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Date: Thu, 19 Mar 2015 19:30:37 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Steven Rostedt <rostedt@...dmis.org> Cc: Daniel Thompson <daniel.thompson@...aro.org>, linux-arm-kernel@...ts.infradead.org, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org, patches@...aro.org, linaro-kernel@...ts.linaro.org, John Stultz <john.stultz@...aro.org>, Sumit Semwal <sumit.semwal@...aro.org>, Marc Zyngier <marc.zyngier@....com>, Andrew Thoelke <andrew.thoelke@....com> Subject: Re: [RFC PATCH 2/7] printk: Simple implementation for NMI backtracing On Thu, Mar 19, 2015 at 01:39:58PM -0400, Steven Rostedt wrote: > > +void printk_nmi_backtrace_complete(void) > > +{ > > + struct nmi_seq_buf *s; > > + int len, cpu, i, last_i; > > + > > + /* > > + * Now that all the NMIs have triggered, we can dump out their > > + * back traces safely to the console. > > + */ > > + for_each_possible_cpu(cpu) { > > + s = &per_cpu(nmi_print_seq, cpu); > > + last_i = 0; > > + > > + len = seq_buf_used(&s->seq); > > + if (!len) > > + continue; > > + > > + /* Print line by line. */ > > + for (i = 0; i < len; i++) { > > + if (s->buffer[i] == '\n') { > > + print_seq_line(s, last_i, i); > > + last_i = i + 1; > > + } > > + } > > + /* Check if there was a partial line. */ > > + if (last_i < len) { > > + print_seq_line(s, last_i, len - 1); > > + pr_cont("\n"); > > + } > > + > > + /* Wipe out the buffer ready for the next time around. */ > > + seq_buf_clear(&s->seq); > > + } > > + > > + clear_bit(0, &nmi_print_flag); > > + smp_mb__after_atomic(); > > Is this really necessary. What is the mb synchronizing? > > [ Added Peter Zijlstra to confirm it's not needed ] It surely looks suspect; and it lacks a comment, which is a clear sign its buggy. Now it if tries to order the accesses to the seqbuf againt the clearing of the bit one would have expected a _before_ barrier, not an _after_. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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