Long time ago, there fixed a similar deadlock in show_slab_objects()
[1]. However, it is apparently due to the commits like 01fb58bcba63
("slab: remove synchronous synchronize_sched() from memcg cache
deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
just reading files in /sys/kernel/slab which will generate a lockdep
splat below.
Since the "mem_hotplug_lock" here is only to obtain a stable online node
mask while racing with NUMA node hotplug, in the worst case, the results
may me miscalculated while doing NUMA node hotplug, but they shall be
corrected by later reads of the same files.
WARNING: possible circular locking dependency detected
------------------------------------------------------
cat/5224 is trying to acquire lock:
ffff900012ac3120 (mem_hotplug_lock.rw_sem){++++}, at:
show_slab_objects+0x94/0x3a8
but task is already holding lock:
b8ff009693eee398 (kn->count#45){++++}, at: kernfs_seq_start+0x44/0xf0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (kn->count#45){++++}:
lock_acquire+0x31c/0x360
__kernfs_remove+0x290/0x490
kernfs_remove+0x30/0x44
sysfs_remove_dir+0x70/0x88
kobject_del+0x50/0xb0
sysfs_slab_unlink+0x2c/0x38
shutdown_cache+0xa0/0xf0
kmemcg_cache_shutdown_fn+0x1c/0x34
kmemcg_workfn+0x44/0x64
process_one_work+0x4f4/0x950
worker_thread+0x390/0x4bc
kthread+0x1cc/0x1e8
ret_from_fork+0x10/0x18
-> #1 (slab_mutex){+.+.}:
lock_acquire+0x31c/0x360
__mutex_lock_common+0x16c/0xf78
mutex_lock_nested+0x40/0x50
memcg_create_kmem_cache+0x38/0x16c
memcg_kmem_cache_create_func+0x3c/0x70
process_one_work+0x4f4/0x950
worker_thread+0x390/0x4bc
kthread+0x1cc/0x1e8
ret_from_fork+0x10/0x18
-> #0 (mem_hotplug_lock.rw_sem){++++}:
validate_chain+0xd10/0x2bcc
__lock_acquire+0x7f4/0xb8c
lock_acquire+0x31c/0x360
get_online_mems+0x54/0x150
show_slab_objects+0x94/0x3a8
total_objects_show+0x28/0x34
slab_attr_show+0x38/0x54
sysfs_kf_seq_show+0x198/0x2d4
kernfs_seq_show+0xa4/0xcc
seq_read+0x30c/0x8a8
kernfs_fop_read+0xa8/0x314
__vfs_read+0x88/0x20c
vfs_read+0xd8/0x10c
ksys_read+0xb0/0x120
__arm64_sys_read+0x54/0x88
el0_svc_handler+0x170/0x240
el0_svc+0x8/0xc
other info that might help us debug this:
Chain exists of:
mem_hotplug_lock.rw_sem --> slab_mutex --> kn->count#45
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(kn->count#45);
lock(slab_mutex);
lock(kn->count#45);
lock(mem_hotplug_lock.rw_sem);
*** DEADLOCK ***
3 locks held by cat/5224:
#0: 9eff00095b14b2a0 (&p->lock){+.+.}, at: seq_read+0x4c/0x8a8
#1: 0eff008997041480 (&of->mutex){+.+.}, at: kernfs_seq_start+0x34/0xf0
#2: b8ff009693eee398 (kn->count#45){++++}, at:
kernfs_seq_start+0x44/0xf0
stack backtrace:
Call trace:
dump_backtrace+0x0/0x248
show_stack+0x20/0x2c
dump_stack+0xd0/0x140
print_circular_bug+0x368/0x380
check_noncircular+0x248/0x250
validate_chain+0xd10/0x2bcc
__lock_acquire+0x7f4/0xb8c
lock_acquire+0x31c/0x360
get_online_mems+0x54/0x150
show_slab_objects+0x94/0x3a8
total_objects_show+0x28/0x34
slab_attr_show+0x38/0x54
sysfs_kf_seq_show+0x198/0x2d4
kernfs_seq_show+0xa4/0xcc
seq_read+0x30c/0x8a8
kernfs_fop_read+0xa8/0x314
__vfs_read+0x88/0x20c
vfs_read+0xd8/0x10c
ksys_read+0xb0/0x120
__arm64_sys_read+0x54/0x88
el0_svc_handler+0x170/0x240
el0_svc+0x8/0xc
[1] http://lkml.iu.edu/hypermail/linux/kernel/1101.0/02850.html
Fixes: 01fb58bcba63 ("slab: remove synchronous synchronize_sched() from memcg cache deactivation path")
Fixes: 03afc0e25f7f ("slab: get_online_mems for kmem_cache_{create,destroy,shrink}")
Cc: stable(a)vger.kernel.org
Acked-by: Michal Hocko <mhocko(a)suse.com>
Signed-off-by: Qian Cai <cai(a)lca.pw>
---
v2: fix the comment alignment and improve the changelog.
