LinuxLists.cc - [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

2020-04-10 11:48:28

Subject: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

From: He Zhe <[email protected]>

commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
introduces a percpu counter that tracks the percpu recursion depth and
warn if it greater than zero, to avoid potential deadlock and stack
overflow.

However sometimes different eventfds may be used in parallel. Specifically,
when heavy network load goes through kvm and vhost, working as below, it
would trigger the following call trace.

- 100.00%
- 66.51%
ret_from_fork
kthread
- vhost_worker
- 33.47% handle_tx_kick
handle_tx
handle_tx_copy
vhost_tx_batch.isra.0
vhost_add_used_and_signal_n
eventfd_signal
- 33.05% handle_rx_net
handle_rx
vhost_add_used_and_signal_n
eventfd_signal
- 33.49%
ioctl
entry_SYSCALL_64_after_hwframe
do_syscall_64
__x64_sys_ioctl
ksys_ioctl
do_vfs_ioctl
kvm_vcpu_ioctl
kvm_arch_vcpu_ioctl_run
vmx_handle_exit
handle_ept_misconfig
kvm_io_bus_write
__kvm_io_bus_write
eventfd_signal

001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
---- snip ----
001: Call Trace:
001: vhost_signal+0x15e/0x1b0 [vhost]
001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
001: handle_rx+0xb9/0x900 [vhost_net]
001: handle_rx_net+0x15/0x20 [vhost_net]
001: vhost_worker+0xbe/0x120 [vhost]
001: kthread+0x106/0x140
001: ? log_used.part.0+0x20/0x20 [vhost]
001: ? kthread_park+0x90/0x90
001: ret_from_fork+0x35/0x40
001: ---[ end trace 0000000000000003 ]---

This patch enlarges the limit to 1 which is the maximum recursion depth we
have found so far.

Signed-off-by: He Zhe <[email protected]>
---
fs/eventfd.c | 3 ++-
include/linux/eventfd.h | 3 +++
2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 78e41c7c3d05..8b9bd6fb08cd 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -70,7 +70,8 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
* it returns true, the eventfd_signal() call should be deferred to a
* safe context.
*/
- if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
+ if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) >
+ EFD_WAKE_COUNT_MAX))
return 0;

spin_lock_irqsave(&ctx->wqh.lock, flags);
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index dc4fd8a6644d..e7684d768e3f 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -29,6 +29,9 @@
#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)

+/* This is the maximum recursion depth we find so far */
+#define EFD_WAKE_COUNT_MAX 1
+
struct eventfd_ctx;
struct file;

--
2.17.1

2020-05-12 07:05:33

by He Zhe

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

Can this be considered for this moment?
This is actually v2 of
"[PATCH 1/2] eventfd: Make wake counter work for single fd instead of all".

