[PATCH v2 0/2] kexec: Use hypercall_create_continuation to protect KEXEC ops

[PATCH v2 0/2] kexec: Use hypercall_create_continuation to protect KEXEC ops

[Eric DeVolder]

During testing (using the script below) we found that multiple
invocations of kexec of unload/load are not safe.

This does not exist in classic Xen kernels in which the kexec-tools
did the kexec via Linux kernel syscall (which in turn made the
hypercall), as the Linux code has a mutex_trylock which would
inhibit multiple concurrent calls.

But with the kexec-tools utilizing xc_kexec_* that is no longer
the case and we need to protect against multiple concurrent
invocations.

Please see the patches and review at your convenience!

==== try-crash.pl from bhavesh.davda@oracle.com ====
#!/usr/bin/perl -w

use strict;
use warnings;
use threads;

sub threaded_task {
    threads->create(sub {
        my $thr_id = threads->self->tid;
        #print "Starting load thread $thr_id\n";
        system("/sbin/kexec  -p --command-line=\"placeholder root=/dev/mapper/nimbula-root ro rhbg console=tty0 console=hvc0 earlyprintk=xen nomodeset printk.time=1 irqpoll maxcpus=1 nr_cpus=1 reset_devices cgroup_disable=memory mce=off selinux=0 console=ttyS1,115200n8\" --initrd=/boot/initrd-4.1.12-61.1.9.el6uek.x86_64kdump.img /boot/vmlinuz-4.1.12-61.1.9.el6uek.x86_64");
        #print "Ending load thread $thr_id\n";
        threads->detach(); #End thread.
    });
    threads->create(sub {
        my $thr_id = threads->self->tid;
        #print "Starting unload thread $thr_id\n";
        system("/sbin/kexec  -p -u");
        #print "Ending unload thread $thr_id\n";
        threads->detach(); #End thread.
    });
}

for my $i (0..99)
{
    threaded_task();
}


Eric DeVolder (2):
  kexec: use hypercall_create_continuation to protect KEXEC ops
  kexec: remove spinlock now that all KEXEC hypercall ops are protected
    at the top-level

 xen/common/kexec.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

-- 
2.7.4


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

[PATCH v2 1/2] kexec: use hypercall_create_continuation to protect KEXEC ops

[Eric DeVolder]

When we concurrently try to unload and load crash
images we eventually get:

 Xen call trace:
    [<ffff82d08018b04f>] machine_kexec_add_page+0x3a0/0x3fa
    [<ffff82d08018b184>] machine_kexec_load+0xdb/0x107
    [<ffff82d080116e8d>] kexec.c#kexec_load_slot+0x11/0x42
    [<ffff82d08011724f>] kexec.c#kexec_load+0x119/0x150
    [<ffff82d080117c1e>] kexec.c#do_kexec_op_internal+0xab/0xcf
    [<ffff82d080117c60>] do_kexec_op+0xe/0x1e
    [<ffff82d08025c620>] pv_hypercall+0x20a/0x44a
    [<ffff82d080260116>] cpufreq.c#test_all_events+0/0x30

 Pagetable walk from ffff820040088320:
  L4[0x104] = 00000002979d1063 ffffffffffffffff
  L3[0x001] = 00000002979d0063 ffffffffffffffff
  L2[0x000] = 00000002979c7063 ffffffffffffffff
  L1[0x088] = 80037a91ede97063 ffffffffffffffff

The interesting thing is that the page bits (063) look legit.

The operation on which we blow up is us trying to write
in the L1 and finding that the L2 entry points to some
bizzare MFN. It stinks of a race, and it looks like
the issue is due to no concurrency locks when dealing
with the crash kernel space.

Specifically we concurrently call kimage_alloc_crash_control_page
which iterates over the kexec_crash_area.start -> kexec_crash_area.size
and once found:

  if ( page )
  {
      image->next_crash_page = hole_end;
      clear_domain_page(_mfn(page_to_mfn(page)));
  }

clears. Since the parameters of what MFN to use are provided
by the callers (and the area to search is bounded) the the 'page'
is probably the same. So #1 we concurrently clear the
'control_code_page'.

The next step is us passing this 'control_code_page' to
machine_kexec_add_page. This function requires the MFNs:
page_to_maddr(image->control_code_page).

And this would always return the same virtual address, as
the MFN of the control_code_page is inside of the
kexec_crash_area.start -> kexec_crash_area.size area.

