[PATCH AUTOSEL 5.7 07/28] arm64/sve: Eliminate data races on sve_default_vl

Sasha Levin
2020-06-23T17:44:02Z

From: Dave Martin <Dave.Martin@arm.com>

[ Upstream commit 1e570f512cbdc5e9e401ba640d9827985c1bea1e ]

sve_default_vl can be modified via the /proc/sys/abi/sve_default_vl
sysctl concurrently with use, and modified concurrently by multiple
threads.

Adding a lock for this seems overkill, and I don't want to think any
more than necessary, so just define wrappers using READ_ONCE()/
WRITE_ONCE().

This will avoid the possibility of torn accesses and repeated loads
and stores.

There's no evidence yet that this is going wrong in practice: this
is just hygiene. For generic sysctl users, it would be better to
build this kind of thing into the sysctl common code somehow.

Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/1591808590-20210-3-git-send-email-Dave.Martin@arm.com
[will: move set_sve_default_vl() inside #ifdef to squash allnoconfig warning]
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm64/kernel/fpsimd.c | 25 ++++++++++++++++++-------
1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 94289d1269933..4a77263c183b3 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -12,6 +12,7 @@
#include <linux/bug.h>
#include <linux/cache.h>
#include <linux/compat.h>
+#include <linux/compiler.h>
#include <linux/cpu.h>
#include <linux/cpu_pm.h>
#include <linux/kernel.h>
@@ -119,10 +120,20 @@ struct fpsimd_last_state_struct {
static DEFINE_PER_CPU(struct fpsimd_last_state_struct, fpsimd_last_state);

/* Default VL for tasks that don't set it explicitly: */
-static int sve_default_vl = -1;
+static int __sve_default_vl = -1;
+
+static int get_sve_default_vl(void)
+{
+ return READ_ONCE(__sve_default_vl);
+}

#ifdef CONFIG_ARM64_SVE

+static void set_sve_default_vl(int val)
+{
+ WRITE_ONCE(__sve_default_vl, val);
+}
+
/* Maximum supported vector length across all CPUs (initially poisoned) */
int __ro_after_init sve_max_vl = SVE_VL_MIN;
int __ro_after_init sve_max_virtualisable_vl = SVE_VL_MIN;
@@ -345,7 +356,7 @@ static int sve_proc_do_default_vl(struct ctl_table *table, int write,
loff_t *ppos)
{
int ret;
- int vl = sve_default_vl;
+ int vl = get_sve_default_vl();
struct ctl_table tmp_table = {
.data = &vl,
.maxlen = sizeof(vl),
@@ -362,7 +373,7 @@ static int sve_proc_do_default_vl(struct ctl_table *table, int write,
if (!sve_vl_valid(vl))
return -EINVAL;

- sve_default_vl = find_supported_vector_length(vl);
+ set_sve_default_vl(find_supported_vector_length(vl));
return 0;
}

@@ -869,7 +880,7 @@ void __init sve_setup(void)
* For the default VL, pick the maximum supported value <= 64.
* VL == 64 is guaranteed not to grow the signal frame.
*/
- sve_default_vl = find_supported_vector_length(64);
+ set_sve_default_vl(find_supported_vector_length(64));

bitmap_andnot(tmp_map, sve_vq_partial_map, sve_vq_map,
SVE_VQ_MAX);
@@ -890,7 +901,7 @@ void __init sve_setup(void)
pr_info("SVE: maximum available vector length %u bytes per vector\n",
sve_max_vl);
pr_info("SVE: default vector length %u bytes per vector\n",
- sve_default_vl);
+ get_sve_default_vl());

/* KVM decides whether to support mismatched systems. Just warn here: */
if (sve_max_virtualisable_vl < sve_max_vl)
@@ -1030,13 +1041,13 @@ void fpsimd_flush_thread(void)
* vector length configured: no kernel task can become a user
* task without an exec and hence a call to this function.
* By the time the first call to this function is made, all
- * early hardware probing is complete, so sve_default_vl
+ * early hardware probing is complete, so __sve_default_vl
* should be valid.
* If a bug causes this to go wrong, we make some noise and
* try to fudge thread.sve_vl to a safe value here.
*/
vl = current->thread.sve_vl_onexec ?
- current->thread.sve_vl_onexec : sve_default_vl;
+ current->thread.sve_vl_onexec : get_sve_default_vl();

if (WARN_ON(!sve_vl_valid(vl)))
vl = SVE_VL_MIN;
--
2.25.1