[Drbd-dev] [PATCH] drbd: when change susp_uuid[NEW] to true, make sure susp_uuid[OLD] is false
Philipp Reisner
philipp.reisner at linbit.com
Thu Dec 7 06:35:28 CET 2023
Hello Zhengbing,
no, I disagree. The code is good as it is since commit e31d63966fac73fc000b.
Here is my explanation:
IO is frozen because the resource lost quorum. The user changes the config
and reduces the quorum setting to 1.
The regular sequence is:
1. in sanitize_state()
susp_uuid = { false, true }
DRBD keeps the resource suspended longer.
2. in finish_state_change()
it sets __NEW_CUR_UUIC and that moves to NEW_CUR_UUID later.
in w_after_state_change()
3a when susp_uuid goes from false -> true then drbd_uuid_new_current()
3b Trigger another state change that sets susp_uuuid[NEW] back to false.
1. sanitize_sate() susp_uuid = { true, false }
2. in finish_state_change() nothing relevant happens
3a no positive edge on susp_uuid no NEW_CUR_UUID bit nothing happens
3b susp_uuid[NEW] == false -> nothing happens
Now let us assume between 3a and 3b, another CPU does an unrelated
state change that usually triggers a new current uuid:
CPU0:
1. in sanitize_state()
susp_uuid = { false, true }
DRBD keeps the resource suspended longer.
2. in finish_state_change()
it sets __NEW_CUR_UUIC and that moves to NEW_CUR_UUID later.
in w_after_state_change()
3a when susp_uuid goes from false -> true
drbd_uuid_new_current()
CPU1:
Here, another thread starts another unrelated state change
that again wants to set NEW_CUR_UUID. But let us go into the details.
1. in sanitize_state() susp_uuid = { true, true }
2. in finish_state_chanage() it does _NOT_ set __NEW_CUR_UUID
because the condition only allows it, when we have a
positive edge on susp_uuid (= susp_uuid = { false, true })
But this time, it is susp_uuid = { true, true }. No edge.
so __NEW_CUR_UUID stays 0 -> NEW_CUR_UUID stays 0
3a nothing, since no NEW_CUR_UUID
3b creates a state change to set susp_uuid[NEW] to false
CPU: continues with w_after_state()
3b Trigger another state change that sets susp_uuuid[NEW] back to false.
This is an "empty state change" as it is false and does nothing.
I think you have a mistake in your reasoning at T3.
best regards,
Philipp
On Mon, Nov 27, 2023 at 3:48 PM 黄正兵 <zhengbing.huang at easystack.cn> wrote:
>
> Hi Philipp,
>
>
> The third point about modification is that it may not cover all scenarios.
>
> In w_after_state_change(), between generating the new current_uuid and
> clearing the susp_uuid, it is possible that another thread set the __NEW_CUR_UUID flag.
>
> The problem scenario is as follows:
>
> T1:
> begin_state_change()
> end_state_change()
> __end_state_change()
> ___end_state_change()
> # susp_uuid value is {True, True}
> T2: w_after_state_change()
> drbd_uuid_new_current()
>
> T3:
> begin_state_change()
> # set __NEW_CUR_UUID flag
> end_state_change()
>
> begin_state_change()
> # set susp_uuid[NEW] = False
> end_state_change()
>
> In the end, the NEW_CUR_UUID flag bit is generated and not cleaned up.
