[ ssic-linux-Bugs-1984656 ] semaphore undo count sometimes screws up

Bugs item #1984656, was opened at 2008-06-04 18:41
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Category: IPC
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: John Hughes (hughesj)
Assigned to: Nobody/Anonymous (nobody)
Summary: semaphore undo count sometimes screws up

Initial Comment:
Testing alsa applications, which use the rather strange idiom that:

DOWN(sem) = SEMOP (wait for semaphore to go to zero, increment semaphore)

UP(sem) = SEMOP (decrement semaphore)

we find that sometimes the semaphore is left at 1 when a process exits, which should be impossible as all operations are done with the SEM_UNDO flag set.

Tracing the ssi_semexit code shows an adjustment of +1 being applied to a semaphore with value 0, which should also be impossible - with this idiom the undo should either be -1 or zero

Argument:

The semaphore is initially zero.  If an application does a DOWN() call then it will have an adjustment of -1.  When it does an UP it will have an adjustment of 0.




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