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authorManfred Spraul <manfred@colorfullife.com>2021-06-30 18:57:15 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2021-07-01 11:06:07 -0700
commit17d056e0bdaab3d3f1fbec1ac154addcc4183aed (patch)
tree1bd3fa260f546c38caab76a9542915d1f26db0c9
parentbc8136a543aa839a848b49af5e101ac6de5f6b27 (diff)
ipc/sem.c: use READ_ONCE()/WRITE_ONCE() for use_global_lock
The patch solves three 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. 3) The comment it sysvipc_sem_proc_show() was hard to understand: The rest of the comments in ipc/sem.c speaks about sem_perm.lock, and suddenly this function speaks about ipc_lock_object(). To solve 1) and 2), use READ_ONCE()/WRITE_ONCE(). Plain C reads are used in code that owns sma->sem_perm.lock. The comment is updated to solve 3) [manfred@colorfullife.com: use READ_ONCE()/WRITE_ONCE() for use_global_lock] Link: https://lkml.kernel.org/r/20210627161919.3196-3-manfred@colorfullife.com Link: https://lkml.kernel.org/r/20210514175319.12195-1-manfred@colorfullife.com Signed-off-by: Manfred Spraul <manfred@colorfullife.com> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Davidlohr Bueso <dbueso@suse.de> Cc: <1vier1@web.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--ipc/sem.c14
1 files changed, 9 insertions, 5 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index 3a58188733d8..971e75d28364 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.
@@ -2436,7 +2439,8 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
/*
* The proc interface isn't aware of sem_lock(), it calls
- * ipc_lock_object() directly (in sysvipc_find_ipc).
+ * ipc_lock_object(), i.e. spin_lock(&sma->sem_perm.lock).
+ * (in sysvipc_find_ipc)
* In order to stay compatible with sem_lock(), we must
* enter / leave complex_mode.
*/