From: Karsten Blees <karsten.blees@gmail.com>
To: Zachary Turner <zturner@chromium.org>
Cc: Stefan Zager <szager@google.com>, Git Mailing List <git@vger.kernel.org>
Subject: Re: Make the git codebase thread-safe
Date: Sat, 15 Feb 2014 00:10:41 +0100 [thread overview]
Message-ID: <52FEA271.2030405@gmail.com> (raw)
In-Reply-To: <CAAErz9j=_FpWLSyUk43pp8A6e7Ej0crT8ghW5-yxBEbGkd6O+A@mail.gmail.com>
Am 14.02.2014 20:16, schrieb Zachary Turner:
> For the mixed read, we wouldn't be looking for another caller of
> pread() (since it doesn't care what the file pointer is), but instead
> a caller of read() or lseek() (since those do depend on the current
> file pointer). In index-pack.c, I see two possible culprits:
>
> 1) A call to xread() from inside fill()
> 2) A call to lseek in parse_pack_objects()
>
> Do you think these could be related? If so, maybe that opens up some
> other solutions?
>
Yeah, I think that's it. The problem is that the single-threaded part (parse_pack_objects/parse_pack_header) _also_ calls pread (via sha1_object -> get_data_from_pack -> unpack_data). So a pread() that modifies the file position would naturally be bad in this single-threaded scenario. Incidentally, that's exactly what the lstat64 in the version below fixes (similar to git_pread).
> BTW, the version you posted isn't thread safe.
It is true that, in a multi-threaded scenario, my version modifies the file position in some indeterministic way. However, as you noted above, the file position is irrelevant to pread(), so that's perfectly thread-safe, as long as all threads use pread() exclusively.
Using [x]read() in one of the threads would _not_ be thread-safe, but we're not doing that here. Both fill()/xread() and parse_pack_objects()/lseek() are unreachable from threaded_second_pass(), and the main thread just waits for the background threads to complete...
>>> 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 23:10 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
[not found] ` <CAAErz9g7ND1htfk=yxRJJLbSEgBi4EV_AHC9uDRptugGWFWcXw@mail.gmail.com>
2014-02-14 19:16 ` Zachary Turner
2014-02-14 23:10 ` Karsten Blees [this message]
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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