>
> Therefore, the third point about modification can be change to this:
>
> diff --git a/drbd/drbd_state.c b/drbd/drbd_state.c
> index e35150340..2101c0ebc 100644
> --- a/drbd/drbd_state.c
> +++ b/drbd/drbd_state.c
> @@ -2553,6 +2553,7 @@ static bool should_try_become_up_to_date(struct drbd_device *device, enum drbd_d
> static void finish_state_change(struct drbd_resource *resource, const char *tag)
> {
> enum drbd_role *role = resource->role;
> + bool *susp_uuid = resource->susp_uuid;
> struct drbd_device *device;
> struct drbd_connection *connection;
> bool starting_resync = false;
> @@ -2828,7 +2829,7 @@ static void finish_state_change(struct drbd_resource *resource, const char *tag)
> if (role[OLD] == R_SECONDARY && role[NEW] == R_PRIMARY)
> create_new_uuid = true;
>
> - if (create_new_uuid)
> + if (create_new_uuid && !susp_uuid[OLD])
> set_bit(__NEW_CUR_UUID, &device->flags);
>
> if (disk_state[NEW] != D_NEGOTIATING && get_ldev_if_state(device, D_DETACHING)) {
>
> best regards,
> Zhengbing
>
>
>
>
>
> From: Philipp Reisner <philipp.reisner at linbit.com>
> Date: 2023-11-25 15:47:16
> To: "黄正兵" <zhengbing.huang at easystack.cn>
> Cc: dongsheng.yang at easystack.cn,drbd-dev at lists.linbit.com
> Subject: Re: Re: [PATCH] drbd: when change susp_uuid[NEW] to true, make sure susp_uuid[OLD] is false>Hi Zhengbing,
> >
> >Lars and I came up with this:
> >
> >commit e31d63966fac73fc000b584ccb2580016eccda17
> >Author: Philipp Reisner <philipp.reisner at linbit.com>
> >Date: Thu Nov 23 06:31:28 2023 +0100
> >
> > drbd: fix susp_uuid clearing
> >
> > I introduced susp_uuid for keeping a resource longer suspended for
> > writing a new current-uuid.
> > But in the following scenario, it failed to clear the susp_uuid bit:
> >
> > 1. The network connection between a primary and secondary node
> > fails. The primary has the setting quorum=2. IO freezes on the
> > primary node.
> >
> > 2. The user changes the quorum setting from 2 to one. The primary
> > node generates a new current-uuid and clears the susp_uuid bit in
> > a second state change.
> >
> > 3. If, during the second state change, another condition for creating
> > a new current UUID evaluates to true, the code fails to clear the
> > suspen_uuid bit.
> >
> > Fixing this in three places:
> > 1. in sanitize_state(): Only set susp_uuid[NEW] if susp_uuid[OLD]
> > was not set. That avoids it happening a second time after
> > one iteration.
> >
> > 2. in w_after_state_chage(): Always end the cycle by setting clearing
> > susp_uuid[NEW] if it is set. (Independently of susp_uuid[OLD].
> >
> > 3. in finish_sate_change(): Do not set NEW_CUR_UUID again if we are
> > in this operation.
> >
> > Strictly speaking, (2) is unnecessary after (1), but I also do it
> > since it shortens the code. Also, (3) alone would fix the issue.
> >
> > Fixes: d47f7456ab ("drbd: create new UUID before resuming IO upon
> > regaining quorum")
> >
> > Co-developed-by: zhengbing.huang <zhengbing.huang at easystack.cn>
> > Co-developed-by: Lars Ellenberg <lars.ellenberg at linbit.com>
> >
> >diff --git a/drbd/drbd_state.c b/drbd/drbd_state.c
> >index e35150340..b884bb622 100644
> >--- a/drbd/drbd_state.c
> >+++ b/drbd/drbd_state.c
> >@@ -2353,7 +2353,8 @@ static void sanitize_state(struct drbd_resource *resource)
> > resource->susp_quorum[NEW] =
> > resource->res_opts.on_no_quorum == ONQ_SUSPEND_IO ?
