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Groups > comp.lang.java.programmer > #11457
| From | Steven Simpson <ss@domain.invalid> |
|---|---|
| Newsgroups | comp.lang.java.programmer |
| Subject | Re: Volatile happens before question |
| Date | 2012-01-18 11:22 +0000 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <fkmiu8-6vc.ln1@news.simpsonst.f2s.com> (permalink) |
| References | (7 earlier) <jf52tt$9b9$1@dont-email.me> <jf5g64$uga$1@dont-email.me> <k0j9x3nt2or.70vxbwtikmz$.dlg@40tude.net> <jf5mm0$ndt$1@dont-email.me> <1pi7kea3zdo0b.1ixhuq3p9ybbu$.dlg@40tude.net> |
On 18/01/12 07:47, Peter Duniho wrote:
> Because the memory model doesn't stipulate what must happen, compilers and
> run-times are permitted to allow the write to "b" to be reordered to occur
> before the write to "a". This reordering then can cause the early-written
> value of "1" to be observed in thread #2, even as it just one program
> statement later observes the still-unwritten variable "a", seeing a value
> of "false".
Let's see if I've followed this...
In the first case, thread 1 writes 'b', then volatile-writes 'a'.
Thread 2 volatile-reads 'a', then reads 'b'. We only care when the read
of 'a' happens after the write of 'a', as otherwise we don't use the
value of 'b':
0 1 2 3 4 5
| |
V V
--------------------------------1
b a
* *
a b
--------------------------------2
| |
V V
'volatile' guarantees that the write to 'b' at time 1 is not optimized
in any way to happen after t2. If it were not guaranteed, it might
happen after t4, which we assert won't happen.
So, 'volatile' prevents the write to 'b' from jumping into the future of
the write to 'a'.
In the second case, thread 1 volatile-writes 'a' (as true), then writes
'b'. Thread 2 reads 'b', then volatile-reads 'a'. Thread 2 copies 'b',
but only uses the copied value if 'a' is later found to be (still)
false, so we only care when the read of 'a' happens before the write of 'a':
0 1 2 3 4 5
| |
V V
--------------------------------1
a b
* *
b a
--------------------------------2
| |
V V
This time, 'volatile' does not guarantee anything for the write to 'b'.
It could be optimized to happen as early as t0, so we cannot assert that
thread 2 will always use the initial value of 'b'.
So, 'volatile' does not prevent the write to 'b' from jumping back into
the past of the write to 'a'.
--
ss at comp dot lancs dot ac dot uk
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Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 04:04 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 08:17 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 08:50 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 09:00 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 08:54 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 11:24 -0800
Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-17 14:49 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 15:49 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 17:14 -0800
Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-17 18:17 -0800
Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 16:45 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 08:18 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 08:33 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 08:49 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 09:21 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 10:10 -0800
Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-17 10:28 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 10:41 -0800
Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-17 11:06 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 12:09 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 12:12 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 12:38 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 13:35 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 14:41 -0800
Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-17 14:55 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 15:27 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 17:37 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 17:42 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 15:46 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-17 16:14 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 20:01 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 20:37 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 21:52 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 23:47 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 00:08 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-18 09:04 -0800
Re: Volatile happens before question Steven Simpson <ss@domain.invalid> - 2012-01-18 11:22 +0000
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 06:24 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 07:32 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 08:34 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-18 10:12 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 11:15 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-18 11:59 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 12:24 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-18 13:16 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-18 15:00 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-18 16:12 -0800
Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-18 16:34 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-18 17:17 -0800
Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-18 19:03 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 15:30 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-18 16:10 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 08:18 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-19 09:40 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 10:10 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-19 10:39 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 12:13 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-19 13:31 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 16:14 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-19 21:16 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-20 03:23 -0800
Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-20 06:46 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-20 08:56 -0800
Re: Volatile happens before question Lew <noone@lewscanon.com> - 2012-01-20 11:12 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 17:06 -0800
Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-19 12:46 -0800
Re: Volatile happens before question markspace <-@.> - 2012-01-19 13:38 -0800
Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 07:32 -0800
Re: Volatile happens before question Lew <noone@lewscanon.com> - 2012-01-18 00:06 -0800
Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-18 09:23 -0800
Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 17:07 -0800
Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 17:12 -0800
Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 19:22 -0800
Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-17 10:24 -0800
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