> >> > > + * Order the stores above in vsnprintf() vs the store > >> > > of the > >> > > + * space below which joins the two strings. Note this > >> > > doesn't > >> > > + * make the code truly race free because there is no > >> > > barrier on > >> > > + * the read side. ie. Another CPU might load the > >> > > uninitialised > >> > > + * tail of the buffer first and then the space below > >> > > (rather > >> > > + * than the NULL that was there previously), and so > >> > > print the > >> > > + * uninitialised tail. But the whole string lives in > >> > > BSS so in > >> > > + * practice it should just see NULLs. > >> > > >> > The comment doesn't say _why_ we need to order these stores: IOW, what > >> > will or can go wrong without this order? This isn't clear to me. > >> > > >> > Another good practice when adding smp_*-constructs (as discussed, e.g., > >> > at KS'18) is to indicate the matching construct/synch. mechanism. > >> > >> Yes, one barrier without a counter-part is suspicious. > > > > As is this silence..., > > > > Michael, what happened to this patch? did you submit a new version? > > No, I'm just busy, it's the merge window next week :)
Got it. > > I thought the comment was pretty clear, if the stores are observed out > of order we might print the uninitialised tail. > > And the barrier on the read side would need to be in printk somewhere, > which is obviously unpleasant. Indeed. > > >> If the parallel access is really needed then we could define the > >> current length as atomic_t and use: > >> > >> + atomic_cmpxchg() to reserve the space for the string > >> + %*s to limit the printed length > >> > >> In the worst case, we would print an incomplete string. > >> See below for a sample code. > > > > Seems worth exploring, IMO; but I'd like to first hear _clear about > > the _intended semantics (before digging into alternatives)... > > It is not my intention to support concurrent updates of the string. The > idea is you setup the string early in boot. Understood, thanks for the clarification. > > The concern with a concurrent reader is simply that the string is dumped > in the panic path, and you never really know when you're going to panic. > Even if you only write to the string before doing SMP bringup you might > still have another CPU go rogue and panic before then. > > But I probably should have just not added the barrier, it's over > paranoid and will almost certainly never matter in practice. Oh, well, I can only echo you: if you don't care about the stores being _observed_ out of order, you could simply remove the barrier; if you do care, then you need "more paranoid" on the readers side. ;-) Andrea > > cheers
