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Date: Wed, 26 Jun 2019 11:08:37 +0900 From: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> To: John Ogness <john.ogness@...utronix.de> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>, Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Andrea Parri <andrea.parri@...rulasolutions.com>, Thomas Gleixner <tglx@...utronix.de>, Sergey Senozhatsky <sergey.senozhatsky@...il.com> Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation On (06/25/19 14:03), John Ogness 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 ;) -ss
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