path: root/ipc
diff options
authorManfred Spraul <manfred@colorfullife.com>2014-06-06 14:37:51 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-06 16:08:15 -0700
commitb220c57aeccf903ec6550b30c52cc0248ddb26bf (patch)
treef0fa284d5819e16f9071ed525d93e40cd1800b7a /ipc
parentipc/sem.c: store which operation blocks in perform_atomic_semop() (diff)
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. Note: 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: Manfred: : - 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. Michael: : * 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>
Diffstat (limited to 'ipc')
1 files changed, 13 insertions, 21 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index d61362b31f1e..910c7a3865c8 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -993,38 +993,30 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
- * check_qop: Test how often a queued operation sleeps on the semaphore semnum
+ * check_qop: Test if a queued operation sleeps on the semaphore semnum
static int check_qop(struct sem_array *sma, int semnum, struct sem_queue *q,
bool count_zero)
- struct sembuf *sops = q->sops;
- int nsops = q->nsops;
- int i, semcnt;
+ struct sembuf *sop = q->blocking;
- semcnt = 0;
+ if (sop->sem_num != semnum)
+ return 0;
- for (i = 0; i < nsops; i++) {
- if (sops[i].sem_num != semnum)
- continue;
- if (sops[i].sem_flg & IPC_NOWAIT)
- continue;
- if (count_zero && sops[i].sem_op == 0)
- semcnt++;
- if (!count_zero && sops[i].sem_op < 0)
- semcnt++;
- }
- return semcnt;
+ if (count_zero && sop->sem_op == 0)
+ return 1;
+ if (!count_zero && sop->sem_op < 0)
+ return 1;
+ return 0;
/* The following counts are associated to each semaphore:
* semncnt number of tasks waiting on semval being nonzero
* semzcnt number of tasks waiting on semval being zero
- * This model assumes that a task waits on exactly one semaphore.
- * Since semaphore operations are to be performed atomically, tasks actually
- * wait on a whole sequence of semaphores simultaneously.
- * The counts we return here are a rough approximation, but still
- * warrant that semncnt+semzcnt>0 if the task is on the pending queue.
+ *
+ * Per definition, a task waits only on the semaphore of the first semop
+ * that cannot proceed, even if additional operation would block, too.
static int count_semcnt(struct sem_array *sma, ushort semnum,
bool count_zero)