Messages in this thread | | | From | Pranith Kumar <> | Date | Fri, 17 Apr 2015 11:55:20 -0400 | Subject | Re: [PATCH] rcu: small rcu_dereference doc update |
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On Fri, Apr 17, 2015 at 6:33 AM, Milos Vyletel <milos@redhat.com> wrote: > Make a note stating that repeated calls of rcu_dereference() may not > return the same pointer if update happens while in critical section.
Might as well make it more explicit with an example then. See below:
> > Reported-by: Jeff Haran <jeff.haran@citrix.com> > Signed-off-by: Milos Vyletel <milos@redhat.com> > --- > Documentation/RCU/whatisRCU.txt | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt > index 88dfce1..82b1b2c 100644 > --- a/Documentation/RCU/whatisRCU.txt > +++ b/Documentation/RCU/whatisRCU.txt > @@ -256,7 +256,9 @@ rcu_dereference() > If you are going to be fetching multiple fields from the > RCU-protected structure, using the local variable is of > course preferred. Repeated rcu_dereference() calls look > - ugly and incur unnecessary overhead on Alpha CPUs. > + ugly, do not guarantee that same pointer will be returned > + if update happened while in critical section and incur > + unnecessary overhead on Alpha CPUs. >
An example like follows:
struct some_ds { int data; bool ready; };
struct some_ds *p = ...;
rcu_read_lock(); if (rcu_dereference(p->ready)) data = rcu_dereference(p->data); // bug rcu_read_unlock();
or some such.
-- Pranith
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