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SubjectRe: [PATCH] ipc/sem.c: use READ_ONCE()/WRITE_ONCE() for use_global_lock
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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@colorfullife.com>
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
>
> 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
>>

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