mm/slub.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 42c1b3af3c98..86bfd9d98af5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4838,7 +4838,13 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
}
}
- get_online_mems();
+ /*
+ * It is impossible to take "mem_hotplug_lock" here with "kernfs_mutex"
+ * already held which will conflict with an existing lock order:
+ *
+ * mem_hotplug_lock->slab_mutex->kernfs_mutex
+ */
+
#ifdef CONFIG_SLUB_DEBUG
if (flags & SO_ALL) {
struct kmem_cache_node *n;
@@ -4879,7 +4885,6 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
x += sprintf(buf + x, " N%d=%lu",
node, nodes[node]);
#endif
- put_online_mems();
kfree(nodes);
return x + sprintf(buf + x, "\n");
}
--
1.8.3.1
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 9b69cab42e5d14b8f0467566e3d97e682365db2d
Gitweb: https://git.kernel.org/tip/9b69cab42e5d14b8f0467566e3d97e682365db2d
Author: Janakarajan Natarajan <Janakarajan.Natarajan(a)amd.com>
AuthorDate: Mon, 07 Oct 2019 19:00:22
Committer: Borislav Petkov <bp(a)suse.de>
CommitterDate: Tue, 08 Oct 2019 09:48:09 +02:00
x86/asm: Fix MWAITX C-state hint value
As per "AMD64 Architecture Programmer's Manual Volume 3: General-Purpose
and System Instructions", MWAITX EAX[7:4]+1 specifies the optional hint
of the optimized C-state. For C0 state, EAX[7:4] should be set to 0xf.
Currently, a value of 0xf is set for EAX[3:0] instead of EAX[7:4]. Fix
this by changing MWAITX_DISABLE_CSTATES from 0xf to 0xf0.
This hasn't had any implications so far because setting reserved bits in
EAX is simply ignored by the CPU.
[ bp: Fixup comment in delay_mwaitx() and massage. ]
Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan(a)amd.com>
Signed-off-by: Borislav Petkov <bp(a)suse.de>
Cc: Frederic Weisbecker <frederic(a)kernel.org>
Cc: Greg Kroah-Hartman <gregkh(a)linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa(a)zytor.com>
Cc: Ingo Molnar <mingo(a)redhat.com>
Cc: Thomas Gleixner <tglx(a)linutronix.de>
Cc: "x86(a)kernel.org" <x86(a)kernel.org>
Cc: Zhenzhong Duan <zhenzhong.duan(a)oracle.com>
Cc: <stable(a)vger.kernel.org>
Link: https://lkml.kernel.org/r/20191007190011.4859-1-Janakarajan.Natarajan@amd.c…
---
arch/x86/include/asm/mwait.h | 2 +-
arch/x86/lib/delay.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index e28f8b7..9d5252c 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -21,7 +21,7 @@
#define MWAIT_ECX_INTERRUPT_BREAK 0x1
#define MWAITX_ECX_TIMER_ENABLE BIT(1)
#define MWAITX_MAX_LOOPS ((u32)-1)
-#define MWAITX_DISABLE_CSTATES 0xf
+#define MWAITX_DISABLE_CSTATES 0xf0
static inline void __monitor(const void *eax, unsigned long ecx,
unsigned long edx)
diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index b7375dc..c126571 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -113,8 +113,8 @@ static void delay_mwaitx(unsigned long __loops)
__monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0);
/*
- * AMD, like Intel, supports the EAX hint and EAX=0xf
- * means, do not enter any deep C-state and we use it
+ * AMD, like Intel's MWAIT version, supports the EAX hint and
+ * EAX=0xf0 means, do not enter any deep C-state and we use it
* here in delay() to minimize wakeup latency.
