| Message ID | 20231013092143.365296-6-f.ebner@proxmox.com |
|---|---|
| State | New |
| Headers | show |
| Series | mirror: allow switching from background to active mode | expand |
Fiona Ebner <f.ebner@proxmox.com> writes: > which allows switching the @copy-mode from 'background' to > 'write-blocking'. > > This is useful for management applications, so they can start out in > background mode to avoid limiting guest write speed and switch to > active mode when certain criteria are fulfilled. > > In presence of an iothread, the copy_mode member is now shared between > the iothread and the main thread, so turn accesses to it atomic. > > Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> [...] > diff --git a/qapi/block-core.json b/qapi/block-core.json > index c6f31a9399..01427c259a 100644 > --- a/qapi/block-core.json > +++ b/qapi/block-core.json > @@ -3044,6 +3044,17 @@ > { 'command': 'block-job-finalize', 'data': { 'id': 'str' }, > 'allow-preconfig': true } > > +## > +# @BlockJobChangeOptionsMirror: > +# > +# @copy-mode: Switch to this copy mode. Currenlty, only the switch Typo: Currently Also, two spaces between sentences for consistency, please. > +# from 'background' to 'write-blocking' is implemented. > +# > +# Since: 8.2 > +## > +{ 'struct': 'BlockJobChangeOptionsMirror', > + 'data': { 'copy-mode' : 'MirrorCopyMode' } } > + > ## > # @BlockJobChangeOptions: > # > @@ -3058,7 +3069,7 @@ > { 'union': 'BlockJobChangeOptions', > 'base': { 'id': 'str', 'type': 'JobType' }, > 'discriminator': 'type', > - 'data': {} } > + 'data': { 'mirror': 'BlockJobChangeOptionsMirror' } } > > ## > # @block-job-change:
Am 13.10.2023 um 11:21 hat Fiona Ebner geschrieben: > which allows switching the @copy-mode from 'background' to > 'write-blocking'. > > This is useful for management applications, so they can start out in > background mode to avoid limiting guest write speed and switch to > active mode when certain criteria are fulfilled. > > In presence of an iothread, the copy_mode member is now shared between > the iothread and the main thread, so turn accesses to it atomic. > > Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> > --- > > Changes in v3: > * turn accesses to copy_mode atomic and... > * ...slightly adapt error handling in mirror_change as a > consequence It would be good to have a comment at the field declaration that it's meant to be accessed with atomics. As we don't have further synchonisation, is the idea that during the switchover it basically doesn't matter if we read the old or the new value? After reading the whole patch, it seems that the field is only ever written under the BQL, while iothreads only read it, and only once per request (after the previous patch). This is why no further synchonisation is needed. If other threads could write it, too, mirror_change() would probably have to be more careful. As the code depends on this, adding that to the comment would be useful, too. > block/mirror.c | 33 ++++++++++++++++++++++++++++++--- > qapi/block-core.json | 13 ++++++++++++- > 2 files changed, 42 insertions(+), 4 deletions(-) > > diff --git a/block/mirror.c b/block/mirror.c > index 8992c09172..889cce5414 100644 > --- a/block/mirror.c > +++ b/block/mirror.c > @@ -1075,7 +1075,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp) > */ > job_transition_to_ready(&s->common.job); > } > - if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) { > + if (qatomic_read(&s->copy_mode) != MIRROR_COPY_MODE_BACKGROUND) { > s->actively_synced = true; > } What resets s->actively_synced when we switch away from active mode? > > @@ -1246,6 +1246,32 @@ static bool commit_active_cancel(Job *job, bool force) > return force || !job_is_ready(job); > } > > +static void mirror_change(BlockJob *job, BlockJobChangeOptions *opts, > + Error **errp) > +{ > + MirrorBlockJob *s = container_of(job, MirrorBlockJob, common); > + BlockJobChangeOptionsMirror *change_opts = &opts->u.mirror; > + MirrorCopyMode current; This is GLOBAL_STATE_CODE(), right? Let's be explicit about it. > + > + if (qatomic_read(&s->copy_mode) == change_opts->copy_mode) { > + return; > + } > + > + if (change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING) { > + error_setg(errp, "Change to copy mode '%s' is not implemented", > + MirrorCopyMode_str(change_opts->copy_mode)); > + return; > + } Ah, ok, we