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Subject[PATCH 1/1] 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@redhat.com>
---
kernel/futex.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 63678b5..b101381 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)) {
put_task_struct(p);
return -EPERM;
}
--
1.5.5.1



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