Subject: Re: [PATCH 1/2] Make xmalloc and xrealloc thread-safe

From: Fredrik Kuivinen <[email-masked]>
Date: Wed, 24 Mar 2010 16:23:29 +0100

On Wed, Mar 24, 2010 at 00:50, Nicolas Pitre <[email-masked]> wrote:
> On Tue, 23 Mar 2010, Fredrik Kuivinen wrote:
>
>> On Tue, Mar 23, 2010 at 19:43, Shawn O. Pearce <[email-masked]> wrote:
>> > If that is what we are doing, disabling the release of pack windows
>> > when malloc fails, why can't we do that all of the time?
>>
>> The idea was that most git programs are single threaded, so they can
>> still benefit from releasing the pack windows when they are low on
>> memory.
>
> This is bobus. The Git program using the most memory is probably
> pack-objects and it is threaded.  Most single-threaded programs don't
> use close to as much memory.

Ok, you are right. But xmalloc/xrealloc cannot be used in multiple
threads simultaneously without some serialization.

For example, I think there are some potential race conditions in the
pack-objects code. In the threaded code we have the following call
chains leading to xcalloc, xmalloc, and xrealloc:

find_deltas -> xcalloc
find_deltas -> do_compress -> xmalloc
find_deltas -> try_delta -> xrealloc
find_deltas -> try_delta -> read_sha1_file -> ... -> xmalloc  (called
with read_lock held, but it can still race with the other calls)

As far as I can see there is no serialization between these calls.

- Fredrik