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authorManfred Spraul <manfred@colorfullife.com>2020-02-03 17:34:42 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2020-02-04 03:05:24 +0000
commit8116b54e7e23ef948ecac0e0ab78d10888265cab (patch)
tree78124f712b73ade494a06d584fe9286f76180076 /ipc
parent0d97a82ba830d89a1e541cc9cd11f1e38c28e416 (diff)
ipc/sem.c: document and update memory barriers
Document and update the memory barriers in ipc/sem.c: - Add smp_store_release() to wake_up_sem_queue_prepare() and document why it is needed. - Read q->status using READ_ONCE+smp_acquire__after_ctrl_dep(). as the pair for the barrier inside wake_up_sem_queue_prepare(). - Add comments to all barriers, and mention the rules in the block regarding locking. - Switch to using wake_q_add_safe(). Link: http://lkml.kernel.org/r/20191020123305.14715-6-manfred@colorfullife.com Signed-off-by: Manfred Spraul <manfred@colorfullife.com> Cc: Waiman Long <longman@redhat.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: <1vier1@web.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Will Deacon <will.deacon@arm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'ipc')
-rw-r--r--ipc/sem.c66
1 files changed, 41 insertions, 25 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index ec97a7072413..4f4303f32077 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -205,15 +205,38 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
*
* Memory ordering:
* Most ordering is enforced by using spin_lock() and spin_unlock().
- * The special case is use_global_lock:
+ *
+ * Exceptions:
+ * 1) use_global_lock: (SEM_BARRIER_1)
* Setting it from non-zero to 0 is a RELEASE, this is ensured by
- * using smp_store_release().
+ * using smp_store_release(): Immediately after setting it to 0,
+ * a simple op can start.
* Testing if it is non-zero is an ACQUIRE, this is ensured by using
* smp_load_acquire().
* Setting it from 0 to non-zero must be ordered with regards to
* this smp_load_acquire(), this is guaranteed because the smp_load_acquire()
* is inside a spin_lock() and after a write from 0 to non-zero a
* spin_lock()+spin_unlock() is done.
+ *
+ * 2) queue.status: (SEM_BARRIER_2)
+ * Initialization is done while holding sem_lock(), so no further barrier is
+ * required.
+ * Setting it to a result code is a RELEASE, this is ensured by both a
+ * smp_store_release() (for case a) and while holding sem_lock()
+ * (for case b).
+ * The AQUIRE when reading the result code without holding sem_lock() is
+ * achieved by using READ_ONCE() + smp_acquire__after_ctrl_dep().
+ * (case a above).
+ * Reading the result code while holding sem_lock() needs no further barriers,
+ * the locks inside sem_lock() enforce ordering (case b above)
+ *
+ * 3) current->state:
+ * current->state is set to TASK_INTERRUPTIBLE while holding sem_lock().
+ * The wakeup is handled using the wake_q infrastructure. wake_q wakeups may
+ * happen immediately after calling wake_q_add. As wake_q_add_safe() is called
+ * when holding sem_lock(), no further barriers are required.
+ *
+ * See also ipc/mqueue.c for more details on the covered races.
*/
#define sc_semmsl sem_ctls[0]
@@ -344,12 +367,8 @@ static void complexmode_tryleave(struct sem_array *sma)
return;
}
if (sma->use_global_lock == 1) {
- /*
- * Immediately after setting use_global_lock to 0,
- * a simple op can start. Thus: all memory writes
- * performed by the current operation must be visible
- * before we set use_global_lock to 0.
- */
+
+ /* See SEM_BARRIER_1 for purpose/pairing */
smp_store_release(&sma->use_global_lock, 0);
} else {
sma->use_global_lock--;
@@ -400,7 +419,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
*/
spin_lock(&sem->lock);
- /* pairs with smp_store_release() */
+ /* see SEM_BARRIER_1 for purpose/pairing */
if (!smp_load_acquire(&sma->use_global_lock)) {
/* fast path successful! */
return sops->sem_num;
@@ -766,15 +785,12 @@ would_block:
static inline void wake_up_sem_queue_prepare(struct sem_queue *q, int error,
struct wake_q_head *wake_q)
{
- wake_q_add(wake_q, q->sleeper);
- /*
- * Rely on the above implicit barrier, such that we can
- * ensure that we hold reference to the task before setting
- * q->status. Otherwise we could race with do_exit if the
- * task is awoken by an external event before calling
- * wake_up_process().
- */
- WRITE_ONCE(q->status, error);
+ get_task_struct(q->sleeper);
+
+ /* see SEM_BARRIER_2 for purpuse/pairing */
+ smp_store_release(&q->status, error);
+
+ wake_q_add_safe(wake_q, q->sleeper);
}
static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
@@ -2148,9 +2164,11 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
}
do {
+ /* memory ordering ensured by the lock in sem_lock() */
WRITE_ONCE(queue.status, -EINTR);
queue.sleeper = current;
+ /* memory ordering is ensured by the lock in sem_lock() */
__set_current_state(TASK_INTERRUPTIBLE);
sem_unlock(sma, locknum);
rcu_read_unlock();
@@ -2173,13 +2191,8 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
*/
error = READ_ONCE(queue.status);
if (error != -EINTR) {
- /*
- * User space could assume that semop() is a memory
- * barrier: Without the mb(), the cpu could
- * speculatively read in userspace stale data that was
- * overwritten by the previous owner of the semaphore.
- */
- smp_mb();
+ /* see SEM_BARRIER_2 for purpose/pairing */
+ smp_acquire__after_ctrl_dep();
goto out_free;
}
@@ -2189,6 +2202,9 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
if (!ipc_valid_object(&sma->sem_perm))
goto out_unlock_free;
+ /*
+ * No necessity for any barrier: We are protect by sem_lock()
+ */
error = READ_ONCE(queue.status);
/*

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