On Thu, 21 Jul 2016 19:54:55 +0200 Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:
> Next update:
> - switch to smp_store_mb() instead of WRITE_ONCE();smp_mb();
> - introduce SEM_GLOBAL_LOCK instead of magic -1.
> - do not use READ_ONCE() for the unlocked&unordered test:
> READ_ONCE doesn't make sense for unlocked&unordered code.
> - document why smp_mb() is required after spin_lock().
I assume "ipc/sem.c: remove duplicated memory barriers" is still
relevant?
From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Subject: ipc/sem.c: remove duplicated memory barriers
With 2c610022711 ("locking/qspinlock: Fix spin_unlock_wait() some more"),
memory barriers were added into spin_unlock_wait(). Thus another barrier
is not required.
And as explained in 055ce0fd1b8 ("locking/qspinlock: Add comments"),
spin_lock() provides a barrier so that reads within the critical section
cannot happen before the write for the lock is visible. i.e. spin_lock
provides an acquire barrier after the write of the lock variable, this
barrier pairs with the smp_mb() in complexmode_enter().
Link: http://lkml.kernel.org/r/1468386412-3608-3-git-send-email-manfred@xxxxxxxxxxxxxxxx
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: <1vier1@xxxxxx>
Cc: <felixh@xxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
ipc/sem.c | 16 ----------------
1 file changed, 16 deletions(-)
diff -puN ipc/sem.c~ipc-semc-remove-duplicated-memory-barriers ipc/sem.c
--- a/ipc/sem.c~ipc-semc-remove-duplicated-memory-barriers
+++ a/ipc/sem.c
@@ -290,14 +290,6 @@ static void complexmode_enter(struct sem
sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock);
}
- /*
- * spin_unlock_wait() is not a memory barriers, it is only a
- * control barrier. The code must pair with spin_unlock(&sem->lock),
- * thus just the control barrier is insufficient.
- *
- * smp_rmb() is sufficient, as writes cannot pass the control barrier.
- */
- smp_rmb();
}
/*
@@ -363,14 +355,6 @@ static inline int sem_lock(struct sem_ar
*/
spin_lock(&sem->lock);
- /*
- * See 51d7d5205d33
- * ("powerpc: Add smp_mb() to arch_spin_is_locked()"):
- * A full barrier is required: the write of sem->lock
- * must be visible before the read is executed
- */
- smp_mb();
-
if (!smp_load_acquire(&sma->complex_mode)) {
/* fast path successful! */
return sops->sem_num;
_
--
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