@@ -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);
Under perf, the fp unwinding scheme requires access to user space memory and can provoke a pagefault via call to __copy_from_user_inatomic from user_backtrace. This unwinding can take place in response to an interrupt (__perf_event_overflow). This is undesirable as we may already have mmap_sem held for write. One example being a process that calls mprotect just as a the PMU counters overflow. An example that can provoke this behaviour: perf record -e event:tocapture --call-graph fp ./application_to_test This patch addresses this issue by disabling pagefaults briefly in user_backtrace (as is done in the other architectures: ARM64, x86, Sparc etc.). Without the patch a deadlock occurs when __perf_event_overflow is called while reading the data from the user space: [ INFO: possible recursive locking detected ] 3.16.0-rc2-00038-g0ed7ff6 #46 Not tainted --------------------------------------------- stress/1634 is trying to acquire lock: (&mm->mmap_sem){++++++}, at: [<c001dc04>] do_page_fault+0xa8/0x428 but task is already holding lock: (&mm->mmap_sem){++++++}, at: [<c00f4098>] SyS_mprotect+0xa8/0x1c8 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(&mm->mmap_sem); lock(&mm->mmap_sem); *** DEADLOCK *** May be due to missing lock nesting notation 2 locks held by stress/1634: #0: (&mm->mmap_sem){++++++}, at: [<c00f4098>] SyS_mprotect+0xa8/0x1c8 #1: (rcu_read_lock){......}, at: [<c00c29dc>] __perf_event_overflow+0x120/0x294 stack backtrace: CPU: 1 PID: 1634 Comm: stress Not tainted 3.16.0-rc2-00038-g0ed7ff6 #46 [<c0017c8c>] (unwind_backtrace) from [<c0012eec>] (show_stack+0x20/0x24) [<c0012eec>] (show_stack) from [<c04de914>] (dump_stack+0x7c/0x98) [<c04de914>] (dump_stack) from [<c006a360>] (__lock_acquire+0x1484/0x1cf0) [<c006a360>] (__lock_acquire) from [<c006b14c>] (lock_acquire+0xa4/0x11c) [<c006b14c>] (lock_acquire) from [<c04e3880>] (down_read+0x40/0x7c) [<c04e3880>] (down_read) from [<c001dc04>] (do_page_fault+0xa8/0x428) [<c001dc04>] (do_page_fault) from [<c00084ec>] (do_DataAbort+0x44/0xa8) [<c00084ec>] (do_DataAbort) from [<c0013a1c>] (__dabt_svc+0x3c/0x60) Exception stack(0xed7c5ae0 to 0xed7c5b28) 5ae0: ed7c5b5c b6dadff4 ffffffec 00000000 b6dadff4 ebc08000 00000000 ebc08000 5b00: 0000007e 00000000 ed7c4000 ed7c5b94 00000014 ed7c5b2c c001a438 c0236c60 5b20: 00000013 ffffffff [<c0013a1c>] (__dabt_svc) from [<c0236c60>] (__copy_from_user+0xa4/0x3a4) Signed-off-by: Jean Pihet <jean.pihet@linaro.org> Cc: Will Deacon <will.deacon@arm.com> Acked-by: Steve Capper <steve.capper@linaro.org> --- arch/arm/kernel/perf_event.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)