Messages in this thread | | | Date | Wed, 16 Jul 2014 12:43:56 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH v3] ring-buffer: Race when writing and swapping cpu buffer in parallel |
| |
On Wed, 16 Jul 2014 10:58:04 +0200 Petr Mladek <pmladek@suse.cz> wrote:
> +/** > + * ring_buffer_swap_cpu - swap a CPU buffer between two ring buffers > + * @buffer_a: One buffer to swap with > + * @buffer_b: The other buffer to swap with > + * > + * This function is useful for tracers that want to take a "snapshot" > + * of a CPU buffer and has another back up buffer lying around. > + * It is expected that the tracer handles the cpu buffer not being > + * used at the moment. > + */ > +int ring_buffer_swap_cpu(struct ring_buffer *buffer_a, > + struct ring_buffer *buffer_b, int cpu) > +{ > + struct ring_buffer_swap_info rb_swap_info = { > + .buffer_a = buffer_a, > + .buffer_b = buffer_b, > + }; > + int ret; > + > + /* > + * Swap the CPU buffer on the same CPU. Recording has to be fast > + * and and this helps to avoid memory barriers. > + */ > + ret = smp_call_function_single(cpu, ring_buffer_swap_this_cpu, > + (void *)&rb_swap_info, 1); > + if (ret) > + return ret; > + > + return rb_swap_info.ret;
We need to check if the cpu is on the current CPU and if so, just call the function directly. Otherwise this can't be done from interrupt disabled context.
Enable IRQSOFF_TRACER and FTRACE_STARTUP_TEST to see why.
[ 2.098008] Testing tracer irqsoff: [ 2.351020] ------------[ cut here ]------------ [ 2.352000] WARNING: CPU: 3 PID: 0 at /home/rostedt/work/git/linux-trace.git/kernel/smp.c:283 smp_call_function_single+0x66/0xad() [ 2.352000] Modules linked in: [ 2.352000] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 3.16.0-rc3-test+ #93 [ 2.352000] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS SDBLI944.86P 05/08/2007 [ 2.352000] 0000000000000000 ffffffff810058cb ffffffff81503d20 0000000000000000 [ 2.352000] ffffffff81040ca1 0000000000000000 ffffffff8109712f 0000000000000001 [ 2.352000] 0000000000000003 ffffffff810bf9db ffff88007a3c3e30 0000000000000001 [ 2.352000] Call Trace: [ 2.352000] [<ffffffff810058cb>] ? show_stack+0x5/0x3e [ 2.352000] [<ffffffff81503d20>] ? dump_stack+0x4a/0x75 [ 2.352000] [<ffffffff81040ca1>] ? warn_slowpath_common+0x7e/0x97 [ 2.352000] [<ffffffff8109712f>] ? smp_call_function_single+0x66/0xad [ 2.352000] [<ffffffff810bf9db>] ? ring_buffer_swap_cpu+0x45/0x45 [ 2.352000] [<ffffffff8109712f>] ? smp_call_function_single+0x66/0xad [ 2.352000] [<ffffffff810970ce>] ? smp_call_function_single+0x5/0xad [ 2.352000] [<ffffffff810bf9ce>] ? ring_buffer_swap_cpu+0x38/0x45 [ 2.352000] [<ffffffff810c6c64>] ? update_max_tr_single+0xb6/0x11f [ 2.352000] [<ffffffff8100ae3e>] ? default_idle+0x1d/0x30 [ 2.352000] [<ffffffff810cd2ac>] ? stop_critical_timing+0x139/0x20d [ 2.352000] [<ffffffff8100ae3e>] ? default_idle+0x1d/0x30 [ 2.352000] [<ffffffff81072d35>] ? cpu_startup_entry+0x115/0x220 [ 2.352000] [<ffffffff81027610>] ? start_secondary+0x21c/0x222 [ 2.352000] ---[ end trace a712676a9f9a53a0 ]---
-- Steve
| |