Re: PROBLEM: pthread-safety bug in write(2) on Linux 2.6.x

[Kyle Moffett]

On Apr 13, 2006, at 05:56:08, Andrew Morton wrote:
> Dan Bonachea <bonachead@xxxxxxxxxxx> wrote:
> > This problem arose in the parallel runtime system for a scientific  
> > language compiler (nearly a million lines of code total -  
> > definitely a "real-world" program) - the example code is merely a  
> > pared-down demonstration of the problem. In parallel scientific  
> > computing, it's very common for many threads to be writing to  
> > stdout (usually for monitoring purposes) and it's expected and  
> > normal for output from separate threads to be arbitrarily  
> > interleaved, but it's *not* ok for output to be lost entirely.  
> > This is essentially equivalent to the real-world example you gave  
> > of many threads logging to a file.
>
> Interesting - afaik that's the first time this has been hit in a  
> real application.

I would guess that it could also be a problem with a wide variety of  
Perl CGIs running under Apache2+mod_perl with the worker threading  
model.  In fact this may actually explain some odd behavior I got  
from one such module that I "fixed" by switching to logging via  
syslog.  I don't remember which module it was or what the exact  
problem was, but I think it seemed similar in nature.  That is  
somewhat of a surprising and nonintuitive failure mode for logging,  
IMHO it would be nice to get fixed.  I would imagine that this has  
probably been hit a number of times before, mysteriously fixed by  
changed userspace locking or subtle thread ordering, and written off  
as the mysterious effects of cosmic rays on RAM.

Cheers,
Kyle Moffett

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