Messages in this thread | | | Date | Fri, 14 May 2021 14:00:27 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] ipc/sem.c: use READ_ONCE()/WRITE_ONCE() for use_global_lock |
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On Fri, May 14, 2021 at 10:25:17PM +0200, Manfred Spraul wrote: > 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.
"It is a service that I provide." ;-)
> > /* > > * 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.
OK, so the sequence of events is as follow?
o sysvipc_proc_start() is invoked to start, as the name implies.
o sysvipc_proc_start() invokes sysvipc_find_ipc(), which scans the IDs and invokes ipc_lock_object() on the one at pos.
o ipc_lock_object() acquires the corresponding lock, which seems unlikely to be sem_perm.lock, though I freely admit that I do not know this code very well.
Ah, I see it now. The kernel_ipc_perm that sysvipc_find_ipc is looking at is the first member of the sem_array structure, and that member is named sem_perm.
> 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)).
As usual, it seems obvious once you know the trick. ;-)
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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