From: Karsten Blees <karsten.blees@gmail.com>
To: Zachary Turner <zturner@chromium.org>, Stefan Zager <szager@google.com>
Cc: Git Mailing List <git@vger.kernel.org>
Subject: Re: Make the git codebase thread-safe
Date: Fri, 14 Feb 2014 20:04:41 +0100 [thread overview]
Message-ID: <52FE68C9.3060403@gmail.com> (raw)
In-Reply-To: <CAAErz9hzeiJ9f9tJ+Z-kOHvrPqgcZrpvrpBpa_tMjnKm4YWSXA@mail.gmail.com>
Am 14.02.2014 00:09, schrieb Zachary Turner:
> To elaborate a little bit more, you can verify with a sample program
> that ReadFile with OVERLAPPED does in fact modify the HANDLE's file
> position. The documentation doesn't actually state one way or
> another. My original attempt at a patch didn't have the ReOpenFile,
> and we experienced regular read corruption. We scratched our heads
> over it for a bit, and then hypothesized that someone must be mixing
> read styles, which led to this ReOpenFile workaround, which
> incidentally also solved the corruption problems. We wrote a similar
> sample program to verify that when using ReOpenHandle, and changing
> the file pointer of the duplicated handle, that the file pointer of
> the original handle is not modified.
>
> We did not actually try to identify the source of the mixed read
> styles, but it seems like the only possible explanation.
>
> On Thu, Feb 13, 2014 at 2:53 PM, Stefan Zager <szager@google.com> wrote:
>> On Thu, Feb 13, 2014 at 2:51 PM, Karsten Blees <karsten.blees@gmail.com> wrote:
>>> Am 13.02.2014 19:38, schrieb Zachary Turner:
>>>
>>>> The only reason ReOpenFile is necessary at
>>>> all is because some code somewhere is mixing read-styles against the same
>>>> fd.
>>>>
>>>
>>> I don't understand...ReadFile with OVERLAPPED parameter doesn't modify the HANDLE's file position, so you should be able to mix read()/pread() however you like (as long as read() is only called from one thread).
>>
>> That is, apparently, a bald-faced lie in the ReadFile API doc. First
>> implementation didn't use ReOpenFile, and it crashed all over the
>> place. ReOpenFile fixed it.
>>
>> Stefan
Damn...you're right, multi-threaded git-index-pack works fine, but some tests fail badly. Mixed reads would have to be from git_mmap, which is the only other caller of pread().
A simple alternative to ReOpenHandle is to reset the file pointer to its original position, as in compat/pread.c::git_pread. Thus single-theaded code can mix read()/pread() at will, but multi-threaded code has to use pread() exclusively (which is usually the case anyway). A main thread using read() and background threads using pread() (which is technically allowed by POSIX) will fail with this solution.
This version passes the test suite on msysgit:
----8<----
ssize_t mingw_pread(int fd, void *buf, size_t count, off64_t offset)
{
DWORD bytes_read;
OVERLAPPED overlapped;
off64_t current;
memset(&overlapped, 0, sizeof(overlapped));
overlapped.Offset = (DWORD) offset;
overlapped.OffsetHigh = (DWORD) (offset >> 32);
current = lseek64(fd, 0, SEEK_CUR);
if (!ReadFile((HANDLE)_get_osfhandle(fd), buf, count, &bytes_read, &overlapped)) {
errno = err_win_to_posix(GetLastError());
return -1;
}
lseek64(fd, current, SEEK_SET);
return (ssize_t) bytes_read;
}
next prev parent reply other threads:[~2014-02-14 19:04 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-12 1:54 Make the git codebase thread-safe Stefan Zager
2014-02-12 2:02 ` Robin H. Johnson
2014-02-12 3:43 ` Duy Nguyen
2014-02-12 11:00 ` Karsten Blees
2014-02-12 23:03 ` Mike Hommey
2014-02-13 0:06 ` Karsten Blees
2014-02-12 18:15 ` Stefan Zager
2014-02-12 2:11 ` Duy Nguyen
2014-02-12 18:12 ` Stefan Zager
2014-02-12 18:33 ` Matthieu Moy
2014-02-12 18:39 ` Stefan Zager
2014-02-12 18:50 ` David Kastrup
2014-02-12 19:02 ` Stefan Zager
2014-02-12 19:15 ` David Kastrup
2014-02-12 23:09 ` Mike Hommey
2014-02-13 6:04 ` David Kastrup
2014-02-13 9:34 ` Mike Hommey
2014-02-13 9:48 ` Mike Hommey
2014-02-13 8:30 ` David Kastrup
2014-02-12 20:06 ` Junio C Hamano
2014-02-12 20:27 ` Stefan Zager
2014-02-12 23:05 ` Junio C Hamano
2014-02-12 11:59 ` Erik Faye-Lund
2014-02-12 18:20 ` Stefan Zager
2014-02-12 18:27 ` Erik Faye-Lund
2014-02-12 18:34 ` Stefan Zager
2014-02-12 18:37 ` Erik Faye-Lund
2014-02-12 19:22 ` Karsten Blees
2014-02-12 19:30 ` Stefan Zager
2014-02-13 8:27 ` Johannes Sixt
2014-02-13 8:38 ` David Kastrup
2014-02-13 18:40 ` Stefan Zager
2014-02-13 18:38 ` Zachary Turner
2014-02-13 22:51 ` Karsten Blees
2014-02-13 22:53 ` Stefan Zager
2014-02-13 23:09 ` Zachary Turner
2014-02-14 19:04 ` Karsten Blees [this message]
[not found] ` <CAAErz9g7ND1htfk=yxRJJLbSEgBi4EV_AHC9uDRptugGWFWcXw@mail.gmail.com>
2014-02-14 19:16 ` Zachary Turner
2014-02-14 23:10 ` Karsten Blees
2014-02-15 0:45 ` Duy Nguyen
2014-02-15 0:50 ` Stefan Zager
2014-02-15 0:56 ` Duy Nguyen
2014-02-15 1:15 ` Zachary Turner
2014-02-15 1:39 ` Duy Nguyen
2014-02-18 17:55 ` Junio C Hamano
2014-02-18 18:14 ` Zachary Turner
2014-02-14 19:52 ` Stefan Zager
2014-02-14 21:49 ` Stefan Zager
2014-02-13 1:42 ` brian m. carlson
2019-04-02 0:52 ` Matheus Tavares
2019-04-02 1:07 ` Duy Nguyen
2019-04-02 10:30 ` David Kastrup
2019-04-02 11:35 ` Duy Nguyen
2019-04-02 11:52 ` David Kastrup
2019-04-02 19:06 ` Matheus Tavares Bernardino
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