*/
__mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 9b69cab42e5d14b8f0467566e3d97e682365db2d
Gitweb: https://git.kernel.org/tip/9b69cab42e5d14b8f0467566e3d97e682365db2d
Author: Janakarajan Natarajan <Janakarajan.Natarajan(a)amd.com>
AuthorDate: Mon, 07 Oct 2019 19:00:22
Committer: Borislav Petkov <bp(a)suse.de>
CommitterDate: Tue, 08 Oct 2019 09:48:09 +02:00
x86/asm: Fix MWAITX C-state hint value
As per "AMD64 Architecture Programmer's Manual Volume 3: General-Purpose
and System Instructions", MWAITX EAX[7:4]+1 specifies the optional hint
of the optimized C-state. For C0 state, EAX[7:4] should be set to 0xf.
Currently, a value of 0xf is set for EAX[3:0] instead of EAX[7:4]. Fix
this by changing MWAITX_DISABLE_CSTATES from 0xf to 0xf0.
This hasn't had any implications so far because setting reserved bits in
EAX is simply ignored by the CPU.
[ bp: Fixup comment in delay_mwaitx() and massage. ]
Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan(a)amd.com>
Signed-off-by: Borislav Petkov <bp(a)suse.de>
Cc: Frederic Weisbecker <frederic(a)kernel.org>
Cc: Greg Kroah-Hartman <gregkh(a)linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa(a)zytor.com>
Cc: Ingo Molnar <mingo(a)redhat.com>
Cc: Thomas Gleixner <tglx(a)linutronix.de>
Cc: "x86(a)kernel.org" <x86(a)kernel.org>
Cc: Zhenzhong Duan <zhenzhong.duan(a)oracle.com>
Cc: <stable(a)vger.kernel.org>
Link: https://lkml.kernel.org/r/20191007190011.4859-1-Janakarajan.Natarajan@amd.c…
---
arch/x86/include/asm/mwait.h | 2 +-
arch/x86/lib/delay.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index e28f8b7..9d5252c 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -21,7 +21,7 @@
#define MWAIT_ECX_INTERRUPT_BREAK 0x1
#define MWAITX_ECX_TIMER_ENABLE BIT(1)
#define MWAITX_MAX_LOOPS ((u32)-1)
-#define MWAITX_DISABLE_CSTATES 0xf
+#define MWAITX_DISABLE_CSTATES 0xf0
static inline void __monitor(const void *eax, unsigned long ecx,
unsigned long edx)
diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index b7375dc..c126571 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -113,8 +113,8 @@ static void delay_mwaitx(unsigned long __loops)
__monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0);
/*
- * AMD, like Intel, supports the EAX hint and EAX=0xf
- * means, do not enter any deep C-state and we use it
+ * AMD, like Intel's MWAIT version, supports the EAX hint and
+ * EAX=0xf0 means, do not enter any deep C-state and we use it
* here in delay() to minimize wakeup latency.
*/
__mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);
udev stored in ep->hcpriv might be NULL if tt buffer is cleared
due to a halted control endpoint during device enumeration
xhci_clear_tt_buffer_complete is called by hub_tt_work() once it's
scheduled, and by then usb core might have freed and allocated a
new udev for the next enumeration attempt.
