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    SubjectRe: [PATCH 2/2] rcu: Remove needless preemption disablement in rcu_all_qs()
    On Tue, Jul 06, 2021 at 02:30:58PM +0200, Frederic Weisbecker wrote:
    > On Tue, Jul 06, 2021 at 09:51:01AM +0200, Peter Zijlstra wrote:
    > > On Tue, Jul 06, 2021 at 01:43:44AM +0200, Frederic Weisbecker wrote:
    > > > The preemption is already disabled when we write rcu_data.rcu_urgent_qs.
    > > > We can use __this_cpu_write() directly, although that path is mostly
    > > > used when CONFIG_PREEMPT=n.
    > > >
    > > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    > > > Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
    > > > Cc: Joel Fernandes <joel@joelfernandes.org>
    > > > Cc: Uladzislau Rezki <urezki@gmail.com>
    > > > Cc: Boqun Feng <boqun.feng@gmail.com>
    > > > ---
    > > > kernel/rcu/tree_plugin.h | 2 +-
    > > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > > >
    > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
    > > > index 27b74352cccf..38b3d01424d7 100644
    > > > --- a/kernel/rcu/tree_plugin.h
    > > > +++ b/kernel/rcu/tree_plugin.h
    > > > @@ -871,7 +871,7 @@ void rcu_all_qs(void)
    > > > preempt_enable();
    > > > return;
    > > > }
    > > > - this_cpu_write(rcu_data.rcu_urgent_qs, false);
    > > > + __this_cpu_write(rcu_data.rcu_urgent_qs, false);
    > >
    > > There's another subtle difference between this_cpu_write() and
    > > __this_cpu_write() aside from preempt. this_cpu_write() is also
    > > IRQ-safe, while __this_cpu_write() is not.
    > >
    > > I've not looked at the usage here to see if that is relevant, but the
    > > Changelog only mentioned the preempt side of things, and that argument
    > > is incomplete in general.
    >
    > You're right, I missed that. I see this rcu_urgent_qs is set by
    > RCU TASKS from rcu_tasks_wait_gp() (did I missed another path?).
    > Not sure if this is called from IRQ nor if it actually matters to
    > protect against IRQs for that single write.

    I think __this_cpu_write() being IRQ-unsafe means it may overwrite
    percpu writes to other bytes in the same word? Let's say the
    rcu_urgent_qs is the lowest byte in the word, the pseduo asm code of
    __this_cpu_write() may be:

    __this_cpu_write(ptr, v):
    long tmp = *ptr;
    tmp &= ~(0xff);
    tmp |= v;
    *ptr = tmp;

    and the following sequence introduces an overwrite:

    __this_cpu_write(ptr, v): // v is 0, and *ptr is 1
    long tmp = *ptr; // tmp is 1
    <interrupted>
    this_cpu_write() // modify another byte of *ptr, make it
    // 0xff01
    <ret from interrupt>
    tmp &= ~(0xff) // tmp is 0
    tmp |=v; // tmp is 0
    *ptr = tmp; // *ptr is 0, overwrite a percpu write on
    // another field.

    I know that many archs have byte-wise store, so compilers don't really
    have the reason to generate code as above, but __this_cpu_write() is
    just a normal write, nothing prevents this from happenning, unless I'm
    missing something here?

    Regards,
    Boqun

    >
    > I'm not quite used to rcu_tasks. Paul?

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