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Re: [Qemu-devel] [PATCH v2] blkdebug: fix "once" rule


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2] blkdebug: fix "once" rule
Date: Fri, 06 Feb 2015 16:15:07 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 2015-02-06 at 13:23, John Snow wrote:
Background:
   The blkdebug scripts are currently engineered so that when a debug
event occurs, a prefilter browses a master list of parsed rules for a
certain event and adds them to an "active list" of rules to be used for
the forthcoming action, provided the events and state numbers match.

   Then, once the request is received, the last active rule is used to
inject an error if certain parameters match.

   This active list is cleared every time the prefilter injects a new
rule for the first time during a debug event.

   The "once" rule currently causes the error injection, if it is
triggered, to only clear the active list. This is insufficient for
preventing future injections of the same rule.

Remedy:
   This patch /deletes/ the rule from the list that the prefilter
browses, so it is gone for good. In V2, we remove only the rule of
interest from the active list instead of allowing the "once" rule to
clear the entire list of active rules.

Impact:
   This affects iotests 026. Several ENOSPC tests that used "once" can
be seen to have output that shows multiple failure messages. After
this patch, the error messages tend to be smaller and less severe, but
the injection can still be seen to be working. I have patched the
expected output to expect the smaller error messages.

V2:
   - Remove only the offending "once" rule from the active list instead
     of clearing the entire active list.

Could you move this version information under the three dashes so it won't be included in the commit message?

Signed-off-by: John Snow <address@hidden>
---

Here, that is.

  block/blkdebug.c           |  9 +++++----
  tests/qemu-iotests/026.out | 24 ++++--------------------
  2 files changed, 9 insertions(+), 24 deletions(-)

diff --git a/block/blkdebug.c b/block/blkdebug.c
index 9ce35cd..185695b 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -473,14 +473,15 @@ static BlockAIOCB *inject_error(BlockDriverState *bs,
      struct BlkdebugAIOCB *acb;
      QEMUBH *bh;
- if (rule->options.inject.once) {
-        QSIMPLEQ_INIT(&s->active_rules);
-    }
-
      if (rule->options.inject.immediately) {
          return NULL;
      }
+ if (rule->options.inject.once) {
+        QSIMPLEQ_REMOVE(&s->active_rules, rule, BlkdebugRule, active_next);
+        remove_rule(rule);
+    }
+
      acb = qemu_aio_get(&blkdebug_aiocb_info, bs, cb, opaque);
      acb->ret = -error;

The new code itself looks good, but why did you move it after the immediately block? Because now, if "immediately" is set, "once" will be ignored:

$ ./qemu-img create -f qcow2 test.qcow2 64M; ./qemu-io -c 'aio_write 0 64k' "json:{'driver':'qcow2','file':{'driver':'blkdebug','image':{'driver':'file','filename':'test.qcow2'},'inject-error':[{'event':'write_aio','once':true,'immediately':true}]}}" Formatting 'test.qcow2', fmt=qcow2 size=67108864 encryption=off cluster_size=65536 lazy_refcounts=off
aio_write failed: Input/output error
Failed to flush the L2 table cache: Input/output error
Failed to flush the refcount block cache: Input/output error

(I know, I know, I should have noticed in v1...)

Max



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