[Patches] [ python-Patches-1366311 ] SRE engine do not release the GIL

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Patches item #1366311, was opened at 2005-11-25 14:57
Message generated for change (Comment added) made by loewis
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Category: None
Group: None
>Status: Pending
>Resolution: Rejected
Priority: 5
Private: No
Submitted By: Eric Noyau (eric_noyau)
Assigned to: Fredrik Lundh (effbot)
Summary: SRE engine do not release the GIL

Initial Comment:
In a multi-threaded program that does lots of regular
expression searching, some of them on very long strings
with complex regex we've noticed that everything stops
when a regular expression is searching.

One of the issue is that the re engine does not release
the interpreter lock while it is running. All the
other threads are therefore blocked for the entire time
it takes to do the regular expression search.

See the thread in python-dev about it:

http://mail.python.org/pipermail/python-dev/2005-November/058316.html



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>Comment By: Martin v. Löwis (loewis)
Date: 2006-11-25 16:13

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I believe the patch is incorrect. While matching, sre may allocate memory
through Python API, and it may raise exceptions through Python API.
Neither is allowed when the GIL is released

Tentatively rejecting the patch.

Eric, if you think the patch is correct or can be corrected, please update
it to the current subversion trunk.

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Comment By: Georg Brandl (birkenfeld)
Date: 2006-02-19 00:41

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Fredrik, do you have time to review this?

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Comment By: Eric Noyau (eric_noyau)
Date: 2005-11-28 15:11

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Thanks for your comments. I've updated the patch to fix your
issues, but without introducing a per-state object lock.

What I did instead is to mark a state as not supporting
concurrency when a scanner object creates it. So the GIL
will not be released for scanners objects at all.

For consistency match also release the GIL now, if possible.


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Comment By: Armin Rigo (arigo)
Date: 2005-11-25 22:38

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The patch looks good, but I wonder if it is safe.  The SRE_STATE structure
that SRE_SEARCH_INNER uses is potentially visible to the application-level
Python code, via the (undocumented) scanner objects:

>>> r = re.compile(r"hello")
>>> s = r.scanner("big string in which to search")
>>> s.search()
<_sre.SRE_Match object at 0x12345678>

Each call to s.search() continues the previous search with the same
SRE_STATE.  The problem with releasing the GIL as you do is that several
threads could call s.search() concurrently, which would most probably
crash CPython.

This probably means that you need to add a lock in SRE_STATE and acquire
it while searching, to serialize its usage.  Of course, we should then be
careful about what overhead this gives to applications that use regexps on
a lot of small strings...

Another note: for consistency, match() should also release the GIL if
search() does.

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Comment By: Eric Noyau (eric_noyau)
Date: 2005-11-25 15:02

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I'm attaching a diff to this bug that remove this limitation
if it sane to do so. If a search is done on a string or a
unicode object (which by definition are immutable) the GIL
is released and reacquired everytime a search is done.

I've tested this patch in both a simple tests (start a
thread with a greedy regex on a monstruous string and verify
that the othe python threads are still active) and by
running our internal application verifying that nothing is
blocking anymore.

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