On 4/8/14 11:33 AM, Chris Hegarty wrote:
On 8 Apr 2014, at 19:12, Pavel Rappo <pavel.ra...@oracle.com> wrote:
Hi Sean, Sherman
as far as the client is using ZipFileInputStream in a multithreaded fashion (de
facto), don't we have to rethink synchronization for the machinery of
ZipFileInputStream.read? As far as I understand it's not enough to put this
particular read of 'rem' into the synchronized block, as there are
at least 2 more accesses to it (and only in the 'read' method). We're reading
it in the synchronized block, fine, but all the corresponding reads/writes have
to be done with respect to 'happens-before' relationship between them.
Agreed. There doesn’t appear, or at least I cannot find, any statement in
ZipFile about its thread-safety ( or lack there of ), but the implementation of
ZipFile appears to be “somewhat” thread-safe. The InputStreams returned by
getInputStream(ZipEntry) are not. Sean’s testcase clearly shows that a single
InputStream is being shared across multiple threads. Unless we want to make
these streams thread-safe then we probably should not make any changes here.
It is a reasonable use scenario to close a java.io.InputStream via a
different thread and it
definitely should not lead to a SEGV. But java.io.InputStream itself is
not "thread-safe". To
read from a java.io.InputStream via multiple thread is kinda of useless
use scenario, which
normally means unpredictable reading result for those read operation.
-Sherman
-Pavel
On 8 Apr 2014, at 18:29, Seán Coffey <sean.cof...@oracle.com> wrote:
Sherman,
I see rem == 0 condition becoming true before the zentry variable is set to 0
(in close()) - in a multi threaded scenario like this one, we could have zero
remaining bytes in zentry and yet have a zentry != 0 - your suggestion would
prevent the SEGV (since we're in the sync block) but we would needlessly cycle
into the ensureOpenOrZipException() and ZipFile.read code (line 713) if another
thread tasked with the close() call was knocked off by the dispatcher during
the various rem == 0 checks that we make : e.g
722 if (rem == 0) {
723 close();
724 }
Am I reading this correctly ?
regards,
Sean.
On 08/04/2014 16:59, Xueming Shen wrote:
Hi Sean,
It might be more explicit to check "if (zentry == 0)" here?
-Sherman
On 4/8/14 8:21 AM, Seán Coffey wrote:
A recently reported bug shows a race condition is possible on the rem == 0
check in ZipFile.read(byte b[], int off, int len). A bad check can result in
referencing a jzentry structure that might already be freed and hence result in
a SEGV. The fix is trivial and involves moving the rem == 0 check into a
synchronized block. The ZipFile API itself is not thread safe so having mutiple
threads operate on the same ZipFileInputStream is something that should never
be performed.
webrev : http://cr.openjdk.java.net/~coffeys/webrev.8038491/webrev/
regards,
Sean.
