[tip:locking/core] locking/futex: Check PF_KTHREAD rather than !p->mm to filter out kthreads

From: tip-bot for Oleg Nesterov
Date: Wed Feb 18 2015 - 12:12:12 EST

Commit-ID: a21294644623ee41034db60e93aaebed4db0e57b
Gitweb: http://git.kernel.org/tip/a21294644623ee41034db60e93aaebed4db0e57b
Author: Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate: Mon, 2 Feb 2015 15:05:36 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 18 Feb 2015 16:57:09 +0100

locking/futex: Check PF_KTHREAD rather than !p->mm to filter out kthreads

attach_to_pi_owner() checks p->mm to prevent attaching to kthreads and
this looks doubly wrong:

1. It should actually check PF_KTHREAD, kthread can do use_mm().

2. If this task is not kthread and it is actually the lock owner we can
wrongly return -EPERM instead of -ESRCH or retry-if-EAGAIN.

And note that this wrong EPERM is the likely case unless the exiting
task is (auto)reaped quickly, we check ->mm before PF_EXITING.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Darren Hart <darren@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Jerome Marchand <jmarchan@xxxxxxxxxx>
Cc: Larry Woodman <lwoodman@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mateusz Guzik <mguzik@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20150202140536.GA26406@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/futex.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 4eeb63d..1f6d646 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -900,7 +900,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
if (!p)
return -ESRCH;

- if (!p->mm) {
+ if (unlikely(p->flags & PF_KTHREAD)) {
return -EPERM;
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