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

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@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
---
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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