Thanks,
Zhe

On 4/10/20 7:47 PM, [email protected] wrote:
> From: He Zhe <[email protected]>
>
> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> introduces a percpu counter that tracks the percpu recursion depth and
> warn if it greater than zero, to avoid potential deadlock and stack
> overflow.
>
> However sometimes different eventfds may be used in parallel. Specifically,
> when heavy network load goes through kvm and vhost, working as below, it
> would trigger the following call trace.
>
> - 100.00%
> - 66.51%
> ret_from_fork
> kthread
> - vhost_worker
> - 33.47% handle_tx_kick
> handle_tx
> handle_tx_copy
> vhost_tx_batch.isra.0
> vhost_add_used_and_signal_n
> eventfd_signal
> - 33.05% handle_rx_net
> handle_rx
> vhost_add_used_and_signal_n
> eventfd_signal
> - 33.49%
> ioctl
> entry_SYSCALL_64_after_hwframe
> do_syscall_64
> __x64_sys_ioctl
> ksys_ioctl
> do_vfs_ioctl
> kvm_vcpu_ioctl
> kvm_arch_vcpu_ioctl_run
> vmx_handle_exit
> handle_ept_misconfig
> kvm_io_bus_write
> __kvm_io_bus_write
> eventfd_signal
>
> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
> ---- snip ----
> 001: Call Trace:
> 001: vhost_signal+0x15e/0x1b0 [vhost]
> 001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
> 001: handle_rx+0xb9/0x900 [vhost_net]
> 001: handle_rx_net+0x15/0x20 [vhost_net]
> 001: vhost_worker+0xbe/0x120 [vhost]
> 001: kthread+0x106/0x140
> 001: ? log_used.part.0+0x20/0x20 [vhost]
> 001: ? kthread_park+0x90/0x90
> 001: ret_from_fork+0x35/0x40
> 001: ---[ end trace 0000000000000003 ]---
>
> This patch enlarges the limit to 1 which is the maximum recursion depth we
> have found so far.
>
> Signed-off-by: He Zhe <[email protected]>
> ---
> fs/eventfd.c | 3 ++-
> include/linux/eventfd.h | 3 +++
> 2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 78e41c7c3d05..8b9bd6fb08cd 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -70,7 +70,8 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
> * it returns true, the eventfd_signal() call should be deferred to a
> * safe context.
> */
> - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
> + if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) >
> + EFD_WAKE_COUNT_MAX))
> return 0;
>
> spin_lock_irqsave(&ctx->wqh.lock, flags);
> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> index dc4fd8a6644d..e7684d768e3f 100644
> --- a/include/linux/eventfd.h
> +++ b/include/linux/eventfd.h
> @@ -29,6 +29,9 @@
> #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
> #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
>
> +/* This is the maximum recursion depth we find so far */
> +#define EFD_WAKE_COUNT_MAX 1
> +
> struct eventfd_ctx;
> struct file;
>

2020-06-22 09:13:25

by He Zhe

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

2020-07-03 08:12:54

by Juri Lelli

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

Hi,

On 10/04/20 19:47, [email protected] wrote:
> From: He Zhe <[email protected]>
>
> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> introduces a percpu counter that tracks the percpu recursion depth and
> warn if it greater than zero, to avoid potential deadlock and stack
> overflow.
>
> However sometimes different eventfds may be used in parallel. Specifically,
> when heavy network load goes through kvm and vhost, working as below, it
> would trigger the following call trace.
>
> - 100.00%
> - 66.51%
> ret_from_fork
> kthread
> - vhost_worker
> - 33.47% handle_tx_kick
> handle_tx
> handle_tx_copy
> vhost_tx_batch.isra.0
> vhost_add_used_and_signal_n
> eventfd_signal
> - 33.05% handle_rx_net
> handle_rx
> vhost_add_used_and_signal_n
> eventfd_signal
> - 33.49%
> ioctl
> entry_SYSCALL_64_after_hwframe
> do_syscall_64
> __x64_sys_ioctl
> ksys_ioctl
> do_vfs_ioctl
> kvm_vcpu_ioctl
> kvm_arch_vcpu_ioctl_run
> vmx_handle_exit
> handle_ept_misconfig
> kvm_io_bus_write
> __kvm_io_bus_write
> eventfd_signal
>
> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
> ---- snip ----
> 001: Call Trace:
> 001: vhost_signal+0x15e/0x1b0 [vhost]
> 001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
> 001: handle_rx+0xb9/0x900 [vhost_net]
> 001: handle_rx_net+0x15/0x20 [vhost_net]
> 001: vhost_worker+0xbe/0x120 [vhost]
> 001: kthread+0x106/0x140
> 001: ? log_used.part.0+0x20/0x20 [vhost]
> 001: ? kthread_park+0x90/0x90
> 001: ret_from_fork+0x35/0x40
> 001: ---[ end trace 0000000000000003 ]---
>
> This patch enlarges the limit to 1 which is the maximum recursion depth we
> have found so far.
>
> Signed-off-by: He Zhe <[email protected]>
> ---

Not sure if this approch can fly, but I also encountered the same
warning (which further caused hangs during VM install) and this change
addresses that.

I'd be interested in understanding what is the status of this problem/fix.

On a side note, by looking at the code, I noticed that (apart from
samples) all callers don't actually check eventfd_signal() return value
and I'm wondering why is that the case and if is it safe to do so.