> >!resource_has_quorum : false;
> >
> >- if (resource_is_suspended(resource, OLD) &&
> >!resource_is_suspended(resource, NEW)) {
> >+ if (!resource->susp_uuid[OLD] &&
> >+ resource_is_suspended(resource, OLD) &&
> >!resource_is_suspended(resource, NEW)) {
> > idr_for_each_entry(&resource->devices, device, vnr) {
> > if (test_bit(NEW_CUR_UUID, &device->flags)) {
> > resource->susp_uuid[NEW] = true;
> >@@ -2553,6 +2554,7 @@ static bool should_try_become_up_to_date(struct
> >drbd_device *device, enum drbd_d
> >static void finish_state_change(struct drbd_resource *resource, const char *tag)
> >{
> > enum drbd_role *role = resource->role;
> >+ bool *susp_uuid = resource->susp_uuid;
> > struct drbd_device *device;
> > struct drbd_connection *connection;
> > bool starting_resync = false;
> >@@ -2828,7 +2830,10 @@ static void finish_state_change(struct
> >drbd_resource *resource, const char *tag)
> > if (role[OLD] == R_SECONDARY && role[NEW] == R_PRIMARY)
> > create_new_uuid = true;
> >
> >- if (create_new_uuid)
> >+ /* When susp_uuid goes from true to false, we just created a new
> >+ * current-uuid, it is pointless to do this one more time
> >+ */
> >+ if (create_new_uuid && !(susp_uuid[OLD] && !susp_uuid[NEW]))
> > set_bit(__NEW_CUR_UUID, &device->flags);
> >
> > if (disk_state[NEW] != D_NEGOTIATING &&
> >get_ldev_if_state(device, D_DETACHING)) {
> >@@ -4189,7 +4194,7 @@ static int w_after_state_change(struct drbd_work
> >*w, int unused)
> > still_connected = true;
> > }
> >
> >- if (!susp_uuid[OLD] && susp_uuid[NEW]) {
> >+ if (susp_uuid[NEW]) {
> > unsigned long irq_flags;
> >
> > begin_state_change(resource, &irq_flags, CS_VERBOSE);
> >
> >On Thu, Nov 23, 2023 at 9:48 AM 黄正兵 <zhengbing.huang at easystack.cn> wrote:
> >>
> >> Hi Philipp,
> >>
> >> Such modifications may cause new problems.
> >>
> >> If, during the second state change, the NEW_CUR_UUID flag was not cleared.
> >> clears the susp_uuid bit in a thirdly state change.
> >>
> >> Eventually, it becomes an endless cycle.
> >>
> >> So, is it possible to add the following modifications：
> >>
> >> diff --git a/drbd/drbd_state.c b/drbd/drbd_state.c
> >> index 2fbd0b1..6bd6431 100644
> >> --- a/drbd/drbd_state.c
> >> +++ b/drbd/drbd_state.c
> >> @@ -3938,9 +3938,13 @@ static int w_after_state_change(struct drbd_work *w, int unused)
> >> drbd_maybe_khelper(device, connection, "disconnected");
> >> }
> >>
> >> - if (!susp_uuid[OLD] && susp_uuid[NEW] &&
> >> - test_and_clear_bit(NEW_CUR_UUID, &device->flags))
> >> - new_current_uuid = true;
> >> + if (susp_uuid[NEW]) {
> >> + if (susp_uuid[OLD])
> >> + clear_bit(NEW_CUR_UUID, &device->flags);
> >> + else if (test_and_clear_bit(NEW_CUR_UUID, &device->flags))) {
> >> + new_curr_uuid = true;
> >> + }
> >> + }
> >>
> >> if (new_current_uuid)
> >> drbd_uuid_new_current(device, false);
> >>
> >> best regards,
> >> Zhengbing
> >>
> >>
> >>
> >>
> >>
> >> From: Philipp Reisner <philipp.reisner at linbit.com>
> >> Date: 2023-11-23 14:06:30
> >> To: "zhengbing.huang" <zhengbing.huang at easystack.cn>
> >> Cc: drbd-dev at lists.linbit.com,joel.colledge at linbit.com
> >> Subject: Re: [PATCH] drbd: when change susp_uuid[NEW] to true, make sure susp_uuid[OLD] is false>Hello Zhengbing,
> >> >
> >> >Thank you for pointing this out. I created a little bit different patch.
> >> >First, the sanitize_state() function should clean up the new state,
> >> >but I think it should never change the old state.
> >> >It can not rewrite history.
> >> >Second, we do not want to create two new current-uuids while I/O is frozen.
> >> >
> >> > drbd: fix susp_uuid clearing
> >> >
> >> > I introduced susp_uuid for keeping a resource longer suspended for
> >> > writing a new current-uuid.
