Subject: [PATCH 2/3] ARM: perf: disable the pagefault handler when reading from user space

From: [email-masked] (Will Deacon)
Date: Thu, 3 Jul 2014 18:52:31 +0100

Hi Jean,

On Fri, Jun 27, 2014 at 03:57:46PM +0100, Jean Pihet wrote:
> As done on other architectures (ARM64, x86, Sparc etc.).
> 
> This prevents a deadlock on down_read in do_page_fault when unwinding
> using fp and triggering on kernel tracepoints:

So is this an issue because you could try setting tracepoints on the
pagefault path? If so, the patch is a little brutal as it would break user
backtracing as soon as we take any old page fault, no?

Or am I missing something obvious?

Will

>   INFO: task stress:2116 blocked for more than 120 seconds.
>         Not tainted 3.15.0-rc4-00364-g3401dfb-dirty #43
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   stress          D c04b41e8     0  2116   2115 0x00000000
>   [<c04b41e8>] (__schedule) from [<c04b46dc>] (schedule+0x40/0x90)
>   [<c04b46dc>] (schedule) from [<c04b6ec8>] (__down_read+0xc4/0xfc)
>   [<c04b6ec8>] (__down_read) from [<c04b69c0>] (down_read+0x18/0x1c)
>   [<c04b69c0>] (down_read) from [<c001d41c>] (do_page_fault+0xac/0x420)
>   [<c001d41c>] (do_page_fault) from [<c0008444>] (do_DataAbort+0x44/0xa8)
>   [<c0008444>] (do_DataAbort) from [<c00136b8>] (__dabt_svc+0x38/0x60)
>   Exception stack(0xecbc3af8 to 0xecbc3b40)
>   3ae0:                                                       ecbc3b74 b6d72ff4
>   3b00: ffffffec 00000000 b6d72ff4 ec0fc000 00000000 ec0fc000 0000007e 00000000
>   3b20: ecbc2000 ecbc3bac 00000014 ecbc3b44 c0019e78 c021ef44 00000013 ffffffff
>   [<c00136b8>] (__dabt_svc) from [<c021ef44>] (__copy_from_user+0xa4/0x3a0)
> 
> Signed-off-by: Jean Pihet <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> ---
>  arch/arm/kernel/perf_event.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
> index 6493c4c..f5aeca2 100644
> --- a/arch/arm/kernel/perf_event.c
> +++ b/arch/arm/kernel/perf_event.c
> @@ -560,11 +560,16 @@ user_backtrace(struct frame_tail __user *tail,
>  	       struct perf_callchain_entry *entry)
>  {
>  	struct frame_tail buftail;
> +	unsigned long err;
>  
> -	/* Also check accessibility of one struct frame_tail beyond */
>  	if (!access_ok(VERIFY_READ, tail, sizeof(buftail)))
>  		return NULL;
> -	if (__copy_from_user_inatomic(&buftail, tail, sizeof(buftail)))
> +
> +	pagefault_disable();
> +	err = __copy_from_user_inatomic(&buftail, tail, sizeof(buftail));
> +	pagefault_enable();
> +
> +	if (err)
>  		return NULL;
>  
>  	perf_callchain_store(entry, buftail.lr);
> -- 
> 1.8.1.2
> 
>