Thanks,

Juri

2020-07-03 11:15:10

by He Zhe

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

On 7/3/20 4:12 PM, Juri Lelli wrote:
> Hi,
>
> On 10/04/20 19:47, [email protected] wrote:
>> From: He Zhe <[email protected]>
>>
>> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
>> introduces a percpu counter that tracks the percpu recursion depth and
>> warn if it greater than zero, to avoid potential deadlock and stack
>> overflow.
>>
>> However sometimes different eventfds may be used in parallel. Specifically,
>> when heavy network load goes through kvm and vhost, working as below, it
>> would trigger the following call trace.
>>
>> - 100.00%
>> - 66.51%
>> ret_from_fork
>> kthread
>> - vhost_worker
>> - 33.47% handle_tx_kick
>> handle_tx
>> handle_tx_copy
>> vhost_tx_batch.isra.0
>> vhost_add_used_and_signal_n
>> eventfd_signal
>> - 33.05% handle_rx_net
>> handle_rx
>> vhost_add_used_and_signal_n
>> eventfd_signal
>> - 33.49%
>> ioctl
>> entry_SYSCALL_64_after_hwframe
>> do_syscall_64
>> __x64_sys_ioctl
>> ksys_ioctl
>> do_vfs_ioctl
>> kvm_vcpu_ioctl
>> kvm_arch_vcpu_ioctl_run
>> vmx_handle_exit
>> handle_ept_misconfig
>> kvm_io_bus_write
>> __kvm_io_bus_write
>> eventfd_signal
>>
>> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
>> ---- snip ----
>> 001: Call Trace:
>> 001: vhost_signal+0x15e/0x1b0 [vhost]
>> 001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
>> 001: handle_rx+0xb9/0x900 [vhost_net]
>> 001: handle_rx_net+0x15/0x20 [vhost_net]
>> 001: vhost_worker+0xbe/0x120 [vhost]
>> 001: kthread+0x106/0x140
>> 001: ? log_used.part.0+0x20/0x20 [vhost]
>> 001: ? kthread_park+0x90/0x90
>> 001: ret_from_fork+0x35/0x40
>> 001: ---[ end trace 0000000000000003 ]---
>>
>> This patch enlarges the limit to 1 which is the maximum recursion depth we
>> have found so far.
>>
>> Signed-off-by: He Zhe <[email protected]>
>> ---
> Not sure if this approch can fly, but I also encountered the same
> warning (which further caused hangs during VM install) and this change
> addresses that.
>
> I'd be interested in understanding what is the status of this problem/fix.

This is actually v2 of the patch and has not got any reply yet. Here is the v1. FYI.
https://lore.kernel.org/lkml/[email protected]/

> On a side note, by looking at the code, I noticed that (apart from
> samples) all callers don't actually check eventfd_signal() return value
> and I'm wondering why is that the case and if is it safe to do so.

Checking the return value right after sending the signal can tell us if the
event counter has just overflowed, that is, exceeding ULLONG_MAX. I guess the
authors of the callers listed in the commit log just don't worry about that,
since they add only one to a dedicated eventfd.