Then machine_kexec_add_page updates the L1 .. which can be done
concurrently and on subsequent calls we mangle it up.

This is all a theory at this time, but testing reveals
that adding the hypercall_create_continuation() at the
kexec hypercall fixes the crash.

NOTE: This patch follows 5c5216 (kexec: clear kexec_image slot
when unloading kexec image) to prevent crashes during
simultaneous load/unloads.

NOTE: Consideration was given to using the existing flag
KEXEC_FLAG_IN_PROGRESS to denote a kexec hypercall in
progress. This, however, overloads the original intent of
the flag which is to denote that we are about-to/have made
the jump to the crash path. The overloading would lead to
failures in existing checks on this flag as the flag would
always be set at the top level in do_kexec_op_internal().
For this reason, the new flag KEXEC_FLAG_HC_IN_PROGRESS
was introduced.

While at it, fixed the #define mismatched spacing

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
v2: 04/17/2017
 - Patch titled 'kexec: use hypercall_create_continuation to protect KEXEC ops'
 - Jan Beulich directed me to use a continuation method instead
   of spinlock.
 - Incorporated feedback from Daniel Kiper <daniel.kiper@oracle.com>
 - Incorporated feedback from Konrad Wilk <konrad.wilk@oracle.com>

v1: 04/10/2017
 - Patch titled 'kexec: Add spinlock for the whole hypercall'
 - Used spinlock in do_kexec_op_internal()
---
 xen/common/kexec.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/xen/common/kexec.c b/xen/common/kexec.c
index 072cc8e..3f96eb2 100644
--- a/xen/common/kexec.c
+++ b/xen/common/kexec.c
@@ -50,9 +50,10 @@ static cpumask_t crash_saved_cpus;
 
 static struct kexec_image *kexec_image[KEXEC_IMAGE_NR];
 
-#define KEXEC_FLAG_DEFAULT_POS   (KEXEC_IMAGE_NR + 0)
-#define KEXEC_FLAG_CRASH_POS     (KEXEC_IMAGE_NR + 1)
-#define KEXEC_FLAG_IN_PROGRESS   (KEXEC_IMAGE_NR + 2)
+#define KEXEC_FLAG_DEFAULT_POS    (KEXEC_IMAGE_NR + 0)
+#define KEXEC_FLAG_CRASH_POS      (KEXEC_IMAGE_NR + 1)
+#define KEXEC_FLAG_IN_PROGRESS    (KEXEC_IMAGE_NR + 2)
+#define KEXEC_FLAG_HC_IN_PROGRESS (KEXEC_IMAGE_NR + 3)
 
 static unsigned long kexec_flags = 0; /* the lowest bits are for KEXEC_IMAGE... */
 
@@ -1193,6 +1194,9 @@ static int do_kexec_op_internal(unsigned long op,
     if ( ret )
         return ret;
 
+    if ( test_and_set_bit(KEXEC_FLAG_HC_IN_PROGRESS, &kexec_flags))
+        return hypercall_create_continuation(__HYPERVISOR_kexec_op, "lh", op, uarg);
+
     switch ( op )
     {
     case KEXEC_CMD_kexec_get_range:
@@ -1227,6 +1231,8 @@ static int do_kexec_op_internal(unsigned long op,
         break;
     }
 
+    clear_bit(KEXEC_FLAG_HC_IN_PROGRESS, &kexec_flags);
+
     return ret;
 }
 
-- 
2.7.4


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 1/2] kexec: use hypercall_create_continuation to protect KEXEC ops

[Jan Beulich]

>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
> --- a/xen/common/kexec.c
> +++ b/xen/common/kexec.c
> @@ -50,9 +50,10 @@ static cpumask_t crash_saved_cpus;
>  
>  static struct kexec_image *kexec_image[KEXEC_IMAGE_NR];
>  
> -#define KEXEC_FLAG_DEFAULT_POS   (KEXEC_IMAGE_NR + 0)
> -#define KEXEC_FLAG_CRASH_POS     (KEXEC_IMAGE_NR + 1)
> -#define KEXEC_FLAG_IN_PROGRESS   (KEXEC_IMAGE_NR + 2)
> +#define KEXEC_FLAG_DEFAULT_POS    (KEXEC_IMAGE_NR + 0)
> +#define KEXEC_FLAG_CRASH_POS      (KEXEC_IMAGE_NR + 1)
> +#define KEXEC_FLAG_IN_PROGRESS    (KEXEC_IMAGE_NR + 2)
> +#define KEXEC_FLAG_HC_IN_PROGRESS (KEXEC_IMAGE_NR + 3)

