Linus, This code is a fork from the trace-3.19 pull as it needed the trace_seq clean ups from that branch.
This code solves the issue of performing stack dumps from NMI context. The issue is that printk() is not safe from NMI context as if the NMI were to trigger when a printk() was being performed, the NMI could deadlock from the printk() internal locks. This has been seen in practice. With lots of review from Petr Mladek, this code went through several iterations, and we feel that it is now at a point of quality to be accepted into mainline. Here's what is contained in this patch set: o Creates a "seq_buf" generic buffer utility that allows a descriptor to be passed around where functions can write their own "printk()" formatted strings into it. The generic version was pulled out of the trace_seq() code that was made specifically for tracing. o The seq_buf code was change to model the seq_file code. I have a patch (not included for 3.19) that converts the seq_file.c code over to use seq_buf.c like the trace_seq.c code does. This was done to make sure that seq_buf.c is compatible with seq_file.c. I may try to get that patch in for 3.20. o The seq_buf.c file was moved to lib/ to remove it from being dependent on CONFIG_TRACING. o The printk() was updated to allow for a per_cpu "override" of the internal calls. That is, instead of writing to the console, a call to printk() may do something else. This made it easier to allow the NMI to change what printk() does in order to call dump_stack() without needing to update that code as well. o Finally, the dump_stack from all CPUs via NMI code was converted to use the seq_buf code. The caller to trigger the NMI code would wait till all the NMIs finished, and then it would print the seq_buf data to the console safely from a non NMI context. [ Updated to remove unnecessary preempt_disable in printk() ] Note, I only showed the difference between V1, as that full diff was already posted here: http://lkml.kernel.org/r/20141208100528.5f971...@gandalf.local.home Please pull the latest trace-seq-buf-3.19-v2 tree, which can be found at: git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git trace-seq-buf-3.19-v2 Tag SHA1: b7a8e382200950f8f596376bfef82754569ccbaf Head SHA1: 1fb8915b9876a80f43732980208b39d013f8da9d Steven Rostedt (Red Hat) (1): printk: Do not disable preemption for accessing printk_func ---- kernel/printk/printk.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --------------------------- commit 1fb8915b9876a80f43732980208b39d013f8da9d Author: Steven Rostedt (Red Hat) <rost...@goodmis.org> Date: Thu Dec 11 09:12:01 2014 -0500 printk: Do not disable preemption for accessing printk_func As printk_func will either be the default function, or a per_cpu function for the current CPU, there's no reason to disable preemption to access it from printk. That's because if the printk_func is not the default then the caller had better disabled preemption as they were the one to change it. Link: http://lkml.kernel.org/r/ca+55afz5-_lkw4jhebowinn9_ouncgrwaf2fua35u46frn-...@mail.gmail.com Suggested-by: Linus Torvalds <torva...@linux-foundation.org> Signed-off-by: Steven Rostedt <rost...@goodmis.org> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 5af2b8bc88f0..9b896e7a50a9 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1859,10 +1859,16 @@ asmlinkage __visible int printk(const char *fmt, ...) int r; va_start(args, fmt); - preempt_disable(); + + /* + * If a caller overrides the per_cpu printk_func, then it needs + * to disable preemption when calling printk(). Otherwise + * the printk_func should be set to the default. No need to + * disable preemption here. + */ vprintk_func = this_cpu_read(printk_func); r = vprintk_func(fmt, args); - preempt_enable(); + va_end(args); return r; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
