From: Ian Campbell <ian.campbell@citrix.com>
To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>,
George Dunlap <george.dunlap@eu.citrix.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Ian Jackson <Ian.Jackson@eu.citrix.com>,
Don Slutz <dslutz@verizon.com>,
"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
Subject: Re: QEMU bumping memory bug analysis
Date: Tue, 9 Jun 2015 13:45:05 +0100 [thread overview]
Message-ID: <1433853905.7108.550.camel@citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.02.1506091049160.19838@kaball.uk.xensource.com>
On Tue, 2015-06-09 at 11:14 +0100, Stefano Stabellini wrote:
> I don't think that the current solution is inherently racy. If we are
> really interested in an honest evaluation of the current solution in
> terms of races, I would be happy to do so.
Consider a domain with 1M of RAM (==target and maxmem for the sake of
argument) and two simultaneous calls to libxl_set_memory_target, both
with relative=0 and enforce=1, one with target=3 and the other with
target=5.
target=3 call target=5 call
get ptr.max_memkb, gets 1
get ptr.max_memkb, gets 1
start transaction
do xenstore stuff, seeing target=1, setting target=3
memorykb = ptr.max_memkb - current_target_memkb + new_target_memkb;
memorykb = 1 - 1 + 3
xc_setmaxmem(3)
transaction commit (success)
Now target=maxmem=3
start transaction
do xenstore stuff, seeing target=3, setting target=5
memorykb = ptr.max_memkb - current_target_memkb + new_target_memkb;
memorykb = 1 - 3 + 5
xc_setmaxmem(3)
transaction commit (success)
Now target=5, maxmem=3.
The obvious fix of moving the get ptr.max_memkb inside the transaction
fails in a different way in the case where the first transaction commit
fails and is retried after the second one, I think.
Prior to 0c029c4da21 "libxl_set_memory_target: retain the same maxmem
offset on top of the current target" this issue didn't exist because
memorykb was just memorykb = new_target_memkb + videoram.
BTW, I noticed some other (unrelated) dubious stuff in there while
looking at this, in particular the setmaxmem is not rolled back if
set_pod_target fails. Also the transiently "wrong" maxmem between the
setmaxmem and a failed transaction getting redone might possibly give
rise to some interesting cases, especially if anything else fails the
second time around the loop.
> When I committed it, I didn't do it without thinking.
Likewise I thought about it when reviewing, but it seems we have both
missed an important aspect.
Ian.
next prev parent reply other threads:[~2015-06-09 12:45 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-05 16:43 QEMU bumping memory bug analysis Wei Liu
2015-06-05 16:58 ` Ian Campbell
2015-06-05 17:13 ` Stefano Stabellini
2015-06-05 19:06 ` Wei Liu
2015-06-05 17:17 ` Andrew Cooper
2015-06-05 17:39 ` Wei Liu
2015-06-05 17:10 ` Stefano Stabellini
2015-06-05 18:10 ` Wei Liu
2015-06-08 11:39 ` Stefano Stabellini
2015-06-08 12:14 ` Andrew Cooper
2015-06-08 13:01 ` Stefano Stabellini
2015-06-08 13:33 ` Jan Beulich
2015-06-08 13:10 ` Wei Liu
2015-06-08 13:27 ` Stefano Stabellini
2015-06-08 13:32 ` Wei Liu
2015-06-08 13:38 ` Stefano Stabellini
2015-06-08 13:44 ` Andrew Cooper
2015-06-08 13:45 ` Stefano Stabellini
2015-06-05 18:49 ` Ian Campbell
2015-06-08 11:40 ` Stefano Stabellini
2015-06-08 12:11 ` Ian Campbell
2015-06-08 13:22 ` Stefano Stabellini
2015-06-08 13:52 ` Ian Campbell
2015-06-08 14:20 ` George Dunlap
2015-06-08 15:01 ` Don Slutz
2015-06-08 15:37 ` George Dunlap
2015-06-08 16:06 ` Don Slutz
2015-06-09 10:00 ` George Dunlap
2015-06-09 10:17 ` Wei Liu
2015-06-09 10:14 ` Stefano Stabellini
2015-06-09 11:20 ` George Dunlap
2015-06-16 16:44 ` Stefano Stabellini
2015-06-09 12:45 ` Ian Campbell [this message]
2015-06-17 13:35 ` Stefano Stabellini
2015-06-08 14:53 ` Konrad Rzeszutek Wilk
2015-06-08 15:20 ` George Dunlap
2015-06-08 15:42 ` Konrad Rzeszutek Wilk
2015-06-08 14:14 ` George Dunlap
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