don't even allow the switch I was wondering about above. What would be needed, apart from removing this check, to make it work? > + current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, > + change_opts->copy_mode); > + if (current != MIRROR_COPY_MODE_BACKGROUND) { > + error_setg(errp, "Expected current copy mode '%s', got '%s'", > + MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND), > + MirrorCopyMode_str(current)); > + } The error path is strange. We return an error, but the new mode is still set. On the other hand, this is probably also the old mode unless someone added a new value to the enum, so it didn't actually change. And because this function is the only place that changes copy_mode and we're holding the BQL, the case can't even happen and this could be an assertion. > +} > + > static const BlockJobDriver mirror_job_driver = { > .job_driver = { > .instance_size = sizeof(MirrorBlockJob), > @@ -1260,6 +1286,7 @@ static const BlockJobDriver mirror_job_driver = { > .cancel = mirror_cancel, > }, > .drained_poll = mirror_drained_poll, > + .change = mirror_change, > }; Kevin
Am 18.10.23 um 18:59 schrieb Kevin Wolf: > Am 13.10.2023 um 11:21 hat Fiona Ebner geschrieben: >> which allows switching the @copy-mode from 'background' to >> 'write-blocking'. >> >> This is useful for management applications, so they can start out in >> background mode to avoid limiting guest write speed and switch to >> active mode when certain criteria are fulfilled. >> >> In presence of an iothread, the copy_mode member is now shared between >> the iothread and the main thread, so turn accesses to it atomic. >> >> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> >> --- >> >> Changes in v3: >> * turn accesses to copy_mode atomic and... >> * ...slightly adapt error handling in mirror_change as a >> consequence > > It would be good to have a comment at the field declaration that it's > meant to be accessed with atomics. > > As we don't have further synchonisation, is the idea that during the > switchover it basically doesn't matter if we read the old or the new > value? > > After reading the whole patch, it seems that the field is only ever > written under the BQL, while iothreads only read it, and only once per > request (after the previous patch). This is why no further > synchonisation is needed. If other threads could write it, too, > mirror_change() would probably have to be more careful. As the code > depends on this, adding that to the comment would be useful, too. > Will do in v4. >> block/mirror.c | 33 ++++++++++++++++++++++++++++++--- >> qapi/block-core.json | 13 ++++++++++++- >> 2 files changed, 42 insertions(+), 4 deletions(-) >> >> diff --git a/block/mirror.c b/block/mirror.c >> index 8992c09172..889cce5414 100644 >> --- a/block/mirror.c >> +++ b/block/mirror.c >> @@ -1075,7 +1075,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp) >> */ >> job_transition_to_ready(&s->common.job); >> } >> - if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) { >> + if (qatomic_read(&s->copy_mode) != MIRROR_COPY_MODE_BACKGROUND) { >> s->actively_synced = true; >> } > > What resets s->actively_synced when we switch away from active mode? > >> >> @@ -1246,6 +1246,32 @@ static bool commit_active_cancel(Job *job, bool force) >> return force || !job_is_ready(job); >> } >> >> +static void mirror_change(BlockJob *job, BlockJobChangeOptions *opts, >> + Error **errp) >> +{ >> + MirrorBlockJob *s = container_of(job, MirrorBlockJob, common); >> + BlockJobChangeOptionsMirror *change_opts = &opts->u.mirror; >> + MirrorCopyMode current; > > This is GLOBAL_STATE_CODE(), right? Let's be explicit about it. > Maybe it wouldn't need to be if we also set actively_synced to false in bdrv_mirror_top_do_write() if/when setting the bitmap. Thinking about it, that change shouldn't hurt in any case. But sure, I'll add the GLOBAL_STATE_CODE annotation here. If ever required not to be GLOBAL_STATE_CODE code, it can still be adapted later. >> + >> + if (qatomic_read(&s->copy_mode) == change_opts->copy_mode) { >> + return; >> + } >> + >> + if (change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING) { >> + error_setg(errp, "Change to copy mode '%s' is not