Zhe

>
> Thanks,
>
> Juri
>

2020-07-06 06:48:42

by Juri Lelli

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

On 03/07/20 19:11, He Zhe wrote:
>
>
> On 7/3/20 4:12 PM, Juri Lelli wrote:
> > Hi,
> >
> > On 10/04/20 19:47, [email protected] wrote:
> >> From: He Zhe <[email protected]>
> >>
> >> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> >> introduces a percpu counter that tracks the percpu recursion depth and
> >> warn if it greater than zero, to avoid potential deadlock and stack
> >> overflow.
> >>
> >> However sometimes different eventfds may be used in parallel. Specifically,
> >> when heavy network load goes through kvm and vhost, working as below, it
> >> would trigger the following call trace.
> >>
> >> - 100.00%
> >> - 66.51%
> >> ret_from_fork
> >> kthread
> >> - vhost_worker
> >> - 33.47% handle_tx_kick
> >> handle_tx
> >> handle_tx_copy
> >> vhost_tx_batch.isra.0
> >> vhost_add_used_and_signal_n
> >> eventfd_signal
> >> - 33.05% handle_rx_net
> >> handle_rx
> >> vhost_add_used_and_signal_n
> >> eventfd_signal
> >> - 33.49%
> >> ioctl
> >> entry_SYSCALL_64_after_hwframe
> >> do_syscall_64
> >> __x64_sys_ioctl
> >> ksys_ioctl
> >> do_vfs_ioctl
> >> kvm_vcpu_ioctl
> >> kvm_arch_vcpu_ioctl_run
> >> vmx_handle_exit
> >> handle_ept_misconfig
> >> kvm_io_bus_write
> >> __kvm_io_bus_write
> >> eventfd_signal
> >>
> >> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
> >> ---- snip ----
> >> 001: Call Trace:
> >> 001: vhost_signal+0x15e/0x1b0 [vhost]
> >> 001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
> >> 001: handle_rx+0xb9/0x900 [vhost_net]
> >> 001: handle_rx_net+0x15/0x20 [vhost_net]
> >> 001: vhost_worker+0xbe/0x120 [vhost]
> >> 001: kthread+0x106/0x140
> >> 001: ? log_used.part.0+0x20/0x20 [vhost]
> >> 001: ? kthread_park+0x90/0x90
> >> 001: ret_from_fork+0x35/0x40
> >> 001: ---[ end trace 0000000000000003 ]---
> >>
> >> This patch enlarges the limit to 1 which is the maximum recursion depth we
> >> have found so far.
> >>
> >> Signed-off-by: He Zhe <[email protected]>
> >> ---
> > Not sure if this approch can fly, but I also encountered the same
> > warning (which further caused hangs during VM install) and this change
> > addresses that.
> >
> > I'd be interested in understanding what is the status of this problem/fix.
>
> This is actually v2 of the patch and has not got any reply yet. Here is the v1. FYI.
> https://lore.kernel.org/lkml/[email protected]/

I see, thanks. Hope this gets reviewed soon! :-)

> > On a side note, by looking at the code, I noticed that (apart from
> > samples) all callers don't actually check eventfd_signal() return value
> > and I'm wondering why is that the case and if is it safe to do so.
>
> Checking the return value right after sending the signal can tell us if the
> event counter has just overflowed, that is, exceeding ULLONG_MAX. I guess the
> authors of the callers listed in the commit log just don't worry about that,
> since they add only one to a dedicated eventfd.

OK. I was mostly wondering if returning early in case the WARN_ON_ONCE
fires would cause a missing wakeup for the eventfd_ctx wait queue.

