Hi Akashi,
On Fri, Sep 26, 2014 at 12:54:13PM +0100, AKASHI Takahiro wrote:
I tried to verify kgdb in vanilla kernel on fast model, but it seems that the single stepping with kgdb doesn't work correctly since its first appearance at v3.15.
On v3.15, 'stepi' command after breaking the kernel at some breakpoint steps forward to the next instruction, but the succeeding 'stepi' never goes beyond that. On v3.16, 'stepi' moves forward and stops at the next instruction just after enable_dbg in el1_dbg, and never goes beyond that. This variance of behavior seems to come in with the following patch in v3.16:
commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault paths where possible")This patch (1) moves kgdb_disable_single_step() from 'c' command handling to single step handler. This makes sure that single stepping gets effective at every 's' command. Please note that, under the current implementation, single step bit in spsr, which is cleared by the first single stepping, will not be set again for the consecutive 's' commands because single step bit in mdscr is still kept on (that is, kernel_active_single_step() in kgdb_arch_handle_exception() is true). (2) removes 'enable_dbg' in el1_dbg. Single step bit in mdscr is turned on in do_handle_exception()-> kgdb_handle_expection() before returning to debugged context, and if debug exception is enabled in el1_dbg, we will see unexpected single- stepping in el1_dbg. (3) masks interrupts while single-stepping one instruction. If an interrupt is caught during processing a single-stepping, debug exception is unintentionally enabled by el1_irq's 'enable_dbg' before returning to debugged context. Thus, like in (2), we will see unexpected single-stepping in el1_irq.
Basically (1) is for v3.15, (2) and (3) with (1) for v3.16.
With those changes, we will see another problem if a breakpoint is set at interrupt-sensible places, like gic_handle_irq():
So it seems to me like kgdb is a complete mess in this area. The low-level debug exception code for arm64 will single-step *into* interrupt handlers. I believe that this is the correct behaviour, as otherwise we're artifically restricting what you can and can't debug (for example, leaving debug exceptions masked on the interrupt path means that you can't put breakpoints in interrupt handlers).
KGDB: re-enter error: breakpoint removed ffffffc000081258 ------------[ cut here ]------------ WARNING: CPU: 0 PID: 650 at kernel/debug/debug_core.c:435 kgdb_handle_exception+0x1dc/0x1f4() Modules linked in: CPU: 0 PID: 650 Comm: sh Not tainted 3.17.0-rc2+ #177 Call trace: [<ffffffc000087fac>] dump_backtrace+0x0/0x130 [<ffffffc0000880ec>] show_stack+0x10/0x1c [<ffffffc0004d683c>] dump_stack+0x74/0xb8 [<ffffffc0000ab824>] warn_slowpath_common+0x8c/0xb4 [<ffffffc0000ab90c>] warn_slowpath_null+0x14/0x20 [<ffffffc000121bfc>] kgdb_handle_exception+0x1d8/0x1f4 [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28 [<ffffffc0000821c8>] brk_handler+0x9c/0xe8 [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac Exception stack(0xffffffc07e027650 to 0xffffffc07e027770) ... [<ffffffc000083cac>] el1_dbg+0x14/0x68 [<ffffffc00012178c>] kgdb_cpu_enter+0x464/0x5c0 [<ffffffc000121bb4>] kgdb_handle_exception+0x190/0x1f4 [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28 [<ffffffc0000821c8>] brk_handler+0x9c/0xe8 [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac Exception stack(0xffffffc07e027ac0 to 0xffffffc07e027be0) ... [<ffffffc000083cac>] el1_dbg+0x14/0x68 [<ffffffc00032e4b4>] __handle_sysrq+0x11c/0x190 [<ffffffc00032e93c>] write_sysrq_trigger+0x4c/0x60 [<ffffffc0001e7d58>] proc_reg_write+0x54/0x84 [<ffffffc000192fa4>] vfs_write+0x98/0x1c8 [<ffffffc0001939b0>] SyS_write+0x40/0xa0Once some interrupt occurs, a breakpoint at gic_handle_irq() triggers kgdb. Kgdb then calls kgdb_roundup_cpus() to sync with other cpus. Current kgdb_roundup_cpus() unmasks interrupts temporarily to use smp_call_function(). This eventually allows another interrupt to occur and likely results in hitting a breakpoint at gic_handle_irq() again since debug exception is always enabled in el1_irq.
We can avoid this issue by specifying "nokgdbroundup" in kernel parameter, but this will also leave other cpus be in unknown state in terms of kgdb, and may result in interfering with kgdb activity.
Yuck. This really sounds like kgdb is broken in its SMP synchronisation for arm64. On x86, they use a NMI and powerpc uses an IPI which can run with irqs disabled. Since we don't have an NMI, how about we do the following to avoid the panic?
(1) Change our kgdb_roundup_cpus to use smp_call_function_single_async, which will avoid the need to enable interrupts
(2) Introduce a timeout into the waiting loop in kgdb_cpu_enter, where we spin on &slaves_in_kgdb and warn if the timeout expires.
Will