ipc/sem.c: make semctl(,,{GETNCNT,GETZCNT}) standard compliant

SUSv4 clearly defines how semncnt and semzcnt must be calculated: A task
waits on exactly one semaphore: The semaphore from the first operation
in the sop array that cannot proceed.

The Linux implementation never followed the standard, it tried to count
all semaphores that might be the reason why a task sleeps.

This patch fixes that.

a) The implementation assumes that GETNCNT and GETZCNT are rare operations,
   therefore the code counts them only on demand.
   (If they wouldn't be rare, then the non-compliance would have
   been found earlier)

b) compared to the initial version of the patch, the BUG_ONs were removed
   and it was clarified that the new behavior conforms to SUS.

Back-compatibility concerns:


: - there is no application in Fedora that uses GETNCNT or GETZCNT.
: - application that use only single-sop semop() are also safe, the
:   difference only affects complex apps.
: - portable application are also safe, the new behavior is standard
:   compliant.
: But that's it.  The old behavior existed in Linux from 0.99.something
: until now.


: * These operations seem to be very little used.  Grepping the public
:   source that is contained Fedora 20 source DVD, there appear to be no
:   uses.  Of course, this says nothing about uses in private /
:   non-mainstream FOSS code, but it seems likely that the same pattern
:   is followed there.
: * The existing behavior is hard enough to understand that I suspect
:   that no one understood it well enough to rely on it anyway
:   (especially as that behavior contradicted both man page and POSIX).
: So, there's a chance of breakage, but I estimate that it's minute.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 file changed