&req);
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+        waited = wait_serialising_requests(&req);
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+        assert(!waited || !use_local_qiov);
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         tail_buf = qemu_blockalign(bs, align);
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         tail_iov = (struct iovec) {
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-- 
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1.7.1
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