Fixes: ef513be0a905 ("usb: xhci: Add Clear_TT_Buffer")
Cc: <stable(a)vger.kernel.org> # v5.3
Reported-by: Johan Hovold <johan(a)kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman(a)linux.intel.com>
---
drivers/usb/host/xhci.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index 00f3804f7aa7..517ec3206f6e 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -5238,8 +5238,16 @@ static void xhci_clear_tt_buffer_complete(struct usb_hcd *hcd,
unsigned int ep_index;
unsigned long flags;
+ /*
+ * udev might be NULL if tt buffer is cleared during a failed device
+ * enumeration due to a halted control endpoint. Usb core might
+ * have allocated a new udev for the next enumeration attempt.
+ */
+
xhci = hcd_to_xhci(hcd);
udev = (struct usb_device *)ep->hcpriv;
+ if (!udev)
+ return;
slot_id = udev->slot_id;
ep_index = xhci_get_endpoint_index(&ep->desc);
--
2.7.4
The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable(a)vger.kernel.org>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
>From b9023b91dd020ad7e093baa5122b6968c48cc9e0 Mon Sep 17 00:00:00 2001
From: Balasubramani Vivekanandan <balasubramani_vivekanandan(a)mentor.com>
Date: Thu, 26 Sep 2019 15:51:01 +0200
Subject: [PATCH] tick: broadcast-hrtimer: Fix a race in bc_set_next
When a cpu requests broadcasting, before starting the tick broadcast
hrtimer, bc_set_next() checks if the timer callback (bc_handler) is active
using hrtimer_try_to_cancel(). But hrtimer_try_to_cancel() does not provide
the required synchronization when the callback is active on other core.
The callback could have already executed tick_handle_oneshot_broadcast()
and could have also returned. But still there is a small time window where
the hrtimer_try_to_cancel() returns -1. In that case bc_set_next() returns
without doing anything, but the next_event of the tick broadcast clock
device is already set to a timeout value.
In the race condition diagram below, CPU #1 is running the timer callback
and CPU #2 is entering idle state and so calls bc_set_next().
In the worst case, the next_event will contain an expiry time, but the
hrtimer will not be started which happens when the racing callback returns
HRTIMER_NORESTART. The hrtimer might never recover if all further requests
from the CPUs to subscribe to tick broadcast have timeout greater than the
next_event of tick broadcast clock device. This leads to cascading of
failures and finally noticed as rcu stall warnings
Here is a depiction of the race condition
CPU #1 (Running timer callback) CPU #2 (Enter idle
and subscribe to
tick broadcast)
--------------------- ---------------------
__run_hrtimer() tick_broadcast_enter()
bc_handler() __tick_broadcast_oneshot_control()
tick_handle_oneshot_broadcast()
raw_spin_lock(&tick_broadcast_lock);
dev->next_event = KTIME_MAX; //wait for tick_broadcast_lock
//next_event for tick broadcast clock
set to KTIME_MAX since no other cores
subscribed to tick broadcasting
raw_spin_unlock(&tick_broadcast_lock);
if (dev->next_event == KTIME_MAX)
return HRTIMER_NORESTART
// callback function exits without
restarting the hrtimer //tick_broadcast_lock acquired
raw_spin_lock(&tick_broadcast_lock);
tick_broadcast_set_event()
clockevents_program_event()
dev->next_event = expires;
bc_set_next()
hrtimer_try_to_cancel()
//returns -1 since the timer
callback is active. Exits without
restarting the timer
cpu_base->running = NULL;
The comment that hrtimer cannot be armed from within the callback is
wrong. It is fine to start the hrtimer from within the callback. Also it is
safe to start the hrtimer from the enter/exit idle code while the broadcast
handler is active. The enter/exit idle code and the broadcast handler are
synchronized using tick_broadcast_lock. So there is no need for the
existing try to cancel logic. All this can be removed which will eliminate
the race condition as well.
