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Subject[PATCH] rdma/siw: avoid smp_store_mb() on a u64
The new siw driver fails to build on i386 with

drivers/infiniband/sw/siw/siw_qp.c:1025:3: error: invalid output size for constraint '+q'
smp_store_mb(*cq->notify, SIW_NOTIFY_NOT);
include/asm-generic/barrier.h:141:35: note: expanded from macro 'smp_store_mb'
#define smp_store_mb(var, value) __smp_store_mb(var, value)
arch/x86/include/asm/barrier.h:65:47: note: expanded from macro '__smp_store_mb'
#define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
include/asm-generic/atomic-instrumented.h:1648:2: note: expanded from macro 'xchg'
arch_xchg(__ai_ptr, __VA_ARGS__); \
arch/x86/include/asm/cmpxchg.h:78:27: note: expanded from macro 'arch_xchg'
#define arch_xchg(ptr, v) __xchg_op((ptr), (v), xchg, "")
arch/x86/include/asm/cmpxchg.h:48:19: note: expanded from macro '__xchg_op'
: "+q" (__ret), "+m" (*(ptr)) \
drivers/infiniband/sw/siw/siw_qp.o: In function `siw_sqe_complete':
siw_qp.c:(.text+0x1450): undefined reference to `__xchg_wrong_size'
drivers/infiniband/sw/siw/siw_qp.o: In function `siw_rqe_complete':
siw_qp.c:(.text+0x15b0): undefined reference to `__xchg_wrong_size'
drivers/infiniband/sw/siw/siw_verbs.o: In function `siw_req_notify_cq':
siw_verbs.c:(.text+0x18ff): undefined reference to `__xchg_wrong_size'

Since smp_store_mb() has to be an atomic store, but the architecture
can only do this on 32-bit quantities or smaller, but 'cq->notify'
is a 64-bit word.

Apparently the smp_store_mb() is paired with a READ_ONCE() here, which
seems like an odd choice because there is only a barrier on the writer
side and not the reader, and READ_ONCE() is already not atomic on
quantities larger than a CPU register.

I suspect it is sufficient to use the (possibly nonatomic) WRITE_ONCE()
and an SMP memory barrier here. If it does need to be atomic as well
as 64-bit quantities, using an atomic64_set_release()/atomic64_read_acquire()
may be a better choice.

Fixes: 303ae1cdfdf7 ("rdma/siw: application interface")
Fixes: f29dd55b0236 ("rdma/siw: queue pair methods")
Cc: Peter Zijlstra <>
Signed-off-by: Arnd Bergmann <>
drivers/infiniband/sw/siw/siw_qp.c | 4 +++-
drivers/infiniband/sw/siw/siw_verbs.c | 5 +++--
2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/infiniband/sw/siw/siw_qp.c b/drivers/infiniband/sw/siw/siw_qp.c
index 11383d9f95ef..a2c08f17f13d 100644
--- a/drivers/infiniband/sw/siw/siw_qp.c
+++ b/drivers/infiniband/sw/siw/siw_qp.c
@@ -1016,13 +1016,15 @@ static bool siw_cq_notify_now(struct siw_cq *cq, u32 flags)
if (!cq->base_cq.comp_handler)
return false;

+ smp_rmb();
cq_notify = READ_ONCE(*cq->notify);

if ((cq_notify & SIW_NOTIFY_NEXT_COMPLETION) ||
((cq_notify & SIW_NOTIFY_SOLICITED) &&
(flags & SIW_WQE_SOLICITED))) {
/* dis-arm CQ */
- smp_store_mb(*cq->notify, SIW_NOTIFY_NOT);
+ WRITE_ONCE(*cq->notify, SIW_NOTIFY_NOT);
+ smp_wmb();

return true;
diff --git a/drivers/infiniband/sw/siw/siw_verbs.c b/drivers/infiniband/sw/siw/siw_verbs.c
index 32dc79d0e898..41c5ab293fe1 100644
--- a/drivers/infiniband/sw/siw/siw_verbs.c
+++ b/drivers/infiniband/sw/siw/siw_verbs.c
@@ -1142,10 +1142,11 @@ int siw_req_notify_cq(struct ib_cq *base_cq, enum ib_cq_notify_flags flags)

/* CQ event for next solicited completion */
- smp_store_mb(*cq->notify, SIW_NOTIFY_SOLICITED);
/* CQ event for any signalled completion */
- smp_store_mb(*cq->notify, SIW_NOTIFY_ALL);
+ WRITE_ONCE(*cq->notify, SIW_NOTIFY_ALL);
+ smp_wmb();

return cq->cq_put - cq->cq_get;
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