From: Randy Dunlap <rdunlap@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>,
Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>,
broonie@kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-next@vger.kernel.org, mhocko@suse.cz,
mm-commits@vger.kernel.org, sfr@canb.auug.org.au,
Linus Torvalds <torvalds@linux-foundation.org>,
viro@zeniv.linux.org.uk, x86@kernel.org,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] x86/uaccess: Remove redundant likely/unlikely annotations
Date: Fri, 29 May 2020 11:29:14 -0700 [thread overview]
Message-ID: <77d5741f-4b84-4cb8-1b01-3e411d3b8a70@infradead.org> (raw)
In-Reply-To: <20200529172505.fdjppgquujab7ayv@treble>
On 5/29/20 10:25 AM, Josh Poimboeuf wrote:
> On Fri, May 29, 2020 at 06:54:19PM +0200, Peter Zijlstra wrote:
>> On Fri, May 29, 2020 at 11:50:11AM -0500, Josh Poimboeuf wrote:
>>> The nested likelys seem like overkill anyway -- user_access_begin() is
>>> __always_inline and it already has unlikely(), which should be
>>> propagated.
>>>
>>> So just remove the outer likelys?
>>
>> That fixes it. Ack!
>
> If there are no objections to the patch, I can add it to my objtool-core
> branch unless anybody else wants to take it. It only affects
> linux-next.
>
> ---8<---
>
> From: Josh Poimboeuf <jpoimboe@redhat.com>
> Subject: [PATCH] x86/uaccess: Remove redundant likely/unlikely annotations
>
> Since user_access_begin() already has an unlikely() annotation for its
> access_ok() check, "if (likely(user_access_begin))" results in nested
> likely annotations. When combined with CONFIG_TRACE_BRANCH_PROFILING,
> GCC converges the error/success paths of the nested ifs, using a
> register value to distinguish between them.
>
> While the code is technically uaccess safe, it complicates the
> branch-profiling generated code. It also confuses objtool, because it
> doesn't do register value tracking, resulting in the following warnings:
>
> arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x2a4: call to memset() with UACCESS enabled
> arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_to_user()+0x243: return with UACCESS enabled
>
> The outer likely annotations aren't actually needed anyway, since the
> compiler propagates the error path coldness when it inlines
> user_access_begin().
>
> Fixes: 18372ef87665 ("x86_64: csum_..._copy_..._user(): switch to unsafe_..._user()")
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Acked-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Thanks.
> ---
> arch/x86/lib/csum-wrappers_64.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/lib/csum-wrappers_64.c b/arch/x86/lib/csum-wrappers_64.c
> index a12b8629206d..ee63d7576fd2 100644
> --- a/arch/x86/lib/csum-wrappers_64.c
> +++ b/arch/x86/lib/csum-wrappers_64.c
> @@ -27,7 +27,7 @@ csum_and_copy_from_user(const void __user *src, void *dst,
> might_sleep();
> *errp = 0;
>
> - if (!likely(user_access_begin(src, len)))
> + if (!user_access_begin(src, len))
> goto out_err;
>
> /*
> @@ -89,7 +89,7 @@ csum_and_copy_to_user(const void *src, void __user *dst,
>
> might_sleep();
>
> - if (unlikely(!user_access_begin(dst, len))) {
> + if (!user_access_begin(dst, len)) {
> *errp = -EFAULT;
> return 0;
> }
>
--
~Randy
next prev parent reply other threads:[~2020-05-29 18:29 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200513175005.1f4839360c18c0238df292d1@linux-foundation.org>
2020-05-14 3:31 ` mmotm 2020-05-13-20-30 uploaded Andrew Morton
2020-05-14 15:30 ` mmotm 2020-05-13-20-30 uploaded (gpu/drm/bridge/sil-sii8620) Randy Dunlap
2020-05-14 15:32 ` mmotm 2020-05-13-20-30 uploaded (objtool warnings) Randy Dunlap
2020-05-14 15:33 ` Randy Dunlap
2020-05-28 15:54 ` Josh Poimboeuf
2020-05-28 17:04 ` Josh Poimboeuf
2020-05-28 17:20 ` Peter Zijlstra
2020-05-29 13:57 ` Christoph Hellwig
2020-05-29 14:35 ` Peter Zijlstra
2020-05-29 14:53 ` Peter Zijlstra
2020-05-29 15:33 ` Peter Zijlstra
2020-05-29 16:05 ` Josh Poimboeuf
2020-05-29 16:12 ` Peter Zijlstra
2020-05-29 16:50 ` Josh Poimboeuf
2020-05-29 16:54 ` Peter Zijlstra
2020-05-29 17:25 ` [PATCH] x86/uaccess: Remove redundant likely/unlikely annotations Josh Poimboeuf
2020-05-29 18:29 ` Randy Dunlap [this message]
2020-05-29 19:31 ` mmotm 2020-05-13-20-30 uploaded (objtool warnings) Linus Torvalds
2020-05-29 20:08 ` Al Viro
2020-05-29 20:14 ` Al Viro
2020-05-15 23:30 ` mmotm 2020-05-15-16-29 uploaded Andrew Morton
2020-05-16 5:53 ` Stephen Rothwell
2020-05-18 23:15 ` Stephen Rothwell
2020-05-20 4:48 ` mmotm 2020-05-19-21-47 uploaded Andrew Morton
2020-05-22 3:43 ` mmotm 2020-05-21-20-42 uploaded Andrew Morton
2020-05-22 16:26 ` mmotm 2020-05-21-20-42 uploaded (atomisp) Randy Dunlap
2020-05-23 3:36 ` mmotm 2020-05-22-20-35 uploaded Andrew Morton
2020-05-23 15:08 ` mmotm 2020-05-22-20-35 uploaded (phy/intel/phy-intel-combo.c) Randy Dunlap
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