> >> > But in the following scenario, it failed to clear the susp_uuid bit:
> >> >
> >> > 1. The network connection between a primary and secondary node
> >> > fails. The primary has the setting quorum=2. IO freezes on the
> >> > primary node.
> >> >
> >> > 2. The user changes the quorum setting from 2 to one. The primary
> >> > node generates a new current-uuid and clears the susp_uuid bit in a
> >> > second state change.
> >> >
> >> > 3. If, during the second state change, another condition for creating
> >> > a new current UUID evaluates to true, the code fails to clear the
> >> > suspen_uuid bit.
> >> >
> >> > The second attempt to create a new current-uuid gets ignored since it
> >> > is pointless to create two current-uuids while I/O is frozen.
> >> >
> >> > Fixes: d47f7456ab ("drbd: create new UUID before resuming IO upon
> >> > regaining quorum")
> >> >
> >> > Co-developed-by: zhengbing.huang <zhengbing.huang at easystack.cn>
> >> >
> >> >diff --git a/drbd/drbd_state.c b/drbd/drbd_state.c
> >> >index e35150340..316d3fd39 100644
> >> >--- a/drbd/drbd_state.c
> >> >+++ b/drbd/drbd_state.c
> >> >@@ -4189,7 +4189,7 @@ static int w_after_state_change(struct drbd_work
> >> >*w, int unused)
> >> > still_connected = true;
> >> > }
> >> >
> >> >- if (!susp_uuid[OLD] && susp_uuid[NEW]) {
> >> >+ if (susp_uuid[NEW]) {
> >> > unsigned long irq_flags;
> >> >
> >> > begin_state_change(resource, &irq_flags, CS_VERBOSE);
> >> >
> >> >
> >> >I give this to internal review for merging.
> >> >
> >> >best regards,
> >> > Philipp
> >> >
> >> >
> >> >On Wed, Nov 22, 2023 at 4:25 AM zhengbing.huang
> >> ><zhengbing.huang at easystack.cn> wrote:
> >> >>
> >> >> The problem scenario is as follows:
> >> >>
> >> >> 1. drbd is built on two nodes, role is primary and secondary, quorum is 2.
> >> >> then drbd's network fails. IO will be suspended.
> >> >> 2. primary modify quorum to 1, during this state change,
> >> >> drbd will set susp_uuid[NEW] to true and generate a new UUID.
> >> >> 3. then in w_after_state_change, start the second state change,
> >> >> set susp_uuid[NEW] to false. but during the second state change,
> >> >> it's possible to find NEW_CUR_UUID flag was set by others.
> >> >> then sanitize_state() will set susp_uuid[NEW] to true.
> >> >>
> >> >> Finally susp_uuid value is {true, true}, IO is frozen.
> >> >> And there is no way to set susp_uuid to false after that.
> >> >>
> >> >> So, while susp_uuid[NEW] is set to true, we want susp_uuid[OLD] to be false.
> >> >>
> >> >> Fixes: d47f7456ab ("drbd: create new UUID before resuming IO upon regaining quorum")
> >> >> Signed-off-by: zhengbing.huang <zhengbing.huang at easystack.cn>
> >> >> ---
> >> >> drbd/drbd_state.c | 1 +
> >> >> 1 file changed, 1 insertion(+)
> >> >>
> >> >> diff --git a/drbd/drbd_state.c b/drbd/drbd_state.c
> >> >> index e35150340..0dedd2dae 100644
> >> >> --- a/drbd/drbd_state.c
> >> >> +++ b/drbd/drbd_state.c
> >> >> @@ -2356,6 +2356,7 @@ static void sanitize_state(struct drbd_resource *resource)
> >> >> if (resource_is_suspended(resource, OLD) && !resource_is_suspended(resource, NEW)) {
> >> >> idr_for_each_entry(&resource->devices, device, vnr) {
> >> >> if (test_bit(NEW_CUR_UUID, &device->flags)) {
> >> >> + resource->susp_uuid[OLD] = false;
> >> >> resource->susp_uuid[NEW] = true;
> >> >> break;
> >> >> }
> >> >> --
> >> >> 2.17.1
> >> >>
> >>
> >>
>
>
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