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authorDouglas Anderson <dianders@chromium.org>2018-12-05 04:38:27 +0100
committerDaniel Thompson <daniel.thompson@linaro.org>2018-12-30 09:29:13 +0100
commit87b095928584da7d5cb3149016f00b0b139c2292 (patch)
treedf3b1997e74319ec8c1459916a4fda5b9f9279aa /kernel
parentkgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function() (diff)
downloadlinux-87b095928584da7d5cb3149016f00b0b139c2292.tar.xz
linux-87b095928584da7d5cb3149016f00b0b139c2292.zip
kgdb: Don't round up a CPU that failed rounding up before
If we're using the default implementation of kgdb_roundup_cpus() that uses smp_call_function_single_async() we can end up hanging kgdb_roundup_cpus() if we try to round up a CPU that failed to round up before. Specifically smp_call_function_single_async() will try to wait on the csd lock for the CPU that we're trying to round up. If the previous round up never finished then that lock could still be held and we'll just sit there hanging. There's not a lot of use trying to round up a CPU that failed to round up before. Let's keep a flag that indicates whether the CPU started but didn't finish to round up before. If we see that flag set then we'll skip the next round up. In general we have a few goals here: - We never want to end up calling smp_call_function_single_async() when the csd is still locked. This is accomplished because flush_smp_call_function_queue() unlocks the csd _before_ invoking the callback. That means that when kgdb_nmicallback() runs we know for sure the the csd is no longer locked. Thus when we set "rounding_up = false" we know for sure that the csd is unlocked. - If there are no timeouts rounding up we should never skip a round up. NOTE #1: In general trying to continue running after failing to round up CPUs doesn't appear to be supported in the debugger. When I simulate this I find that kdb reports "Catastrophic error detected" when I try to continue. I can overrule and continue anyway, but it should be noted that we may be entering the land of dragons here. Possibly the "Catastrophic error detected" was added _because_ of the future failure to round up, but even so this is an area of the code that hasn't been strongly tested. NOTE #2: I did a bit of testing before and after this change. I introduced a 10 second hang in the kernel while holding a spinlock that I could invoke on a certain CPU with 'taskset -c 3 cat /sys/...". Before this change if I did: - Invoke hang - Enter debugger - g (which warns about Catastrophic error, g again to go anyway) - g - Enter debugger ...I'd hang the rest of the 10 seconds without getting a debugger prompt. After this change I end up in the debugger the 2nd time after only 1 second with the standard warning about 'Timed out waiting for secondary CPUs.' I'll also note that once the CPU finished waiting I could actually debug it (aka "btc" worked) I won't promise that everything works perfectly if the errant CPU comes back at just the wrong time (like as we're entering or exiting the debugger) but it certainly seems like an improvement. NOTE #3: setting 'kgdb_info[cpu].rounding_up = false' is in kgdb_nmicallback() instead of kgdb_call_nmi_hook() because some implementations override kgdb_call_nmi_hook(). It shouldn't hurt to have it in kgdb_nmicallback() in any case. NOTE #4: this logic is really only needed because there is no API call like "smp_try_call_function_single_async()" or "smp_csd_is_locked()". If such an API existed then we'd use it instead, but it seemed a bit much to add an API like this just for kgdb. Signed-off-by: Douglas Anderson <dianders@chromium.org> Acked-by: Daniel Thompson <daniel.thompson@linaro.org> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/debug/debug_core.c20
-rw-r--r--kernel/debug/debug_core.h1
2 files changed, 20 insertions, 1 deletions
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 10db2833a423..1fb8b239e567 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -247,6 +247,7 @@ void __weak kgdb_roundup_cpus(void)
call_single_data_t *csd;
int this_cpu = raw_smp_processor_id();
int cpu;
+ int ret;
for_each_online_cpu(cpu) {
/* No need to roundup ourselves */
@@ -254,8 +255,23 @@ void __weak kgdb_roundup_cpus(void)
continue;
csd = &per_cpu(kgdb_roundup_csd, cpu);
+
+ /*
+ * If it didn't round up last time, don't try again
+ * since smp_call_function_single_async() will block.
+ *
+ * If rounding_up is false then we know that the
+ * previous call must have at least started and that
+ * means smp_call_function_single_async() won't block.
+ */
+ if (kgdb_info[cpu].rounding_up)
+ continue;
+ kgdb_info[cpu].rounding_up = true;
+
csd->func = kgdb_call_nmi_hook;
- smp_call_function_single_async(cpu, csd);
+ ret = smp_call_function_single_async(cpu, csd);
+ if (ret)
+ kgdb_info[cpu].rounding_up = false;
}
}
@@ -788,6 +804,8 @@ int kgdb_nmicallback(int cpu, void *regs)
struct kgdb_state kgdb_var;
struct kgdb_state *ks = &kgdb_var;
+ kgdb_info[cpu].rounding_up = false;
+
memset(ks, 0, sizeof(struct kgdb_state));
ks->cpu = cpu;
ks->linux_regs = regs;
diff --git a/kernel/debug/debug_core.h b/kernel/debug/debug_core.h
index 127d9bc49fb4..b4a7c326d546 100644
--- a/kernel/debug/debug_core.h
+++ b/kernel/debug/debug_core.h
@@ -42,6 +42,7 @@ struct debuggerinfo_struct {
int ret_state;
int irq_depth;
int enter_kgdb;
+ bool rounding_up;
};
extern struct debuggerinfo_struct kgdb_info[];