Fixes: 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast")
Originally-by: Thomas Gleixner <tglx(a)linutronix.de>
Signed-off-by: Balasubramani Vivekanandan <balasubramani_vivekanandan(a)mentor.com>
Signed-off-by: Thomas Gleixner <tglx(a)linutronix.de>
Cc: stable(a)vger.kernel.org
Link: https://lkml.kernel.org/r/20190926135101.12102-2-balasubramani_vivekanandan…
diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
index c1f5bb590b5e..b5a65e212df2 100644
--- a/kernel/time/tick-broadcast-hrtimer.c
+++ b/kernel/time/tick-broadcast-hrtimer.c
@@ -42,39 +42,39 @@ static int bc_shutdown(struct clock_event_device *evt)
*/
static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
{
- int bc_moved;
/*
- * We try to cancel the timer first. If the callback is on
- * flight on some other cpu then we let it handle it. If we
- * were able to cancel the timer nothing can rearm it as we
- * own broadcast_lock.
+ * This is called either from enter/exit idle code or from the
+ * broadcast handler. In all cases tick_broadcast_lock is held.
*
- * However we can also be called from the event handler of
- * ce_broadcast_hrtimer itself when it expires. We cannot
- * restart the timer because we are in the callback, but we
- * can set the expiry time and let the callback return
- * HRTIMER_RESTART.
+ * hrtimer_cancel() cannot be called here neither from the
+ * broadcast handler nor from the enter/exit idle code. The idle
+ * code can run into the problem described in bc_shutdown() and the
+ * broadcast handler cannot wait for itself to complete for obvious
+ * reasons.
*
- * Since we are in the idle loop at this point and because
- * hrtimer_{start/cancel} functions call into tracing,
- * calls to these functions must be bound within RCU_NONIDLE.
+ * Each caller tries to arm the hrtimer on its own CPU, but if the
+ * hrtimer callbback function is currently running, then
+ * hrtimer_start() cannot move it and the timer stays on the CPU on
+ * which it is assigned at the moment.
+ *
+ * As this can be called from idle code, the hrtimer_start()
+ * invocation has to be wrapped with RCU_NONIDLE() as
+ * hrtimer_start() can call into tracing.
*/
- RCU_NONIDLE(
- {
- bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
- if (bc_moved) {
- hrtimer_start(&bctimer, expires,
- HRTIMER_MODE_ABS_PINNED_HARD);
- }
- }
- );
-
- if (bc_moved) {
- /* Bind the "device" to the cpu */
- bc->bound_on = smp_processor_id();
- } else if (bc->bound_on == smp_processor_id()) {
- hrtimer_set_expires(&bctimer, expires);
- }
+ RCU_NONIDLE( {
+ hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
+ /*
+ * The core tick broadcast mode expects bc->bound_on to be set
+ * correctly to prevent a CPU which has the broadcast hrtimer
+ * armed from going deep idle.
+ *
+ * As tick_broadcast_lock is held, nothing can change the cpu
+ * base which was just established in hrtimer_start() above. So
+ * the below access is safe even without holding the hrtimer
+ * base lock.
+ */
+ bc->bound_on = bctimer.base->cpu_base->cpu;
+ } );
return 0;
}
@@ -100,10 +100,6 @@ static enum hrtimer_restart bc_handler(struct hrtimer *t)
{
ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
- if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
- if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
- return HRTIMER_RESTART;
-
return HRTIMER_NORESTART;
}
The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable(a)vger.kernel.org>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
>From b9023b91dd020ad7e093baa5122b6968c48cc9e0 Mon Sep 17 00:00:00 2001
From: Balasubramani Vivekanandan <balasubramani_vivekanandan(a)mentor.com>
Date: Thu, 26 Sep 2019 15:51:01 +0200
Subject: [PATCH] tick: broadcast-hrtimer: Fix a race in bc_set_next
When a cpu requests broadcasting, before starting the tick broadcast
hrtimer, bc_set_next() checks if the timer callback (bc_handler) is active
using hrtimer_try_to_cancel(). But hrtimer_try_to_cancel() does not provide
the required synchronization when the callback is active on other core.
