----- On Aug 17, 2019, at 11:42 AM, rostedt rost...@goodmis.org wrote: > On Sat, 17 Aug 2019 10:27:39 -0400 (EDT) > Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote: > >> I get your point wrt WRITE_ONCE(): since it's a cache it should not have >> user-visible effects if a temporary incorrect value is observed. Well in >> reality, it's not a cache: if the lookup fails, it returns "<...>" instead, >> so cache lookup failure ends up not providing any useful data in the trace. >> Let's assume this is a known and documented tracer limitation. > > Note, this is done at every sched switch, for both next and prev tasks. > And the update is only done at the enabling of a tracepoint (very rare > occurrence) If it missed it scheduling in, it has a really good chance > of getting it while scheduling out. > > And 99.999% of my tracing that I do, the tasks scheduling in when > enabling a tracepoint is not what I even care about, as I enable > tracing then start what I want to trace.
Since it's refcount based, my concern is about the side-effect of incrementing or decrementing that reference count without WRITE_ONCE which would lead to a transient corrupted value observed by _another_ active tracing user. For you use-case, it would lead to a missing comm when you are actively tracing what you want to trace, caused by another user of that refcount incrementing or decrementing it. I agree with you that missing tracing data at the beginning or end of a trace is not important. >> >> However, wrt READ_ONCE(), things are different. The variable read ends up >> being used to control various branches in the code, and the compiler could >> decide to re-fetch the variable (with a different state), and therefore >> cause _some_ of the branches to be inconsistent. See >> tracing_record_taskinfo_sched_switch() and tracing_record_taskinfo() @flags >> parameter. > > I'm more OK with using a READ_ONCE() on the flags so it is consistent. > But the WRITE_ONCE() is going a bit overboard. Hence my request for additional guidance on the usefulness of WRITE_ONCE(), whether it's mainly there for documentation purposes, or if we should consider that it takes care of real-life problems introduced by compiler optimizations in the wild. The LWN article seems to imply that it's not just a theoretical issue, but I'll have to let the article authors justify their conclusions, because I have limited time to investigate this myself. > >> >> AFAIU the current code should not generate any out-of-bound writes in case of >> re-fetch, but no comment in there documents how fragile this is. > > Which part of the code are you talking about here? kernel/trace/trace.c:tracing_record_taskinfo_sched_switch() kernel/trace/trace.c:tracing_record_taskinfo() where @flags is used to control a few branches. I don't think any of those would end up causing corruption if the flags is re-fetched between two branches, but it seems rather fragile. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