Perhaps KEXEC_FLAG_IN_HYPERCALL? Other than that (and this
clearly is subject to Andrew's opinion)
Reviewed-by: Jan Beulich <jbeulich@suse.com>

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 1/2] kexec: use hypercall_create_continuation to protect KEXEC ops

[Daniel Kiper]

On Tue, Apr 18, 2017 at 04:48:06AM -0600, Jan Beulich wrote:
> >>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
> > --- a/xen/common/kexec.c
> > +++ b/xen/common/kexec.c
> > @@ -50,9 +50,10 @@ static cpumask_t crash_saved_cpus;
> >
> >  static struct kexec_image *kexec_image[KEXEC_IMAGE_NR];
> >
> > -#define KEXEC_FLAG_DEFAULT_POS   (KEXEC_IMAGE_NR + 0)
> > -#define KEXEC_FLAG_CRASH_POS     (KEXEC_IMAGE_NR + 1)
> > -#define KEXEC_FLAG_IN_PROGRESS   (KEXEC_IMAGE_NR + 2)
> > +#define KEXEC_FLAG_DEFAULT_POS    (KEXEC_IMAGE_NR + 0)
> > +#define KEXEC_FLAG_CRASH_POS      (KEXEC_IMAGE_NR + 1)
> > +#define KEXEC_FLAG_IN_PROGRESS    (KEXEC_IMAGE_NR + 2)
> > +#define KEXEC_FLAG_HC_IN_PROGRESS (KEXEC_IMAGE_NR + 3)
>
> Perhaps KEXEC_FLAG_IN_HYPERCALL? Other than that (and this

Make sense for me.

> clearly is subject to Andrew's opinion)
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Otherwise Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Daniel

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 1/2] kexec: use hypercall_create_continuation to protect KEXEC ops

[Andrew Cooper]

On 19/04/17 12:00, Daniel Kiper wrote:
> On Tue, Apr 18, 2017 at 04:48:06AM -0600, Jan Beulich wrote:
>>>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
>>> --- a/xen/common/kexec.c
>>> +++ b/xen/common/kexec.c
>>> @@ -50,9 +50,10 @@ static cpumask_t crash_saved_cpus;
>>>
>>>  static struct kexec_image *kexec_image[KEXEC_IMAGE_NR];
>>>
>>> -#define KEXEC_FLAG_DEFAULT_POS   (KEXEC_IMAGE_NR + 0)
>>> -#define KEXEC_FLAG_CRASH_POS     (KEXEC_IMAGE_NR + 1)
>>> -#define KEXEC_FLAG_IN_PROGRESS   (KEXEC_IMAGE_NR + 2)
>>> +#define KEXEC_FLAG_DEFAULT_POS    (KEXEC_IMAGE_NR + 0)
>>> +#define KEXEC_FLAG_CRASH_POS      (KEXEC_IMAGE_NR + 1)
>>> +#define KEXEC_FLAG_IN_PROGRESS    (KEXEC_IMAGE_NR + 2)
>>> +#define KEXEC_FLAG_HC_IN_PROGRESS (KEXEC_IMAGE_NR + 3)
>> Perhaps KEXEC_FLAG_IN_HYPERCALL? Other than that (and this
> Make sense for me.

Agreed.  (I can fix on commit).

>
>> clearly is subject to Andrew's opinion)
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> Otherwise Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

CC'ing Julien.  This is clearly a good bugfix for 4.9

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 1/2] kexec: use hypercall_create_continuation to protect KEXEC ops

[Daniel Kiper]

