Messages in this thread | | | Date | Mon, 7 Dec 2009 13:54:27 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 4/4] copy_signal cleanup: clean tty_audit_fork() |
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On 12/06, Miloslav Trmac wrote: > > ----- "Oleg Nesterov" <oleg@redhat.com> wrote: > > On 12/05, Miloslav Trmac wrote: > > > > Off-topic question to this who understands this code. > > > > > > > > But afaics we can also remove ->siglock from this helper and make > > > > it really trivial for being inline. ->siglock buys nothing, we just > > > > read a boolean. In fact, after the quick grep I do not understand > > > > how ->siglock is connected to ->audit_tty. OK, it protects > > > > tty_audit_buf, > > > > but why we always take ->siglock to access ->audit_tty ? > > > AFAIK there is no explicit documentation of the atomicity semantics > > > expected by the Linux kernel (both from the hardware and from the compiler), > > > so every access to the boolean is protected by a lock, to be on the safe side. > > > > Not sure I understand, but the kernel relies on fact it is always safe > > to load/store a word. > And is "word" an "unsigned", "unsigned long" or "intptr_t"? Must it be > suitably aligned, and if so, what is "suitably"?
Sure, it must be aligned.
> Where is this documented?
Perhaps nowhere, I do not know. If this is not documented, probably it would be nice to add a note.
> > What atomicity semantics do you mean and how ->siglock can help? > At the very least, "any access will read the last value stored and not result > in undefined behavior, even if other threads attempt to access the value". > In user-space, per POSIX, the only way to guarantee this is using explicit > synchronization primitives.
We have numerous examples in kernel code which rely on this fact.
If we are talking about copy_process() pathes, please look at, say, sched_fork(). Say, we read current->normal_prio lockless, while another thread could change ->normal_prio in parallel.
Oleg.
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