Best,

Juri

2020-07-13 13:25:32

by Juri Lelli

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

Hi,

On 06/07/20 08:45, Juri Lelli wrote:
> On 03/07/20 19:11, He Zhe wrote:
> >
> >
> > On 7/3/20 4:12 PM, Juri Lelli wrote:
> > > Hi,
> > >
> > > On 10/04/20 19:47, [email protected] wrote:
> > >> From: He Zhe <[email protected]>
> > >>
> > >> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> > >> introduces a percpu counter that tracks the percpu recursion depth and
> > >> warn if it greater than zero, to avoid potential deadlock and stack
> > >> overflow.
> > >>
> > >> However sometimes different eventfds may be used in parallel. Specifically,
> > >> when heavy network load goes through kvm and vhost, working as below, it
> > >> would trigger the following call trace.
> > >>
> > >> - 100.00%
> > >> - 66.51%
> > >> ret_from_fork
> > >> kthread
> > >> - vhost_worker
> > >> - 33.47% handle_tx_kick
> > >> handle_tx
> > >> handle_tx_copy
> > >> vhost_tx_batch.isra.0
> > >> vhost_add_used_and_signal_n
> > >> eventfd_signal
> > >> - 33.05% handle_rx_net
> > >> handle_rx
> > >> vhost_add_used_and_signal_n
> > >> eventfd_signal
> > >> - 33.49%
> > >> ioctl
> > >> entry_SYSCALL_64_after_hwframe
> > >> do_syscall_64
> > >> __x64_sys_ioctl
> > >> ksys_ioctl
> > >> do_vfs_ioctl
> > >> kvm_vcpu_ioctl
> > >> kvm_arch_vcpu_ioctl_run
> > >> vmx_handle_exit
> > >> handle_ept_misconfig
> > >> kvm_io_bus_write
> > >> __kvm_io_bus_write
> > >> eventfd_signal
> > >>
> > >> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
> > >> ---- snip ----
> > >> 001: Call Trace:
> > >> 001: vhost_signal+0x15e/0x1b0 [vhost]
> > >> 001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
> > >> 001: handle_rx+0xb9/0x900 [vhost_net]
> > >> 001: handle_rx_net+0x15/0x20 [vhost_net]
> > >> 001: vhost_worker+0xbe/0x120 [vhost]
> > >> 001: kthread+0x106/0x140
> > >> 001: ? log_used.part.0+0x20/0x20 [vhost]
> > >> 001: ? kthread_park+0x90/0x90
> > >> 001: ret_from_fork+0x35/0x40
> > >> 001: ---[ end trace 0000000000000003 ]---
> > >>
> > >> This patch enlarges the limit to 1 which is the maximum recursion depth we
> > >> have found so far.
> > >>
> > >> Signed-off-by: He Zhe <[email protected]>
> > >> ---
> > > Not sure if this approch can fly, but I also encountered the same
> > > warning (which further caused hangs during VM install) and this change
> > > addresses that.
> > >
> > > I'd be interested in understanding what is the status of this problem/fix.
> >
> > This is actually v2 of the patch and has not got any reply yet. Here is the v1. FYI.
> > https://lore.kernel.org/lkml/[email protected]/
>
> I see, thanks. Hope this gets reviewed soon! :-)
>
> > > On a side note, by looking at the code, I noticed that (apart from
> > > samples) all callers don't actually check eventfd_signal() return value
> > > and I'm wondering why is that the case and if is it safe to do so.
> >
> > Checking the return value right after sending the signal can tell us if the
> > event counter has just overflowed, that is, exceeding ULLONG_MAX. I guess the
> > authors of the callers listed in the commit log just don't worry about that,
> > since they add only one to a dedicated eventfd.
>
> OK. I was mostly wondering if returning early in case the WARN_ON_ONCE
> fires would cause a missing wakeup for the eventfd_ctx wait queue.

Gentle ping about this issue (mainly addressing relevant maintainers and
potential reviewers). It's easily reproducible with PREEMPT_RT.

Thanks,

Juri

2020-07-22 09:02:32

by Juri Lelli

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

On 13/07/20 15:22, Juri Lelli wrote:

[...]

> Gentle ping about this issue (mainly addressing relevant maintainers and
> potential reviewers). It's easily reproducible with PREEMPT_RT.

Ping. Any comment at all? :-)

Thanks,

Juri

2020-08-20 10:44:31

by He Zhe

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

On 7/22/20 5:01 PM, Juri Lelli wrote:
> On 13/07/20 15:22, Juri Lelli wrote:
>
> [...]
>
>> Gentle ping about this issue (mainly addressing relevant maintainers and
>> potential reviewers). It's easily reproducible with PREEMPT_RT.
> Ping. Any comment at all? :-)

Hi Maintainer(s),

It's been 4 months. Can this be considered this round?

Thanks,
Zhe

>
> Thanks,
>
> Juri
>

2021-05-27 17:45:45

by Nitesh Lal

[permalink] [raw]

Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

On 8/20/20 6:41 AM, He Zhe wrote:
>
> On 7/22/20 5:01 PM, Juri Lelli wrote:
>> On 13/07/20 15:22, Juri Lelli wrote:
>>
>> [...]
>>
>>> Gentle ping about this issue (mainly addressing relevant maintainers and
>>> potential reviewers). It's easily reproducible with PREEMPT_RT.
>> Ping. Any comment at all? :-)
> Hi Maintainer(s),
>
> It's been 4 months. Can this be considered this round?

Gentle ping, is there any update or comments here?

As Juri mentioned the issue is still easily reproducible with PREEMPT_RT.

--
Thanks
Nitesh