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@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Darren Hart <darren@dvhart.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Jerome Marchand <jmarchan@redhat.com> Cc: Larry Woodman <lwoodman@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mateusz Guzik <mguzik@redhat.com> Link: http://lkml.kernel.org/r/20150202140536.GA26406@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -900,7 +900,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
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if (!p)
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if (!p)
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return -ESRCH;
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return -ESRCH;
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if (!p->mm) {
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if (unlikely(p->flags & PF_KTHREAD)) {
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put_task_struct(p);
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put_task_struct(p);
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return -EPERM;
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return -EPERM;
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}
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}
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