On 11/15/2014 08:07 PM, Sven Eckelmann wrote: > The backported (<3.9) version of hlist_for_each_entry_rcu and > hlist_for_each_entry_safe uses the new macro hlist_entry_safe. It is called > with an ACCESS_ONCE parameter for the first parameter ptr. This disallows > merging of the two loads which the current version of the macro uses. > > This is problematic because this macro must only generate one load. Otherwise > with two contexts (or CPUs) following could happen: > > 1. context 1 fetches the ptr to the last entry in hlist_entry_safe() and > accepts this non-NULL ptr > > 2. context 2 deletes the last entry and terminates the list with NULL > > 3. context 1 re-fetches the pointer, doesn't check for zero, calculates the > entry based on a NULL pointer > > 4. context 1 crashes because it tries to load/write data from/to the invalid > address > > Instead use a single load to a temporary variable and do the NULL-check and > calculation based on that one. This is also the approach used in the current > Linux versions and was introduced by Paul E. McKenney. > > Signed-off-by: Sven Eckelmann <[email protected]> > --- > backport/backport-include/linux/list.h | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > Thank you for your patch, it was pushed upstream.
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