On Thu 2014-11-27 19:06:37, Masami Hiramatsu wrote: > (2014/11/27 0:27), Josh Poimboeuf wrote: > > On Wed, Nov 26, 2014 at 10:18:24AM +0100, Jiri Kosina wrote: > >> On Wed, 26 Nov 2014, Masami Hiramatsu wrote: > >> > >>>> Note to Steve: > >>>> Masami's IPMODIFY patch is heading for -next via your tree. Once it > >>>> arrives, > >>>> I'll rebase and make the change to set IPMODIFY. Do not pull this for > >>>> -next > >>>> yet. This version (v4) is for review and gathering acks. > >>> > >>> BTW, as we discussed IPMODIFY is an exclusive flag. So if we allocate > >>> ftrace_ops for each function in each patch, it could be conflict each > >>> other. > >> > >> Yup, this corresponds to what Petr brought up yesterday. There are cases > >> where all solutions (kpatch, kgraft, klp) would allocate multiple > >> ftrace_ops for a single function entry (think of patching one function > >> multiple times in a row). > >> > >> So it's not as easy as just setting the flag. > >> > >>> Maybe we need to have another ops hashtable to find such conflict and > >>> new handler to handle it. > >> > >> If I understand your proposal correctly, that would sound like a hackish > >> workaround, trying to basically trick the IPMODIFY flag semantics you just > >> implemented :) > > > > I think Masami may be proposing something similar to what we do in > > kpatch today. We have a single ftrace_ops and handler which is used for > > all functions. The handler accesses a global hash of kpatch_func > > structs which is indexed by the original function's IP address. > > Hmm, I think both is OK. kpatch method is less memory consuming and > will have a bigger overhead. However, as Steven talked at Plumbers Conf., > he will introduce a direct code modifying interface for ftrace. After > that is introduced, we don't need to care about performance degradation > by patching :)
Yup, I would prefer to have ftrace_ops per (original) function entry. I mean that new patches will reuse the existing ftrace_ops for already patched functions. They will just create new ftrace_ops from the not-yet-patched symbols. Using a single ftrace_ops everywhere would kill the win from Steven's direct ftrace optimization. > > It actually works out pretty well because it nicely encapsulates the > > knowledge about which functions are patched in a single place. And it > > makes it easy to track function versions (for incremental patching and > > rollback). > > > >> What I'd propose instead is to make sure that we always have > >> just a ftrace_ops per function entry, and only update the pointers there > >> as necessary. Fortunately we can do the switch atomically, by making use > >> of ->private. > > > > But how would you update multiple functions atomically, to enforce > > per-thread consistency? > > At this point, both can do it atomically. We just need an atomic flag > for applying patches. By other words, we would need something like the "kgr_immutable" flag from kGraft. It will make sure that everybody stays with the current code until all function entries are updated. Best Regards, Petr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
