On 2019-06-26, Sergey Senozhatsky <[email protected]> wrote: > [..] >> > CPU0 CPU1 >> > printk(...) >> > sz = vscprintf(NULL, "Comm %s\n", current->comm); >> > >> > ia64_mca_modify_comm() >> > >> > snprintf(comm, sizeof(comm), "%s %d", current->comm, >> > previous_current->pid); >> > >> > memcpy(current->comm, comm, sizeof(current->comm)); >> > if ((buf = prb_reserve(... sz))) { >> > vscnprintf(buf, "Comm %s\n", current->comm); >> > ^^^^^^^^^^^^^^ ->comm has changed. >> > Nothing critical, we >> > should not corrupt >> > anything, but we will >> > truncate ->comm if its >> > new size is larger than >> > what it used to be when >> > we did vscprintf(NULL). >> > prb_commit(...); >> > } > > [..] >> In my v1 rfc series, I avoided this issue by having a separate dedicated >> ringbuffer (rb_sprintf) that was used to allocate a temporary max-size >> (2KB) buffer for sprinting to. Then _that_ was used for the real >> ringbuffer input (strlen, prb_reserve, memcpy, prb_commit). That would >> still be the approach of my choice. > > In other words per-CPU buffering, AKA printk_safe ;)
Actually, no. I made use of a printk_ringbuffer (which is global). It was used for temporary memory allocation for sprintf, but the result was immediately written into the printk buffer from the same context. In contrast, printk_safe triggers a different context to handle the insertion. It is still my intention to eliminate the buffering component of printk_safe. After we get a lockless ringbuffer that we are happy with, my next series to integrate the buffer into printk will again use the sprint_rb solution to avoid the issue discussed in this thread. Perhaps it would be best to continue this discussion after I've posted that series. John Ogness