On Wed, Apr 19, 2017 at 12:48:56PM +0100, Andrew Cooper wrote:
> On 19/04/17 12:00, Daniel Kiper wrote:
> > On Tue, Apr 18, 2017 at 04:48:06AM -0600, Jan Beulich wrote:
> >>>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
> >>> --- a/xen/common/kexec.c
> >>> +++ b/xen/common/kexec.c
> >>> @@ -50,9 +50,10 @@ static cpumask_t crash_saved_cpus;
> >>>
> >>>  static struct kexec_image *kexec_image[KEXEC_IMAGE_NR];
> >>>
> >>> -#define KEXEC_FLAG_DEFAULT_POS   (KEXEC_IMAGE_NR + 0)
> >>> -#define KEXEC_FLAG_CRASH_POS     (KEXEC_IMAGE_NR + 1)
> >>> -#define KEXEC_FLAG_IN_PROGRESS   (KEXEC_IMAGE_NR + 2)
> >>> +#define KEXEC_FLAG_DEFAULT_POS    (KEXEC_IMAGE_NR + 0)
> >>> +#define KEXEC_FLAG_CRASH_POS      (KEXEC_IMAGE_NR + 1)
> >>> +#define KEXEC_FLAG_IN_PROGRESS    (KEXEC_IMAGE_NR + 2)
> >>> +#define KEXEC_FLAG_HC_IN_PROGRESS (KEXEC_IMAGE_NR + 3)
> >> Perhaps KEXEC_FLAG_IN_HYPERCALL? Other than that (and this
> > Make sense for me.
>
> Agreed.  (I can fix on commit).
>
> >
> >> clearly is subject to Andrew's opinion)
> >> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> > Otherwise Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
>
> CC'ing Julien.  This is clearly a good bugfix for 4.9

Yep and I think that it is stable material too.

Daniel

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 1/2] kexec: use hypercall_create_continuation to protect KEXEC ops

[Eric DeVolder]

On 04/19/2017 06:48 AM, Andrew Cooper wrote:
> On 19/04/17 12:00, Daniel Kiper wrote:
>> On Tue, Apr 18, 2017 at 04:48:06AM -0600, Jan Beulich wrote:
>>>>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
>>>> --- a/xen/common/kexec.c
>>>> +++ b/xen/common/kexec.c
>>>> @@ -50,9 +50,10 @@ static cpumask_t crash_saved_cpus;
>>>>
>>>>  static struct kexec_image *kexec_image[KEXEC_IMAGE_NR];
>>>>
>>>> -#define KEXEC_FLAG_DEFAULT_POS   (KEXEC_IMAGE_NR + 0)
>>>> -#define KEXEC_FLAG_CRASH_POS     (KEXEC_IMAGE_NR + 1)
>>>> -#define KEXEC_FLAG_IN_PROGRESS   (KEXEC_IMAGE_NR + 2)
>>>> +#define KEXEC_FLAG_DEFAULT_POS    (KEXEC_IMAGE_NR + 0)
>>>> +#define KEXEC_FLAG_CRASH_POS      (KEXEC_IMAGE_NR + 1)
>>>> +#define KEXEC_FLAG_IN_PROGRESS    (KEXEC_IMAGE_NR + 2)
>>>> +#define KEXEC_FLAG_HC_IN_PROGRESS (KEXEC_IMAGE_NR + 3)
>>> Perhaps KEXEC_FLAG_IN_HYPERCALL? Other than that (and this
>> Make sense for me.
>
> Agreed.  (I can fix on commit).
>
>>
>>> clearly is subject to Andrew's opinion)
>>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>> Otherwise Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
>
> CC'ing Julien.  This is clearly a good bugfix for 4.9

I've just posted v3 of this patch with the flag name change.
Eric



_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

[PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Eric DeVolder]

The spinlock in kexec_swap_images() was removed as
this function is only reachable on the kexec hypercall, which is
now protected at the top-level in do_kexec_op_internal(),
thus the local spinlock is no longer necessary.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 xen/common/kexec.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/xen/common/kexec.c b/xen/common/kexec.c
index 3f96eb2..efecf60 100644
--- a/xen/common/kexec.c
+++ b/xen/common/kexec.c
@@ -820,7 +820,6 @@ static int kexec_exec(XEN_GUEST_HANDLE_PARAM(void) uarg)
 static int kexec_swap_images(int type, struct kexec_image *new,
                              struct kexec_image **old)
 {
-    static DEFINE_SPINLOCK(kexec_lock);
     int base, bit, pos;
     int new_slot, old_slot;
 
@@ -832,8 +831,6 @@ static int kexec_swap_images(int type, struct kexec_image *new,
     if ( kexec_load_get_bits(type, &base, &bit) )
         return -EINVAL;
 
-    spin_lock(&kexec_lock);
-
     pos = (test_bit(bit, &kexec_flags) != 0);
     old_slot = base + pos;
     new_slot = base + !pos;
@@ -846,8 +843,6 @@ static int kexec_swap_images(int type, struct kexec_image *new,
     clear_bit(old_slot, &kexec_flags);
     *old = kexec_image[old_slot];
 
-    spin_unlock(&kexec_lock);
-
     return 0;
 }
 
-- 
2.7.4


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Jan Beulich]

>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
> The spinlock in kexec_swap_images() was removed as
> this function is only reachable on the kexec hypercall, which is
> now protected at the top-level in do_kexec_op_internal(),
> thus the local spinlock is no longer necessary.

