On 2019-02-22, Petr Mladek <pmladek@xxxxxxxx> wrote: >>>> + rbuf = prb_reserve(&h, &sprint_rb, PRINTK_SPRINT_MAX); >>> >>> The second ring buffer for temporary buffers is really interesting >>> idea. >>> >>> Well, it brings some questions. For example, how many users might >>> need a reservation in parallel. Or if the nested use might cause >>> some problems when we decide to use printk-specific ring buffer >>> implementation. I still have to think about it. >> >> Keep in mind that it is only used by the writers, which have the >> prb_cpulock. Typically there would only be 2 max users: a non-NMI >> writer that was interrupted during the reserve/commit window and the >> interrupting NMI that does printk. The only exception would be if the >> printk-code code itself triggers a BUG_ON or WARN_ON within the >> reserve/commit window. Then you will have an additional user per >> recursion level. > > I am not sure it is worth to call the ring buffer machinery just > to handle 2-3 buffers. It may be slightly overkill, but: 1. We have the prb_cpulock at this point anyway, so it will be fast. (Both ring buffers share the same prb_cpulock.) 2. Getting a safe buffer is just 1 line of code: prb_reserve() 3. Why should we waste _any_ lines of code implementing the handling of these special 3-4 buffers? > Well, it might be just my mental block. We need to be really careful > to avoid infinite recursion when storing messages into the log > buffer. The recursion works well. I inserted a triggerable BUG_ON() in vprintk_emit() _within_ the reserve/commit window and I see a clean backtrace on the emergency console. John Ogness
