diff options
| author | Manfred Spraul <manfred@colorfullife.com> | 2021-06-30 18:57:15 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2021-07-01 11:06:07 -0700 |
| commit | 17d056e0bdaab3d3f1fbec1ac154addcc4183aed (patch) | |
| tree | 1bd3fa260f546c38caab76a9542915d1f26db0c9 | |
| parent | bc8136a543aa839a848b49af5e101ac6de5f6b27 (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.c | 14 |
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. */ |