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powerpc: Fix smp_send_stop NMI IPI handling

PATCH In review Nicholas Piggin 2 participants 3 messages Apr 25, 2018 – Apr 26, 2018 PowerPC

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Nicholas Piggin

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#0 lore raw

PATCH v2 1/1 powerpc: Fix smp_send_stop NMI IPI handling

The NMI IPI handler for a receiving CPU increments nmi_ipi_busy_count over the handler function call, which causes later smp_send_nmi_ipi() callers to spin until the call is finished.

The smp_send_stop function never returns, so the busy count is never decremeted, which can cause the system to hang in some cases. For example panic() will call smp_send_stop early on, then later in the reboot path, pnv_restart will call smp_send_stop again, which hangs.

Fix this by adding a special case to the smp_send_stop handler to decrement the busy count, because it will never return.

Fixes 6bed3237624e3 ("powerpc: use NMI IPI for smp_send_stop")
Reported-by Abdul Haleem <[email protected]>
Signed-off-by Nicholas Piggin <[email protected]>

---
Changes since v1:
- Reduce #ifdef spaghetti suggested by mpe

 arch/powerpc/kernel/smp.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)
arch/powerpc/kernel/smp.c +29 -7
index e16ec7b3b427..41d42c2f88d4 100644
@@ -566,10 +566,35 @@ void crash_send_ipi(void (*crash_ipi_callback)(struct pt_regs *))
 #endif
 
 #ifdef CONFIG_NMI_IPI
-static void stop_this_cpu(struct pt_regs *regs)
-#else
+static void nmi_stop_this_cpu(struct pt_regs *regs)
+{
+	/*
+	 * This is a special case because it never returns, so the NMI IPI
+	 * handling would never mark it as done, which makes any later
+	 * smp_send_nmi_ipi() call spin forever. Mark it done now.
+	 *
+	 * IRQs are already hard disabled by the smp_handle_nmi_ipi.
+	 */
+	nmi_ipi_lock();
+	nmi_ipi_busy_count--;
+	nmi_ipi_unlock();
+
+	/* Remove this CPU */
+	set_cpu_online(smp_processor_id(), false);
+
+	spin_begin();
+	while (1)
+		spin_cpu_relax();
+}
+
+void smp_send_stop(void)
+{
+	smp_send_nmi_ipi(NMI_IPI_ALL_OTHERS, nmi_stop_this_cpu, 1000000);
+}
+
+#else /* CONFIG_NMI_IPI */
+
 static void stop_this_cpu(void *dummy)
-#endif
 {
 	/* Remove this CPU */
 	set_cpu_online(smp_processor_id(), false);

@@ -582,12 +607,9 @@ static void stop_this_cpu(void *dummy)
 
 void smp_send_stop(void)
 {
-#ifdef CONFIG_NMI_IPI
-	smp_send_nmi_ipi(NMI_IPI_ALL_OTHERS, stop_this_cpu, 1000000);
-#else
 	smp_call_function(stop_this_cpu, NULL, 0);
-#endif
 }
+#endif /* CONFIG_NMI_IPI */
 
 struct thread_info *current_set[NR_CPUS];
Signature · 1 line
2.17.0
reviewer
#1 lore raw
Quoted 12 lines · Nicholas Piggin

The NMI IPI handler for a receiving CPU increments nmi_ipi_busy_count over the handler function call, which causes later smp_send_nmi_ipi() callers to spin until the call is finished.

The smp_send_stop function never returns, so the busy count is never decremeted, which can cause the system to hang in some cases. For example panic() will call smp_send_stop early on, then later in the reboot path, pnv_restart will call smp_send_stop again, which hangs.

Fix this by adding a special case to the smp_send_stop handler to decrement the busy count, because it will never return.

Quoted 6 lines
Fixes 6bed3237624e3 ("powerpc: use NMI IPI for smp_send_stop")
Reported-by Abdul Haleem <[email protected]>
Signed-off-by Nicholas Piggin <[email protected]>

---
Changes since v1:
- Reduce #ifdef spaghetti suggested by mpe

Sorry I missed this.

But then saw it in the patchwork listing, which is why I sent my version as v3. Which I also merged. Sorry for the confusion.

I'm pretty sure my version is equivalent to this, except for the way the #ifdefs are arranged.

The end result of this is nicer than my version, so I'd take a cleanup patch to get us to this final state.

cheers

author reviewer
#2 lore raw
Quoted 22 lines · On Thu, 26 Apr 2018 20:30:37 +1000 Michael Ellerman <[email protected]> wrote:
Quoted 12 lines · Nicholas Piggin

The NMI IPI handler for a receiving CPU increments nmi_ipi_busy_count over the handler function call, which causes later smp_send_nmi_ipi() callers to spin until the call is finished.

The smp_send_stop function never returns, so the busy count is never decremeted, which can cause the system to hang in some cases. For example panic() will call smp_send_stop early on, then later in the reboot path, pnv_restart will call smp_send_stop again, which hangs.

Fix this by adding a special case to the smp_send_stop handler to decrement the busy count, because it will never return.

Quoted 6 lines
Fixes 6bed3237624e3 ("powerpc: use NMI IPI for smp_send_stop")
Reported-by Abdul Haleem <[email protected]>
Signed-off-by Nicholas Piggin <[email protected]>

---
Changes since v1:
- Reduce #ifdef spaghetti suggested by mpe

Sorry I missed this.

Quoted 8 lines

But then saw it in the patchwork listing, which is why I sent my version as v3. Which I also merged. Sorry for the confusion.

I'm pretty sure my version is equivalent to this, except for the way the #ifdefs are arranged.

The end result of this is nicer than my version, so I'd take a cleanup patch to get us to this final state.

That's okay. We end up needing to fix the !NMI case as well, with "powerpc: Fix deadlock with multiple calls to smp_send_stop", so I can rediff that and send it up.

Thanks,
Nick