Messages in this thread | | | Date | Fri, 11 Dec 2015 17:03:44 -0800 | From | "Paul E. McKenney" <> | Subject | Re: new warning on sysrq kernel crash trigger |
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On Fri, Dec 11, 2015 at 04:16:37PM -0800, Ani Sinha wrote: > On Fri, 11 Dec 2015, Paul E. McKenney wrote: > > On Fri, Dec 11, 2015 at 05:10:43PM -0500, Rik van Riel wrote: > > > On 12/11/2015 03:44 PM, Ani Sinha wrote: > > > > > > > > > > > > On Thu, 10 Dec 2015, Paul E. McKenney wrote: > > > > > > > >> On Thu, Dec 10, 2015 at 03:57:09PM -0800, Ani Sinha wrote: > > > >>> Hi guys > > > >>> > > > >>> I am noticing a new warning in linux 3.18 which we did not see before > > > >>> in linux 3.4 : > > > >>> > > > >>> bash-4.1# echo c > /proc/sysrq-trigger > > > >>> [ 978.807185] BUG: sleeping function called from invalid context at > > > >>> ../arch/x86/mm/fault.c:1187 > > > >>> [ 978.909816] in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash > > > >>> [ 978.987358] Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a > > > >>> > > > >>> > > > >>> I have bisected this to the following change : > > > >>> > > > >>> commit 984d74a72076a12b400339973e8c98fd2fcd90e5 > > > >>> Author: Rik van Riel <riel@redhat.com> > > > >>> Date: Fri Jun 6 14:38:13 2014 -0700 > > > >>> > > > >>> sysrq: rcu-ify __handle_sysrq > > > >>> > > > >>> > > > >>> the rcu_read_lock() in handle_sysrq() bumps up > > > >>> current->rcu_read_lock_nesting. Hence, in __do_page_fault() when it > > > >>> calls might_sleep() in x86/mm/fault.c line 1191, > > > >>> preempt_count_equals(0) returns false and hence the warning is > > > >>> printed. > > > >>> > > > >>> One way to handle this would be to do something like this: > > > >>> > > > >>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > > > >>> index eef44d9..d4dbe22 100644 > > > >>> --- a/arch/x86/mm/fault.c > > > >>> +++ b/arch/x86/mm/fault.c > > > >>> @@ -1132,7 +1132,7 @@ __do_page_fault(struct pt_regs *regs, unsigned > > > >>> long error_code, > > > >>> * If we're in an interrupt, have no user context or are running > > > >>> * in a region with pagefaults disabled then we must not take the fault > > > >>> */ > > > >>> - if (unlikely(faulthandler_disabled() || !mm)) { > > > >>> + if (unlikely(faulthandler_disabled() || rcu_preempt_depth() || !mm)) { > > > >> > > > >> This works if CONFIG_PREEMPT=y, but if CONFIG_PREEMPT=n, then > > > >> rcu_preempt_depth() unconditionally returns zero. And if > > > >> CONFIG_PREEMPT_COUNT=y && CONFIG_PREEMPT=n, you would still see > > > >> the might_sleep() splat. > > > >> > > > >> Maybe use SRCU instead of RCU for this purpose? > > > >> > > > > > > > > From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001 > > > > From: Ani Sinha <ani@arista.com> > > > > Date: Fri, 11 Dec 2015 12:07:42 -0800 > > > > Subject: [PATCH 1/1] Fix 'sleeping function called from invalid context' > > > > warning in sysrq generated crash. > > > > > > > > Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") > > > > replaced spin_lock_irqsave() calls with > > > > rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not > > > > disable preemption, faulthandler_disabled() in > > > > __do_page_fault() in x86/fault.c returns false. When the code > > > > later calls might_sleep() in the pagefault handler, we get the > > > > following warning: > > > > > > > > BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187 > > > > in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash > > > > Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a > > > > > > > > To fix this, replace RCU call in handle_sysrq() to use SRCU. > > > > > > The sysrq code can be called from irq context. > > > > > > Trying to use SRCU from an irq context sounds like it could > > > be a bad idea, though admittedly I do not know enough about > > > SRCU to know for sure :) > > > > Indeed, not the best idea! ;-) > > > > I could imagine something like this: > > > > if (in_irq()) > > rcu_read_lock(); > > else > > idx = srcu_read_lock(&sysrq_rcu); > > > > And ditto for unlock. Then, for the update: > > > > synchronize_rcu_mult(call_rcu, call_sysrq_srcu); > > > > Where: > > > > static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func) > > { > > call_srcu(&sysrq_rcu, head, func); > > } > > > > >From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001 > From: Ani Sinha <ani@arista.com> > Date: Fri, 11 Dec 2015 12:07:42 -0800 > Subject: [PATCH 1/1] Fix 'sleeping function called from invalid context' > warning in sysrq generated crash. > > Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") > replaced spin_lock_irqsave() calls with > rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not > disable preemption, faulthandler_disabled() in > __do_page_fault() in x86/fault.c returns false. When the code > later calls might_sleep() in the pagefault handler, we get the > following warning: > > BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187 > in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash > Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a > > To fix this, replace RCU call in handle_sysrq() to use SRCU > in non-irq context. > > Tested this patch on linux 3.18 by booting off one of our boards. > > Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") > > Signed-off-by: Ani Sinha <ani@arista.com>
From an RCU perspective:
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
But I must defer to Rik from an sysrq perspective.
Thanx, Paul
> --- > diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c > index 5381a72..df7d747 100644 > --- a/drivers/tty/sysrq.c > +++ b/drivers/tty/sysrq.c > @@ -54,6 +54,7 @@ > /* Whether we react on sysrq keys or just ignore them */ > static int __read_mostly sysrq_enabled = CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE; > static bool __read_mostly sysrq_always_enabled; > +DEFINE_STATIC_SRCU(sysrq_rcu); > > static bool sysrq_on(void) > { > @@ -519,10 +520,13 @@ void __handle_sysrq(int key, bool check_mask) > { > struct sysrq_key_op *op_p; > int orig_log_level; > - int i; > + int i, idx; > > rcu_sysrq_start(); > - rcu_read_lock(); > + if (in_irq()) > + rcu_read_lock(); > + else > + idx = srcu_read_lock(&sysrq_rcu); > /* > * Raise the apparent loglevel to maximum so that the sysrq header > * is shown to provide the user with positive feedback. We do not > @@ -564,7 +568,10 @@ void __handle_sysrq(int key, bool check_mask) > pr_cont("\n"); > console_loglevel = orig_log_level; > } > - rcu_read_unlock(); > + if (in_irq()) > + rcu_read_unlock(); > + else > + srcu_read_unlock(&sysrq_rcu, idx); > rcu_sysrq_end(); > } > > @@ -1040,6 +1047,11 @@ int sysrq_toggle_support(int enable_mask) > return 0; > } > > +static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func) > +{ > + call_srcu(&sysrq_rcu, head, func); > +} > + > static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p, > struct sysrq_key_op *remove_op_p) > { > @@ -1059,7 +1071,7 @@ static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p, > * Wait for it to go away before returning, so the code for an old > * op is not freed (eg. on module unload) while it is in use. > */ > - synchronize_rcu(); > + synchronize_rcu_mult(call_rcu, call_sysrq_srcu); > > return retval; > } >
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