Re: [PATCH 2/2] arm64/sve: Eliminate data races on sve_default_vl

From: Will Deacon <will@kernel.org>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Qian Cai <cai@lca.pw>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/2] arm64/sve: Eliminate data races on sve_default_vl
Date: Tue, 16 Jun 2020 18:19:27 +0100	[thread overview]
Message-ID: <20200616171926.GA2129@willie-the-truck> (raw)
In-Reply-To: <20200616161704.GN25945@arm.com>

On Tue, Jun 16, 2020 at 05:17:04PM +0100, Dave Martin wrote:
> On Tue, Jun 16, 2020 at 04:04:51PM +0100, Will Deacon wrote:
> > On Tue, Jun 16, 2020 at 09:18:08AM -0400, Qian Cai wrote:
> > > On Wed, Jun 10, 2020 at 06:03:10PM +0100, Dave Martin wrote:
> > > > 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>
> > > 
> > > While this original patch looks correct, linux-next has this,
> > > 
> > > [will: move set_sve_default_vl() inside #ifdef to squash allnoconfig warning]
> > > 
> > > 1e570f512cbd ("arm64/sve: Eliminate data races on sve_default_vl")
> > > 
> > > which causes an error with CONFIG_ARM64_SVE=n,
> > > 
> > > This .config,
> > > https://raw.githubusercontent.com/cailca/linux-mm/master/arm64.config
> > > 
> > > arch/arm64/kernel/fpsimd.c: In function ‘sve_proc_do_default_vl’:
> > > arch/arm64/kernel/fpsimd.c:375:2: error: implicit declaration of
> > > function ‘set_sve_default_vl’; did you mean ‘get_sve_default_vl’?
> > > [-Werror=implicit-function-declaration]
> > >   set_sve_default_vl(find_supported_vector_length(vl));
> > >   ^~~~~~~~~~~~~~~~~~
> > >   get_sve_default_vl
> > 
> > Thanks, I'll take a look.
> 
> I haven't looked in detail at this; I guess the new helpers just
> need to be manually placed in the right #ifdef block.

That was what I did when I merged the patch, but that broke configurations
where SYSCTL is enabled but SVE is disabled. I've ended up with this
diff on top of for-next/fixes.

Will

--->8

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index d9eee9194511..55c8f3ec6705 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -349,7 +349,7 @@ static unsigned int find_supported_vector_length(unsigned int vl)
 	return sve_vl_from_vq(__bit_to_vq(bit));
 }
 
-#ifdef CONFIG_SYSCTL
+#if defined(CONFIG_ARM64_SVE) && defined(CONFIG_SYSCTL)
 
 static int sve_proc_do_default_vl(struct ctl_table *table, int write,
 				  void *buffer, size_t *lenp, loff_t *ppos)
@@ -394,9 +394,9 @@ static int __init sve_sysctl_init(void)
 	return 0;
 }
 
-#else /* ! CONFIG_SYSCTL */
+#else /* ! (CONFIG_ARM64_SVE && CONFIG_SYSCTL) */
 static int __init sve_sysctl_init(void) { return 0; }
-#endif /* ! CONFIG_SYSCTL */
+#endif /* ! (CONFIG_ARM64_SVE && CONFIG_SYSCTL) */
 
 #define ZREG(sve_state, vq, n) ((char *)(sve_state) +		\
 	(SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))

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