linux-kernel - [PATCH] drm/i915: Zero initialize this_cpu in busywait

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Date:   Thu,  7 Mar 2019 18:20:24 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH] drm/i915: Zero initialize this_cpu in busywait_stop

When building with -Wsometimes-uninitialized, Clang warns:

drivers/gpu/drm/i915/i915_request.c:1032:6: warning: variable 'this_cpu'
is used uninitialized whenever '&&' condition is false
[-Wsometimes-uninitialized]

time_after expands to use two typecheck with logical ANDs between them.
typecheck evaluates to 1 but Clang clearly gets confused with the logic
that as semantic analysis happens early in the pipeline. Fix this by
just zero initializing this_cpu as it will always be properly
initialized before the comparison below.

Link: https://github.com/ClangBuiltLinux/linux/issues/415
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---

Alternatively, this can be solved by having the return value of
local_clock_us(&this_cpu) be a local variable but this seems less
controversial.

 drivers/gpu/drm/i915/i915_request.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index c2a5c48c7541..06c0c952191f 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1027,7 +1027,7 @@ static unsigned long local_clock_us(unsigned int *cpu)
 
 static bool busywait_stop(unsigned long timeout, unsigned int cpu)
 {
-	unsigned int this_cpu;
+	unsigned int this_cpu = 0;
 
 	if (time_after(local_clock_us(&this_cpu), timeout))
 		return true;
-- 
2.21.0