From: Dave Martin <Dave.Martin@xxxxxxx> [ 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@xxxxxxxxxx> Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx> Link: https://lore.kernel.org/r/1591808590-20210-3-git-send-email-Dave.Martin@xxxxxxx [will: move set_sve_default_vl() inside #ifdef to squash allnoconfig warning] Signed-off-by: Will Deacon <will@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- 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 14fdbaa6ee3ab..af59b42973141 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -22,6 +22,7 @@ #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> @@ -124,10 +125,20 @@ 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; /* Set of available vector lengths, as vq_to_bit(vq): */ @@ -311,7 +322,7 @@ 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), @@ -328,7 +339,7 @@ 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; } @@ -772,12 +783,12 @@ 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)); 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()); sve_efi_setup(); } @@ -914,13 +925,13 @@ 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
