> On 28 Jun 2018, at 09.59, Matias Bjørling <[email protected]> wrote: > > On 06/28/2018 01:31 AM, Heiner Litz wrote: >> There is a control dependency between two disjoint variables (only read data >> if flags == WRITTEN). Because x86-TSO allows re-ordering of loads the >> control dependency can be violated. > > I'm sorry, I do not see it :) > > Here is my understanding: > > entry->w_ctx.flags is used as a flagging mechanism to wait for data to become > available when "flags" has PBLK_WRITTEN_DATA. > > The later access to entry->data is independent of this. It is assumed that > entry->w_ctx.flags is the guarding barrier.
This is correct. The motivation is to allow several producers to fill the buffer simultaneously. > > Here is the titbit from the control dependency section that makes me > say that the entry->data loads is not possible to be done before > entry->w_ctx.flags: > > " (*) Control dependencies apply only to the then-clause and else-clause > of the if-statement containing the control dependency, including > any functions that these two clauses call. Control dependencies > do -not- apply to code following the if-statement containing the > control dependency." > > The read of entry->data must come after the READ_ONCE of entry->w_ctx.flags. > I also understood it this way when implementing the barrier, but after an offline discussion with Heiner, he convinced me that reordering could occur. >> BTW: ARM has an even more relaxed memory model and also allows to >> re-order writes. If we want to support ARM correctly, I think we also >> need to insert a memory barrier between writing data and writing >> flags (and there might be a couple other places where we need to >> check). Heiner: Can you develop on this? Javier
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