Re: Semaphores and threads anomaly and bug?

[Manfred Spraul]

John wrote:

> I know this isn't defined anywhere but the seems to be an ambiguity and 
> discrepancy between versions of Unix and Linux over threads and semaphores.
>
> Do the "SEM_UNDO"s get applied when a thread terminates or when the 
> "whole thing" terminates?
>  
According to the Unix spec: per-process.
Older Linux kernels applied it per-thread. Newer kernels can handle it 
per-process, and AFAIK it's the default for NPTL.

> I think that in ipc/sem.c line 1062 the line should be made 
> conditional on "u->semadj[i]" being non-zero.
>  
Fixed in 2.6. But there is another bug in that block: undos can increase 
the semaphore value above SEMVMX.

> There is a potential problem here in that the code in ipc/sem.c doesn't 
> allow the adjustment to yield a negative value but what if it starts at 
> zero, thread A increments it, thread B decrements it back to zero (both 
> with SEM_UNDO) and thread A exits first? Thread A's undo won't work and 
> then thread B's undo will increment it again leaving it in an incorrect 
> state which is different from thread B exiting first.
>  
Correct. undo operations should never try to decrease the semaphore 
value - an attempt to decrease below 0 is either silently ignored, or 
the semaphore value is set to 0.

--
   Manfred

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