But perhaps leave an ASSERT() there, making sure the in-hypercall
flag is set?

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Daniel Kiper]

On Tue, Apr 18, 2017 at 04:49:48AM -0600, Jan Beulich wrote:
> >>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
> > The spinlock in kexec_swap_images() was removed as
> > this function is only reachable on the kexec hypercall, which is
> > now protected at the top-level in do_kexec_op_internal(),
> > thus the local spinlock is no longer necessary.
>
> But perhaps leave an ASSERT() there, making sure the in-hypercall
> flag is set?

I am not sure why but if at all I think that we should also consider
other key kexec functions then. Or put ASSERT() into do_kexec_op_internal()
just before "switch ( op )".

Daniel

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Jan Beulich]

>>> On 19.04.17 at 12:56, <daniel.kiper@oracle.com> wrote:
> On Tue, Apr 18, 2017 at 04:49:48AM -0600, Jan Beulich wrote:
>> >>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
>> > The spinlock in kexec_swap_images() was removed as
>> > this function is only reachable on the kexec hypercall, which is
>> > now protected at the top-level in do_kexec_op_internal(),
>> > thus the local spinlock is no longer necessary.
>>
>> But perhaps leave an ASSERT() there, making sure the in-hypercall
>> flag is set?
> 
> I am not sure why but if at all I think that we should also consider
> other key kexec functions then. Or put ASSERT() into do_kexec_op_internal()
> just before "switch ( op )".

The point of my placement suggestion was that the ASSERT()
effectively replaces the lock acquire - the places you name
didn't previously require any synchronization.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Andrew Cooper]

On 19/04/17 12:20, Jan Beulich wrote:
>>>> On 19.04.17 at 12:56, <daniel.kiper@oracle.com> wrote:
>> On Tue, Apr 18, 2017 at 04:49:48AM -0600, Jan Beulich wrote:
>>>>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
>>>> The spinlock in kexec_swap_images() was removed as
>>>> this function is only reachable on the kexec hypercall, which is
>>>> now protected at the top-level in do_kexec_op_internal(),
>>>> thus the local spinlock is no longer necessary.
>>> But perhaps leave an ASSERT() there, making sure the in-hypercall
>>> flag is set?
>> I am not sure why but if at all I think that we should also consider
>> other key kexec functions then. Or put ASSERT() into do_kexec_op_internal()
>> just before "switch ( op )".
> The point of my placement suggestion was that the ASSERT()
> effectively replaces the lock acquire - the places you name
> didn't previously require any synchronization.

I'd recommend adding the ASSERT(), just to be on the safe side.

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Daniel Kiper]

On Wed, Apr 19, 2017 at 05:20:50AM -0600, Jan Beulich wrote:
> >>> On 19.04.17 at 12:56, <daniel.kiper@oracle.com> wrote:
> > On Tue, Apr 18, 2017 at 04:49:48AM -0600, Jan Beulich wrote:
> >> >>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
> >> > The spinlock in kexec_swap_images() was removed as
> >> > this function is only reachable on the kexec hypercall, which is
> >> > now protected at the top-level in do_kexec_op_internal(),
> >> > thus the local spinlock is no longer necessary.
> >>
> >> But perhaps leave an ASSERT() there, making sure the in-hypercall
> >> flag is set?
> >
> > I am not sure why but if at all I think that we should also consider
> > other key kexec functions then. Or put ASSERT() into do_kexec_op_internal()
> > just before "switch ( op )".
>
> The point of my placement suggestion was that the ASSERT()
> effectively replaces the lock acquire - the places you name
> didn't previously require any synchronization.

After the first patch of this series kexec_swap_images() cannot be
started twice in parallel. So, I do not see the point of ASSERT() here.
Or let's say we wish to have it to double check that "the in-hypercall
flag is set". AIUI, it is your original idea. However, then I think that
we should have an ASSERT() at least in kexec_load_slot() because parallel
loads make issues too. And we can go higher to feel more safe. That is
why I suggested do_kexec_op_internal() as the final resting place for
an ASSERT(). Simply it looks to me the safest approach. Am I missing
something?

