linux-kernel - Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock critical sections

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Date:   Fri, 23 Nov 2018 11:40:48 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock
 critical sections

On (11/22/18 11:16), Peter Zijlstra wrote:
> > So maybe we need to switch debug objects print-outs to _always_
> > printk_deferred(). Debug objects can be used in code which cannot
> > do direct printk() - timekeeping is just one example.
> 
> No, printk_deferred() is a disease, it needs to be eradicated, not
> spread around.

deadlock-free printk() is deferred, but OK.


Another idea then:

---

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 70935ed91125..3928c2b2f77c 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -323,10 +323,13 @@ static void debug_print_object(struct debug_obj *obj, char *msg)
 		void *hint = descr->debug_hint ?
 			descr->debug_hint(obj->object) : NULL;
 		limit++;
+
+		bust_spinlocks(1);
 		WARN(1, KERN_ERR "ODEBUG: %s %s (active state %u) "
 				 "object type: %s hint: %pS\n",
 			msg, obj_states[obj->state], obj->astate,
 			descr->name, hint);
+		bust_spinlocks(0);
 	}
 	debug_objects_warnings++;
 }

---

This should make serial consoles re-entrant.
So printk->console_driver_write() hopefully will not deadlock.

IOW, this turns serial consoles into early_consoles, for a while ;)

	-ss