The callback could have already executed tick_handle_oneshot_broadcast()
and could have also returned. But still there is a small time window where
the hrtimer_try_to_cancel() returns -1. In that case bc_set_next() returns
without doing anything, but the next_event of the tick broadcast clock
device is already set to a timeout value.
In the race condition diagram below, CPU #1 is running the timer callback
and CPU #2 is entering idle state and so calls bc_set_next().
In the worst case, the next_event will contain an expiry time, but the
hrtimer will not be started which happens when the racing callback returns
HRTIMER_NORESTART. The hrtimer might never recover if all further requests
from the CPUs to subscribe to tick broadcast have timeout greater than the
next_event of tick broadcast clock device. This leads to cascading of
failures and finally noticed as rcu stall warnings
Here is a depiction of the race condition
CPU #1 (Running timer callback) CPU #2 (Enter idle
and subscribe to
tick broadcast)
--------------------- ---------------------
__run_hrtimer() tick_broadcast_enter()
bc_handler() __tick_broadcast_oneshot_control()
tick_handle_oneshot_broadcast()
raw_spin_lock(&tick_broadcast_lock);
dev->next_event = KTIME_MAX; //wait for tick_broadcast_lock
//next_event for tick broadcast clock
set to KTIME_MAX since no other cores
subscribed to tick broadcasting
raw_spin_unlock(&tick_broadcast_lock);
if (dev->next_event == KTIME_MAX)
return HRTIMER_NORESTART
// callback function exits without
restarting the hrtimer //tick_broadcast_lock acquired
raw_spin_lock(&tick_broadcast_lock);
tick_broadcast_set_event()
clockevents_program_event()
dev->next_event = expires;
bc_set_next()
hrtimer_try_to_cancel()
//returns -1 since the timer
callback is active. Exits without
restarting the timer
cpu_base->running = NULL;
The comment that hrtimer cannot be armed from within the callback is
wrong. It is fine to start the hrtimer from within the callback. Also it is
safe to start the hrtimer from the enter/exit idle code while the broadcast
handler is active. The enter/exit idle code and the broadcast handler are
synchronized using tick_broadcast_lock. So there is no need for the
existing try to cancel logic. All this can be removed which will eliminate
the race condition as well.
Fixes: 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast")
Originally-by: Thomas Gleixner <tglx(a)linutronix.de>
Signed-off-by: Balasubramani Vivekanandan <balasubramani_vivekanandan(a)mentor.com>
Signed-off-by: Thomas Gleixner <tglx(a)linutronix.de>
Cc: stable(a)vger.kernel.org
Link: https://lkml.kernel.org/r/20190926135101.12102-2-balasubramani_vivekanandan…
diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
index c1f5bb590b5e..b5a65e212df2 100644
--- a/kernel/time/tick-broadcast-hrtimer.c
+++ b/kernel/time/tick-broadcast-hrtimer.c
@@ -42,39 +42,39 @@ static int bc_shutdown(struct clock_event_device *evt)
*/
static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
{
- int bc_moved;
/*
- * We try to cancel the timer first. If the callback is on
- * flight on some other cpu then we let it handle it. If we
- * were able to cancel the timer nothing can rearm it as we
- * own broadcast_lock.
+ * This is called either from enter/exit idle code or from the
+ * broadcast handler. In all cases tick_broadcast_lock is held.
*
- * However we can also be called from the event handler of
- * ce_broadcast_hrtimer itself when it expires. We cannot
- * restart the timer because we are in the callback, but we
- * can set the expiry time and let the callback return
- * HRTIMER_RESTART.
+ * hrtimer_cancel() cannot be called here neither from the
+ * broadcast handler nor from the enter/exit idle code. The idle
+ * code can run into the problem described in bc_shutdown() and the
+ * broadcast handler cannot wait for itself to complete for obvious
+ * reasons.
*
- * Since we are in the idle loop at this point and because
- * hrtimer_{start/cancel} functions call into tracing,
- * calls to these functions must be bound within RCU_NONIDLE.
