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ipc/sem.c: use READ_ONCE()/WRITE_ONCE() for use_global_lock

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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>
This commit is contained in:
Manfred Spraul 2021-06-30 18:57:15 -07:00 • committed by Linus Torvalds
parent bc8136a543
commit 17d056e0bd
1 changed files with 9 additions and 5 deletions
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14
ipc/sem.c
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@ -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() * 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 * is inside a spin_lock() and after a write from 0 to non-zero a
* spin_lock()+spin_unlock() is done. * 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) * 2) queue.status: (SEM_BARRIER_2)
* Initialization is done while holding sem_lock(), so no further barrier is * 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 * Nothing to do, just reset the
* counter until we return to simple mode. * 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; 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++) { for (i = 0; i < sma->sem_nsems; i++) {
sem = &sma->sems[i]; sem = &sma->sems[i];
@ -371,7 +373,8 @@ static void complexmode_tryleave(struct sem_array *sma)
/* See SEM_BARRIER_1 for purpose/pairing */ /* See SEM_BARRIER_1 for purpose/pairing */
smp_store_release(&sma->use_global_lock, 0); smp_store_release(&sma->use_global_lock, 0);
} else { } 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, * Initial check for use_global_lock. Just an optimization,
* no locking, no memory barrier. * 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. * It appears that no complex operation is around.
* Acquire the per-semaphore lock. * 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 * 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 * In order to stay compatible with sem_lock(), we must
* enter / leave complex_mode. * enter / leave complex_mode.
*/ */