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

From: Will Deacon
Date: Thu Jul 03 2014 - 13:53:12 EST


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 <jean.pihet@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> ---
> 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
>
>
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