+ * Each caller tries to arm the hrtimer on its own CPU, but if the
+ * hrtimer callbback function is currently running, then
+ * hrtimer_start() cannot move it and the timer stays on the CPU on
+ * which it is assigned at the moment.
+ *
+ * As this can be called from idle code, the hrtimer_start()
+ * invocation has to be wrapped with RCU_NONIDLE() as
+ * hrtimer_start() can call into tracing.
*/
- RCU_NONIDLE(
- {
- bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
- if (bc_moved) {
- hrtimer_start(&bctimer, expires,
- HRTIMER_MODE_ABS_PINNED_HARD);
- }
- }
- );
-
- if (bc_moved) {
- /* Bind the "device" to the cpu */
- bc->bound_on = smp_processor_id();
- } else if (bc->bound_on == smp_processor_id()) {
- hrtimer_set_expires(&bctimer, expires);
- }
+ RCU_NONIDLE( {
+ hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
+ /*
+ * The core tick broadcast mode expects bc->bound_on to be set
+ * correctly to prevent a CPU which has the broadcast hrtimer
+ * armed from going deep idle.
+ *
+ * As tick_broadcast_lock is held, nothing can change the cpu
+ * base which was just established in hrtimer_start() above. So
+ * the below access is safe even without holding the hrtimer
+ * base lock.
+ */
+ bc->bound_on = bctimer.base->cpu_base->cpu;
+ } );
return 0;
}
@@ -100,10 +100,6 @@ static enum hrtimer_restart bc_handler(struct hrtimer *t)
{
ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
- if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
- if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
- return HRTIMER_RESTART;
-
return HRTIMER_NORESTART;
}
The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable(a)vger.kernel.org>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
>From b9023b91dd020ad7e093baa5122b6968c48cc9e0 Mon Sep 17 00:00:00 2001
From: Balasubramani Vivekanandan <balasubramani_vivekanandan(a)mentor.com>
Date: Thu, 26 Sep 2019 15:51:01 +0200
Subject: [PATCH] tick: broadcast-hrtimer: Fix a race in bc_set_next
When a cpu requests broadcasting, before starting the tick broadcast
hrtimer, bc_set_next() checks if the timer callback (bc_handler) is active
using hrtimer_try_to_cancel(). But hrtimer_try_to_cancel() does not provide
the required synchronization when the callback is active on other core.
The callback could have already executed tick_handle_oneshot_broadcast()
and could have also returned. But still there is a small time window where
the hrtimer_try_to_cancel() returns -1. In that case bc_set_next() returns
without doing anything, but the next_event of the tick broadcast clock
device is already set to a timeout value.
In the race condition diagram below, CPU #1 is running the timer callback
and CPU #2 is entering idle state and so calls bc_set_next().
In the worst case, the next_event will contain an expiry time, but the
hrtimer will not be started which happens when the racing callback returns
HRTIMER_NORESTART. The hrtimer might never recover if all further requests
from the CPUs to subscribe to tick broadcast have timeout greater than the
next_event of tick broadcast clock device. This leads to cascading of
failures and finally noticed as rcu stall warnings
Here is a depiction of the race condition
CPU #1 (Running timer callback) CPU #2 (Enter idle
and subscribe to
tick broadcast)
--------------------- ---------------------
__run_hrtimer() tick_broadcast_enter()
bc_handler() __tick_broadcast_oneshot_control()
tick_handle_oneshot_broadcast()
raw_spin_lock(&tick_broadcast_lock);
dev->next_event = KTIME_MAX; //wait for tick_broadcast_lock
//next_event for tick broadcast clock
set to KTIME_MAX since no other cores
subscribed to tick broadcasting
raw_spin_unlock(&tick_broadcast_lock);
if (dev->next_event == KTIME_MAX)
return HRTIMER_NORESTART
// callback function exits without
restarting the hrtimer //tick_broadcast_lock acquired
raw_spin_lock(&tick_broadcast_lock);
tick_broadcast_set_event()
clockevents_program_event()
dev->next_event = expires;
bc_set_next()
hrtimer_try_to_cancel()
//returns -1 since the timer
callback is active. Exits without
restarting the timer
cpu_base->running = NULL;
The comment that hrtimer cannot be armed from within the callback is
wrong. It is fine to start the hrtimer from within the callback. Also it is
safe to start the hrtimer from the enter/exit idle code while the broadcast
handler is active. The enter/exit idle code and the broadcast handler are
synchronized using tick_broadcast_lock. So there is no need for the
existing try to cancel logic. All this can be removed which will eliminate
the race condition as well.
