linux-kernel - [RFC PATCH 1/2] debug: Convert dbg_slave

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Date:   Fri, 22 May 2020 15:55:09 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     sumit.garg@...aro.org, jason.wessel@...driver.com,
        dianders@...omium.org
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        patches@...aro.org, pmladek@...e.com, sergey.senozhatsky@...il.com,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Subject: [RFC PATCH 1/2] debug: Convert dbg_slave_lock to an atomic

Currently the debug core takes and releases dbg_slave_lock, knowing it to
be uncontended, as a means to set and clear a flag that it uses to herd
the other cores in to or out of a holding pen.

Let's convert this to a regular atomic instead. This change is worthwhile
simply for the subtle increase in clarity in a very tangled bit of code.
It is also useful because it removes a raw_spin_lock() from within the
debug trap, which will make it easier to introduce spin lock debugging
code for the debug trap handler.

Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
---
 kernel/debug/debug_core.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 2b7c9b67931d..8f43171ddeac 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -121,12 +121,12 @@ static struct kgdb_bkpt		kgdb_break[KGDB_MAX_BREAKPOINTS] = {
 atomic_t			kgdb_active = ATOMIC_INIT(-1);
 EXPORT_SYMBOL_GPL(kgdb_active);
 static DEFINE_RAW_SPINLOCK(dbg_master_lock);
-static DEFINE_RAW_SPINLOCK(dbg_slave_lock);
 
 /*
  * We use NR_CPUs not PERCPU, in case kgdb is used to debug early
  * bootup code (which might not have percpu set up yet):
  */
+static atomic_t			slaves_must_spin;
 static atomic_t			masters_in_kgdb;
 static atomic_t			slaves_in_kgdb;
 static atomic_t			kgdb_break_tasklet_var;
@@ -615,7 +615,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 			dump_stack();
 			kgdb_info[cpu].exception_state &= ~DCPU_WANT_BT;
 		} else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
-			if (!raw_spin_is_locked(&dbg_slave_lock))
+			if (!atomic_read(&slaves_must_spin))
 				goto return_normal;
 		} else {
 return_normal:
@@ -677,7 +677,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 	 * CPU in a spin state while the debugger is active
 	 */
 	if (!kgdb_single_step)
-		raw_spin_lock(&dbg_slave_lock);
+		atomic_set(&slaves_must_spin, 1);
 
 #ifdef CONFIG_SMP
 	/* If send_ready set, slaves are already waiting */
@@ -741,7 +741,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 		dbg_io_ops->post_exception();
 
 	if (!kgdb_single_step) {
-		raw_spin_unlock(&dbg_slave_lock);
+		atomic_set(&slaves_must_spin, 0);
 		/* Wait till all the CPUs have quit from the debugger. */
 		while (kgdb_do_roundup && atomic_read(&slaves_in_kgdb))
 			cpu_relax();
-- 
2.25.4