Daniel

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Jan Beulich]

>>> On 19.04.17 at 14:13, <daniel.kiper@oracle.com> wrote:
> On Wed, Apr 19, 2017 at 05:20:50AM -0600, Jan Beulich wrote:
>> >>> On 19.04.17 at 12:56, <daniel.kiper@oracle.com> wrote:
>> > On Tue, Apr 18, 2017 at 04:49:48AM -0600, Jan Beulich wrote:
>> >> >>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
>> >> > The spinlock in kexec_swap_images() was removed as
>> >> > this function is only reachable on the kexec hypercall, which is
>> >> > now protected at the top-level in do_kexec_op_internal(),
>> >> > thus the local spinlock is no longer necessary.
>> >>
>> >> But perhaps leave an ASSERT() there, making sure the in-hypercall
>> >> flag is set?
>> >
>> > I am not sure why but if at all I think that we should also consider
>> > other key kexec functions then. Or put ASSERT() into do_kexec_op_internal()
>> > just before "switch ( op )".
>>
>> The point of my placement suggestion was that the ASSERT()
>> effectively replaces the lock acquire - the places you name
>> didn't previously require any synchronization.
> 
> After the first patch of this series kexec_swap_images() cannot be
> started twice in parallel. So, I do not see the point of ASSERT() here.
> Or let's say we wish to have it to double check that "the in-hypercall
> flag is set". AIUI, it is your original idea. However, then I think that
> we should have an ASSERT() at least in kexec_load_slot() because parallel
> loads make issues too. And we can go higher to feel more safe. That is
> why I suggested do_kexec_op_internal() as the final resting place for
> an ASSERT(). Simply it looks to me the safest approach. Am I missing
> something?

The point you're missing is - why don't you then move the ASSERT()
yet one more level up, right next to where the flag is being set? IOW
what you suggest would imo rather mean not adding any assertion
at all.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

Re: [PATCH v2 2/2] kexec: remove spinlock now that all KEXEC hypercall ops are protected at the top-level

[Eric DeVolder]

On 04/19/2017 08:37 AM, Jan Beulich wrote:
>>>> On 19.04.17 at 14:13, <daniel.kiper@oracle.com> wrote:
>> On Wed, Apr 19, 2017 at 05:20:50AM -0600, Jan Beulich wrote:
>>>>>> On 19.04.17 at 12:56, <daniel.kiper@oracle.com> wrote:
>>>> On Tue, Apr 18, 2017 at 04:49:48AM -0600, Jan Beulich wrote:
>>>>>>>> On 17.04.17 at 21:09, <eric.devolder@oracle.com> wrote:
>>>>>> The spinlock in kexec_swap_images() was removed as
>>>>>> this function is only reachable on the kexec hypercall, which is
>>>>>> now protected at the top-level in do_kexec_op_internal(),
>>>>>> thus the local spinlock is no longer necessary.
>>>>>
>>>>> But perhaps leave an ASSERT() there, making sure the in-hypercall
>>>>> flag is set?
>>>>
>>>> I am not sure why but if at all I think that we should also consider
>>>> other key kexec functions then. Or put ASSERT() into do_kexec_op_internal()
>>>> just before "switch ( op )".
>>>
>>> The point of my placement suggestion was that the ASSERT()
>>> effectively replaces the lock acquire - the places you name
>>> didn't previously require any synchronization.
>>
>> After the first patch of this series kexec_swap_images() cannot be
>> started twice in parallel. So, I do not see the point of ASSERT() here.
>> Or let's say we wish to have it to double check that "the in-hypercall
>> flag is set". AIUI, it is your original idea. However, then I think that
>> we should have an ASSERT() at least in kexec_load_slot() because parallel
>> loads make issues too. And we can go higher to feel more safe. That is
>> why I suggested do_kexec_op_internal() as the final resting place for
>> an ASSERT(). Simply it looks to me the safest approach. Am I missing
>> something?
>
> The point you're missing is - why don't you then move the ASSERT()
> yet one more level up, right next to where the flag is being set? IOW
> what you suggest would imo rather mean not adding any assertion
> at all.

I've just posted v3 of this patch with the ASSERT in lieu of the 
spin_lock().

Eric



_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel