linux-kernel - [PATCH 5.5 215/399] mlx5: work around high stack usage with gcc

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Message-Id: <20200221072423.900172139@linuxfoundation.org>
Date:   Fri, 21 Feb 2020 08:39:00 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.5 215/399] mlx5: work around high stack usage with gcc

From: Arnd Bergmann <arnd@...db.de>

[ Upstream commit 42ae1a5c76691928ed217c7e40269db27f5225e9 ]

In some configurations, gcc tries too hard to optimize this code:

drivers/net/ethernet/mellanox/mlx5/core/en_stats.c: In function 'mlx5e_grp_sw_update_stats':
drivers/net/ethernet/mellanox/mlx5/core/en_stats.c:302:1: error: the frame size of 1336 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

As was stated in the bug report, the reason is that gcc runs into a corner
case in the register allocator that is rather hard to fix in a good way.

As there is an easy way to work around it, just add a comment and the
barrier that stops gcc from trying to overoptimize the function.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92657
Cc: Adhemerval Zanella <adhemerval.zanella@...aro.org>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_stats.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_stats.c b/drivers/net/ethernet/mellanox/mlx5/core/en_stats.c
index 9f09253f9f466..a05158472ed11 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_stats.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_stats.c
@@ -297,6 +297,9 @@ static void mlx5e_grp_sw_update_stats(struct mlx5e_priv *priv)
 			s->tx_tls_drop_bypass_req   += sq_stats->tls_drop_bypass_req;
 #endif
 			s->tx_cqes		+= sq_stats->cqes;
+
+			/* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92657 */
+			barrier();
 		}
 	}
 }
-- 
2.20.1