(Round 2) RFR(s): 8150460: (linux|bsd|aix)_close.c: file descriptor table may become large or may not work at all

Thomas Stüfe thomas.stuefe at gmail.com
Thu Mar 10 07:59:47 UTC 2016


may I have a review of this new iteration for this fix?

Thank you!


On Thu, Mar 3, 2016 at 5:26 PM, Thomas Stüfe <thomas.stuefe at gmail.com>

> Hi all,
> https://bugs.openjdk.java.net/browse/JDK-8150460
> thanks to all who took the time to review the first version of this fix!
> This is the new version:
> http://cr.openjdk.java.net/~stuefe/webrevs/8150460-linux_close-fdTable/webrev.02/webrev/
> I reworked the fix, trying to add in all the input I got: This fix uses a
> simple one-dimensional array, preallocated at startup, for low-value file
> descriptors. Like the code did before. Only for large values of file
> descriptors it switches to an overflow table, organized as two dimensional
> sparse array of fixed sized slabs, which are allocated on demand. Only the
> overflow table is protected by a lock.
> For 99% of all cases we will be using the plain simple fdTable structure
> as before. Only for unusually large file descriptor values we will be using
> this overflow table.
> Memory footprint is kept low: for small values of RLIMIT_NOFILE, we will
> only allocate as much space as we need. Only if file descriptor values get
> large, memory is allocated in the overflow table.
> Note that I avoided the proposed double-checked locking solution: I find
> it too risky in this place and also unnecessary. When calling getFdEntry(),
> we will be executing a blocking IO operation afterwards, flanked by two
> mutex locks (in startOp and endOp). So, I do not think the third mutex lock
> in getFdEntry will add much, especially since it is only used in case of
> larger file descriptor values.
> I also added the fix to bsd_close.c and aix_close.c. I do not like this
> code triplication. I briefly played around with unifying this code, but
> this is more difficult than it seems: implementations subtly differ between
> the three platforms, and solaris implementation is completely different. It
> may be a worthwhile cleanup, but that would be a separate issue.
> I did some artificial tests to check how the code does with many and large
> file descriptor values, all seemed to work well. I also ran java/net jtreg
> tests on Linux and AIX.
> Kind Regards, Thomas

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