[PATCH] Initialize IO_STATUS_BLOCK for pread, pwrite
Tue Nov 28 10:28:00 GMT 2017
Corinna Vinschen wrote:
> On Nov 28 00:03, Mark Geisert wrote:
>> Mark Geisert wrote:
>>> winsup/cygwin/fhandler_disk_file.cc | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>> diff --git a/winsup/cygwin/fhandler_disk_file.cc b/winsup/cygwin/fhandler_disk_file.cc
>>> index 5dfcae4d9..2ead9948c 100644
>> Oops, I neglected to include an explanatory comment. Issuing simultaneous
>> pwrite(s) on one file descriptor from multiple threads, as one might do in a
>> forthcoming POSIX aio implementation, sometimes results in garbage status in
>> the IO_STATUS_BLOCK on return from NtWriteFile(). Zeroing beforehand made
>> the issue go away.
>> This is mildly concerning to me because there are many other uses of
>> IO_STATUS_BLOCK in the Cygwin DLL that haven't seemed to have needed
> Ok, let's start with, why did you tweak pread if you only observed
> a problem in pwrite?
Optimism? :-) No, you're correct; I was getting ahead of myself.
> In terms of pread, we already have a very recent
> patch series:
> In this case it turned out that the problem was related to hitting EOF.
> I wonder if we hit a similar problem here.
> Two points:
> - Did you check the status code returned by NtWriteFile? Not all non-0
> status codes fail the !NT_SUCCESS (status) test.
I did check the status code but don't recall what it was. The symptom I was
seeing was outlandish io.Information values being returned by pwrite(). Far
larger than the number requested in the call to pwrite() and NtWriteFile().
> - Do you have a simple, self-contained testcase?
That would be difficult. I can supply an strace excerpt just showing the region
of these simultaneous pwrite() calls, without this patch. If it's too large
I'll put it somewhere and post a link (but I don't think it will be).
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