[SRU][Bionic][PATCH 1/1] powerpc/powernv: Fix concurrency issue with npu->mmio_atsd_usage

Khalid Elmously khalid.elmously at canonical.com
Thu Aug 30 19:44:44 UTC 2018

From: Reza Arbab <arbab at linux.ibm.com>

BugLink: https://bugs.launchpad.net/bugs/1788097

We've encountered a performance issue when multiple processors stress
{get,put}_mmio_atsd_reg(). These functions contend for
mmio_atsd_usage, an unsigned long used as a bitmask.

The accesses to mmio_atsd_usage are done using test_and_set_bit_lock()
and clear_bit_unlock(). As implemented, both of these will require
a (successful) stwcx to that same cache line.

What we end up with is thread A, attempting to unlock, being slowed by
other threads repeatedly attempting to lock. A's stwcx instructions
fail and retry because the memory reservation is lost every time a
different thread beats it to the punch.

There may be a long-term way to fix this at a larger scale, but for
now resolve the immediate problem by gating our call to
test_and_set_bit_lock() with one to test_bit(), which is obviously
implemented without using a store.

Fixes: 1ab66d1fbada ("powerpc/powernv: Introduce address translation services for Nvlink2")
Signed-off-by: Reza Arbab <arbab at linux.ibm.com>
Acked-by: Alistair Popple <alistair at popple.id.au>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
(cherry-picked from 9eab9901b015f489199105c470de1ffc337cfabb)
Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
 arch/powerpc/platforms/powernv/npu-dma.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
index 6c8e168e6571..18226895681e 100644
--- a/arch/powerpc/platforms/powernv/npu-dma.c
+++ b/arch/powerpc/platforms/powernv/npu-dma.c
@@ -434,8 +434,9 @@ static int get_mmio_atsd_reg(struct npu *npu)
 	int i;
 	for (i = 0; i < npu->mmio_atsd_count; i++) {
-		if (!test_and_set_bit_lock(i, &npu->mmio_atsd_usage))
-			return i;
+		if (!test_bit(i, &npu->mmio_atsd_usage))
+			if (!test_and_set_bit_lock(i, &npu->mmio_atsd_usage))
+				return i;
 	return -ENOSPC;

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