Re: [PATCH] ipc/sem.c: use READ_ONCE()/WRITE_ONCE() for use_global_lock
From: Manfred Spraul
Date: Fri May 14 2021 - 16:25:26 EST
Hi Paul,
On 5/14/21 9:44 PM, Paul E. McKenney wrote:
On Fri, May 14, 2021 at 07:53:19PM +0200, Manfred Spraul wrote:
The patch solves two weaknesses in ipc/sem.c:
1) The initial read of use_global_lock in sem_lock() is an
intentional race. KCSAN detects these accesses and prints
a warning.
2) The code assumes that plain C read/writes are not
mangled by the CPU or the compiler.
To solve both issues, use READ_ONCE()/WRITE_ONCE().
Plain C reads are used in code that owns sma->sem_perm.lock.
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
One follow-up question: If I am reading the code correctly, there is
a call to complexmode_enter() from sysvipc_sem_proc_show() that does
not hold the global lock. Does this mean that the first check of
->use_global_lock in complexmode_enter() should be marked?
Now you made me nervous, usually I do not test the proc interface.
According to the documentation in sysvipc_sem_proc_show(),
sysvipc_find_ipc() acquires the global lock.
/*
* The proc interface isn't aware of sem_lock(), it calls
* ipc_lock_object() directly (in sysvipc_find_ipc).
* In order to stay compatible with sem_lock(), we must
* enter / leave complex_mode.
*/
I have just tested it again: Yes, this is still true.
Perhaps, as future improvement: The rest of ipc/sem.c speaks about
"sem_perm.lock", and here we suddenly use a function name instead of the
structure member name.
> "it calls ipc_lock_object() (i.e.: spin_lock(&sma->sem_perm.lock)).
Thanx, Paul
---
ipc/sem.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/ipc/sem.c b/ipc/sem.c
index bf534c74293e..a0ad3a3edde2 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -217,6 +217,8 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
* 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.
+ * To prevent the compiler/cpu temporarily writing 0 to use_global_lock,
+ * READ_ONCE()/WRITE_ONCE() is used.
*
* 2) queue.status: (SEM_BARRIER_2)
* Initialization is done while holding sem_lock(), so no further barrier is
@@ -342,10 +344,10 @@ static void complexmode_enter(struct sem_array *sma)
* Nothing to do, just reset the
* counter until we return to simple mode.
*/
- sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
+ WRITE_ONCE(sma->use_global_lock, USE_GLOBAL_LOCK_HYSTERESIS);
return;
}
- sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
+ WRITE_ONCE(sma->use_global_lock, USE_GLOBAL_LOCK_HYSTERESIS);
for (i = 0; i < sma->sem_nsems; i++) {
sem = &sma->sems[i];
@@ -371,7 +373,8 @@ static void complexmode_tryleave(struct sem_array *sma)
/* See SEM_BARRIER_1 for purpose/pairing */
smp_store_release(&sma->use_global_lock, 0);
} else {
- sma->use_global_lock--;
+ WRITE_ONCE(sma->use_global_lock,
+ sma->use_global_lock-1);
}
}
@@ -412,7 +415,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
* Initial check for use_global_lock. Just an optimization,
* no locking, no memory barrier.
*/
- if (!sma->use_global_lock) {
+ if (!READ_ONCE(sma->use_global_lock)) {
/*
* It appears that no complex operation is around.
* Acquire the per-semaphore lock.
--
2.31.1