Fixes: 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast")
Originally-by: Thomas Gleixner <tglx(a)linutronix.de>
Signed-off-by: Balasubramani Vivekanandan <balasubramani_vivekanandan(a)mentor.com>
Signed-off-by: Thomas Gleixner <tglx(a)linutronix.de>
Cc: stable(a)vger.kernel.org
Link: https://lkml.kernel.org/r/20190926135101.12102-2-balasubramani_vivekanandan…
diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
index c1f5bb590b5e..b5a65e212df2 100644
--- a/kernel/time/tick-broadcast-hrtimer.c
+++ b/kernel/time/tick-broadcast-hrtimer.c
@@ -42,39 +42,39 @@ static int bc_shutdown(struct clock_event_device *evt)
*/
static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
{
- int bc_moved;
/*
- * We try to cancel the timer first. If the callback is on
- * flight on some other cpu then we let it handle it. If we
- * were able to cancel the timer nothing can rearm it as we
- * own broadcast_lock.
+ * This is called either from enter/exit idle code or from the
+ * broadcast handler. In all cases tick_broadcast_lock is held.
*
- * However we can also be called from the event handler of
- * ce_broadcast_hrtimer itself when it expires. We cannot
- * restart the timer because we are in the callback, but we
- * can set the expiry time and let the callback return
- * HRTIMER_RESTART.
+ * hrtimer_cancel() cannot be called here neither from the
+ * broadcast handler nor from the enter/exit idle code. The idle
+ * code can run into the problem described in bc_shutdown() and the
+ * broadcast handler cannot wait for itself to complete for obvious
+ * reasons.
*
- * Since we are in the idle loop at this point and because
- * hrtimer_{start/cancel} functions call into tracing,
- * calls to these functions must be bound within RCU_NONIDLE.
+ * Each caller tries to arm the hrtimer on its own CPU, but if the
+ * hrtimer callbback function is currently running, then
+ * hrtimer_start() cannot move it and the timer stays on the CPU on
+ * which it is assigned at the moment.
+ *
+ * As this can be called from idle code, the hrtimer_start()
+ * invocation has to be wrapped with RCU_NONIDLE() as
+ * hrtimer_start() can call into tracing.
*/
- RCU_NONIDLE(
- {
- bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
- if (bc_moved) {
- hrtimer_start(&bctimer, expires,
- HRTIMER_MODE_ABS_PINNED_HARD);
- }
- }
- );
-
- if (bc_moved) {
- /* Bind the "device" to the cpu */
- bc->bound_on = smp_processor_id();
- } else if (bc->bound_on == smp_processor_id()) {
- hrtimer_set_expires(&bctimer, expires);
- }
+ RCU_NONIDLE( {
+ hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
+ /*
+ * The core tick broadcast mode expects bc->bound_on to be set
+ * correctly to prevent a CPU which has the broadcast hrtimer
+ * armed from going deep idle.
+ *
+ * As tick_broadcast_lock is held, nothing can change the cpu
+ * base which was just established in hrtimer_start() above. So
+ * the below access is safe even without holding the hrtimer
+ * base lock.
+ */
+ bc->bound_on = bctimer.base->cpu_base->cpu;
+ } );
return 0;
}
@@ -100,10 +100,6 @@ static enum hrtimer_restart bc_handler(struct hrtimer *t)
{
ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
- if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
- if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
- return HRTIMER_RESTART;
-
return HRTIMER_NORESTART;
}