op 11-04-14 10:38, Thomas Hellstrom schreef:
Hi, Maarten.
Here I believe we encounter a lot of locking inconsistencies.
First, it seems you're use a number of pointers as RCU pointers without
annotating them as such and use the correct rcu
macros when assigning those pointers.
Some pointers (like the pointers in the shared fence list) are both used
as RCU pointers (in dma_buf_poll()) for example,
or considered protected by the seqlock
(reservation_object_get_fences_rcu()), which I believe is OK, but then
the pointers must
be assigned using the correct rcu macros. In the memcpy in
reservation_object_get_fences_rcu() we might get away with an
ugly typecast, but with a verbose comment that the pointers are
considered protected by the seqlock at that location.
So I've updated (attached) the headers with proper __rcu annotation and
locking comments according to how they are being used in the various
reading functions.
I believe if we want to get rid of this we need to validate those
pointers using the seqlock as well.
This will generate a lot of sparse warnings in those places needing
rcu_dereference()
rcu_assign_pointer()
rcu_dereference_protected()
With this I think we can get rid of all ACCESS_ONCE macros: It's not
needed when the rcu_x() macros are used, and
it's never needed for the members protected by the seqlock, (provided
that the seq is tested). The only place where I think that's
*not* the case is at the krealloc in reservation_object_get_fences_rcu().
Also I have some more comments in the
reservation_object_get_fences_rcu() function below:
I felt that the barriers needed for rcu were already provided by checking the
seqcount lock.
But looking at rcu_dereference makes it seem harmless to add it in more places,
it handles
the ACCESS_ONCE and barrier() for us.
We could probably get away with using RCU_INIT_POINTER on the writer side,
because the smp_wmb is already done by arranging seqcount updates correctly.
diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
index d89a98d2c37b..ca6ef0c4b358 100644
--- a/drivers/base/dma-buf.c
+++ b/drivers/base/dma-buf.c
+int reservation_object_get_fences_rcu(struct reservation_object *obj,
+ struct fence **pfence_excl,
+ unsigned *pshared_count,
+ struct fence ***pshared)
+{
+ unsigned shared_count = 0;
+ unsigned retry = 1;
+ struct fence **shared = NULL, *fence_excl = NULL;
+ int ret = 0;
+
+ while (retry) {
+ struct reservation_object_list *fobj;
+ unsigned seq, retry;
You're shadowing retry?
Oops.
+
+ seq = read_seqcount_begin(&obj->seq);
+
+ rcu_read_lock();
+
+ fobj = ACCESS_ONCE(obj->fence);
+ if (fobj) {
+ struct fence **nshared;
+
+ shared_count = ACCESS_ONCE(fobj->shared_count);
+ nshared = krealloc(shared, sizeof(*shared) *
shared_count, GFP_KERNEL);
krealloc inside rcu_read_lock(). Better to put this first in the loop.
Except that shared_count isn't known until the rcu_read_lock is taken.
Thanks,
Thomas
~Maarten
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