[PATCH 1/2] arm64: entry: Move ct_user_exit before any user of RCU

From: James Morse <james.morse@arm.com>
To: linux-arm-kernel@lists.infradead.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, James Morse <james.morse@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: [PATCH 1/2] arm64: entry: Move ct_user_exit before any user of RCU
Date: Thu,  1 Aug 2019 16:41:49 +0100	[thread overview]
Message-ID: <20190801154150.195959-2-james.morse@arm.com> (raw)
In-Reply-To: <20190801154150.195959-1-james.morse@arm.com>

When taking an SError or Debug exception from EL0, we run the C
handler for these exceptions before updating the context tracking
code and unmasking lower priority interrupts.

When booting with nohz_full lockdep tells us we got this wrong:
| =============================
| WARNING: suspicious RCU usage
| 5.3.0-rc2-00010-gb4b5e9dcb11b-dirty #11271 Not tainted
| -----------------------------
| include/linux/rcupdate.h:643 rcu_read_unlock() used illegally wh!
|
| other info that might help us debug this:
|
|
| RCU used illegally from idle CPU!
| rcu_scheduler_active = 2, debug_locks = 1
| RCU used illegally from extended quiescent state!
| 1 lock held by a.out/432:
|  #0: 00000000c7a79515 (rcu_read_lock){....}, at: brk_handler+0x00
|
| stack backtrace:
| CPU: 1 PID: 432 Comm: a.out Not tainted 5.3.0-rc2-00010-gb4b5e9d1
| Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno De8
| Call trace:
|  dump_backtrace+0x0/0x140
|  show_stack+0x14/0x20
|  dump_stack+0xbc/0x104
|  lockdep_rcu_suspicious+0xf8/0x108
|  brk_handler+0x164/0x1b0
|  do_debug_exception+0x11c/0x278
|  el0_dbg+0x14/0x20

Move the ct_user_exit calls to be before do_debug_exception()
and do_serror() respectively.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 6c81fe7925cc4c42 ("arm64: enable context tracking")
Signed-off-by: James Morse <james.morse@arm.com>
---
The context to this is Masami Hiramatsu's patch adding sanity checks
for this:
Link: https://lore.kernel.org/r/156466954920.8995.14798868044005509434.stgit@devnote2
---
 arch/arm64/kernel/entry.S | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 320a30dbe35e..28681034d599 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -902,13 +902,14 @@ el0_dbg:
 	 * Debug exception handling
 	 */
 	tbnz	x24, #0, el0_inv		// EL0 only
+	mrs	x24, far_el1
 	gic_prio_kentry_setup tmp=x3
-	mrs	x0, far_el1
+	ct_user_exit
+	mov	x0, x24
 	mov	x1, x25
 	mov	x2, sp
 	bl	do_debug_exception
 	enable_da_f
-	ct_user_exit
 	b	ret_to_user
 el0_inv:
 	enable_daif
@@ -958,13 +959,14 @@ ENDPROC(el1_error)
 el0_error:
 	kernel_entry 0
 el0_error_naked:
-	mrs	x1, esr_el1
+	mrs	x25, esr_el1
 	gic_prio_kentry_setup tmp=x2
+	ct_user_exit
 	enable_dbg
 	mov	x0, sp
+	mov	x1, x25
 	bl	do_serror
 	enable_da_f
-	ct_user_exit
 	b	ret_to_user
 ENDPROC(el0_error)
 
-- 
2.20.1


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