Messages in this thread | ![/](/images/icornerl.gif) | | From | Dmitry Vyukov <> | Date | Thu, 10 Jan 2019 13:54:41 +0100 | Subject | Re: seqcount usage in xt_replace_table() |
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On Thu, Jan 10, 2019 at 1:44 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Jan 08, 2019 at 11:33:39AM -0800, Anatol Pomozov wrote: > > Hello folks, > > > > A bit of context what I am doing. I am trying to port KTSAN (Kernel > > Thread Sanitizer) tool to v4.20. That tool tracks shared data usage > > and makes sure it is accessed in a thread-safe manner. > > > > seqlock is a synchronization primitive used by Linux kernel. KTSAN > > annotates read_seqbegin()/read_seqretry() and tracks what data been > > accessed in its critical section. > > > > During KTSAN port I found and interesting seqcount usage introduced in > > commit 80055dab5de0c8677bc148c4717ddfc753a9148e > > > > If I read this commit correctly xt_replace_table() does not use > > seqlock in a canonical way to specify a critical section. Instead the > > code reads the counter and waits until it gets to a specific value. > > (gets away from) > > > Now I want KTSAN to play with this code nicely. I need to tell KTSAN > > something like "this raw_read_seqcount() does not start a critical > > section, just ignore it". So temporary I introduced > > raw_read_seqcount_nocritical() function that is ignored by KTSAN. Is > > it a good solution? > > This code is special enough to just do: READ_ONCE(->sequence) and be > done with it. It doesn't need the smp_rmb() or anything else.
Sounds good to me. From KTSAN perspective it then should work without any additional dance, it's always good when code works as-is.
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