Commit 17d056e0 authored by Manfred Spraul's avatar Manfred Spraul Committed by Linus Torvalds
Browse files

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: default avatarManfred Spraul <manfred@colorfullife.com>
Reviewed-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Reviewed-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: <1vier1@web.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent bc8136a5

ipc/sem.c

+9 −5
Original line number Diff line number Diff line
@@ -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.

	 */