[U-Boot-Users] [PATCH] Use `ln -sf` rather than `rm -f && ln -s`

[J. William Campbell]

Mike Frysinger wrote:
> On Monday 28 January 2008, Wolfgang Denk wrote:
>   
>> In message <200801280638.59012.vapier at gentoo.org> you wrote:
>>     
>>> odd ... git-send-email ate the explanatory text ...
>>> ---
>>> The -f option to `ln` should give the same behavior as the -f option to
>>> the `rm` command.  It is better to do this in one shot so as to avoid
>>> race conditions when building in parallel.  I build on a quad G5 and
>>> without this change, it isn't uncommon for the build to fail when using
>>> -j8 due to this small window where the files don't actually exist.
>>>       
>> Note that "ln -s -f" will come down to two separate system calls as
>> well:
>>
>> 	...
>> 	unlink();
>> 	symlink();
>> 	...
>>
>> So you don't avoid the race condition; you're just making it a little
>> less likely at the cost of reduced portability.
>>     
>
> yes, i know it's much less likely, but that difference on my system has been 
> from 1-in-20 build failures to 1-in-none-so-far-out-of-hundreds.  the real 
> fix is to overhaul the u-boot build system, but that'll take quite a bit of 
> time and this change is for all practical purposes, Good Enough.  i dont know 
> what portability issues you refer to considering the -f flag is in POSIX and 
> has been supported on all Linux systems since before that (pre-2000).
> -mike
>   
>   
I don't think there is a race condition at all in the "ln -s -f" case. 
The race condition exists in the first approach because the shell is 
allowed to execute the rm and the ln in parallel, while in the "ln -s 
-f" case the two system calls are sequential in the same thread of 
execution (inside ln) and therefore happen in a defined order. It is not 
just that the window is small, there is no window.
-Bill
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