implemented", >> + MirrorCopyMode_str(change_opts->copy_mode)); >> + return; >> + } > > Ah, ok, we don't even allow the switch I was wondering about above. What > would be needed, apart from removing this check, to make it work? > Of course, setting actively_synced to false, as you pointed out above. But I think it would also require more synchronization, because I think otherwise the iothread could still read the old value of copy_mode (as MIRROR_COPY_MODE_WRITE_BLOCKING) right afterwards and might set actively_synced to true again. Do you want me to think it through in detail and allow the change in the other direction too? I guess that would also require using the job mutex instead of atomics. Or should we wait until somebody actually requires that? >> + current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, >> + change_opts->copy_mode); >> + if (current != MIRROR_COPY_MODE_BACKGROUND) { >> + error_setg(errp, "Expected current copy mode '%s', got '%s'", >> + MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND), >> + MirrorCopyMode_str(current)); >> + } > > The error path is strange. We return an error, but the new mode is still > set. On the other hand, this is probably also the old mode unless > someone added a new value to the enum, so it didn't actually change. And > because this function is the only place that changes copy_mode and we're > holding the BQL, the case can't even happen and this could be an > assertion. > AFAIU and testing seem to confirm this, the new mode is only set when the current mode is MIRROR_COPY_MODE_BACKGROUND. The error is only set when the current mode is not MIRROR_COPY_MODE_BACKGROUND and thus when the mode wasn't changed. Adding a new copy mode shouldn't cause issues either? It's just not going to be supported to change away from that mode (or to that mode, because of the change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING check above) without adapting the code first. Of course, if we want to allow switching from active to background mode, the function needs to be adapted too. I wanted to make it more future-proof for the case where it might not be the only place changing the value and based it on what Vladimir suggested in the review of v2: https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg03552.html Best Regards, Fiona
Am 23.10.2023 um 13:37 hat Fiona Ebner geschrieben: > Am 18.10.23 um 18:59 schrieb Kevin Wolf: > > Am 13.10.2023 um 11:21 hat Fiona Ebner geschrieben: > >> which allows switching the @copy-mode from 'background' to > >> 'write-blocking'. > >> > >> This is useful for management applications, so they can start out in > >> background mode to avoid limiting guest write speed and switch to > >> active mode when certain criteria are fulfilled. > >> > >> In presence of an iothread, the copy_mode member is now shared between > >> the iothread and the main thread, so turn accesses to it atomic. > >> > >> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> > >> --- > >> > >> Changes in v3: > >> * turn accesses to copy_mode atomic and... > >> * ...slightly adapt error handling in mirror_change as a > >> consequence > > > > It would be good to have a comment at the field declaration that it's > > meant to be accessed with atomics. > > > > As we don't have further synchonisation, is the idea that during the > > switchover it basically doesn't matter if we read the old or the new > > value? > > > > After reading the whole patch, it seems that the field is only ever > > written under the BQL, while iothreads only read it, and only once per > > request (after the previous patch). This is why no further > > synchonisation is needed. If other threads could write it, too, > > mirror_change() would probably have to be more careful. As the code > > depends on this, adding that to the comment would be useful, too. > > > > Will do in v4. > > >> block/mirror.c | 33 ++++++++++++++++++++++++++++++--- > >> qapi/block-core.json | 13 ++++++++++++- > >> 2 files changed, 42 insertions(+), 4 deletions(-) > >> > >> diff --git a/block/mirror.c b/block/mirror.c > >> index 8992c09172..889cce5414 100644 > >> --- a/block/mirror.c > >> +++ b/block/mirror.c > >> @@ -1075,7 +1075,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp) > >> */ > >> job_transition_to_ready(&s->common.job); > >> } > >> - if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) { > >> + if (qatomic_read(&s->copy_mode) != MIRROR_COPY_MODE_BACKGROUND) { > >> s->actively_synced = true; > >> } > > > > What resets s->actively_synced when we switch away from active mode? > > > >> > >> @@ -1246,6 +1246,32 @@ static bool commit_active_cancel(Job *job, bool force) > >> return force || !job_is_ready(job); > >> } > >> > >> +static void mirror_change(BlockJob *job, BlockJobChangeOptions *opts, > >> + Error **errp) > >> +{ > >> + MirrorBlockJob *s = container_of(job, MirrorBlockJob, common); > >> + BlockJobChangeOptionsMirror *change_opts = &opts->u.mirror; > >> + MirrorCopyMode current; > > > > This is GLOBAL_STATE_CODE(), right? Let's be explicit about it. > > > > Maybe it wouldn't need to be if we also set actively_synced to false in > bdrv_mirror_top_do_write() if/when setting the bitmap. Thinking about > it, that change shouldn't hurt in any case. But sure, I'll add the > GLOBAL_STATE_CODE annotation here. If ever required not to be > GLOBAL_STATE_CODE code, it can still be adapted later. Maybe we'll want to change it later so that we have some kind of automatic switch in the job coroutine, then it might not be GLOBAL_STATE_CODE() any more. For now, that's not a concern and it makes reasoning about the rest of the function easier. > >> + > >> + if (qatomic_read(&s->copy_mode) == change_opts->copy_mode) { > >> + return; > >> + } > >> + > >> + if (change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING) { > >> + error_setg(errp, "Change to copy mode '%s' is not implemented", > >> + MirrorCopyMode_str(change_opts->copy_mode)); > >> + return; > >> + } > > > > Ah, ok, we don't even allow the switch I was wondering about above. What > > would be needed, apart from removing this check, to make it work? > > > > Of course, setting actively_synced to false, as you pointed out above. > But I think it would also require more synchronization, because I think > otherwise the iothread could still read the old value of copy_mode (as > MIRROR_COPY_MODE_WRITE_BLOCKING) right afterwards and might set > actively_synced to true again. Do you want me to think it through in > detail and allow the change in the other direction too? I guess that > would also require using the job mutex instead of atomics. Or should we > wait until somebody actually requires that? I won't stop you if you want to implement it, it's an obvious feature to add and I like symmetry. But if nobody has a use for it, there is no reason to make your life harder. I was mostly just curious if you're already aware of any challenges in the other direction that made you tackle a more limited problem for now. > >> + current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, > >> + change_opts->copy_mode); > >> + if (current != MIRROR_COPY_MODE_BACKGROUND) { > >> + error_setg(errp, "Expected current copy mode '%s', got '%s'", > >> + MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND), > >> + MirrorCopyMode_str(current)); > >> + } > > > > The error path is strange. We return an error, but the new mode is still > > set. On the other hand, this is probably also the old mode unless > > someone added a new value to the enum, so it didn't actually change. And > > because this function is the only place that changes copy_mode and we're > > holding the BQL, the case can't even happen and this could be an > > assertion. > > > > AFAIU and testing seem to confirm this, the new mode is only set when > the current mode is MIRROR_COPY_MODE_BACKGROUND. The error is only set > when the current mode is not MIRROR_COPY_MODE_BACKGROUND and thus when > the mode wasn't changed. Yes, the new mode is only set when it was MIRROR_COPY_MODE_BACKGROUND, that's the meaning of cmpxchg. And now that I checked the return value of qatomic_cmpxchg(), it's not the actual value, but it returns the second parameter (the expected old value). As this is a constant in our call, that's what we'll always get back. So the whole check is pointless, even as an assertion. It's trivially true, and I expect it's even obvious enough for the compiler that it might just optimise it away. Just qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, change_opts->copy_mode); without using the (constant) result should be enough. > Adding a new copy mode shouldn't cause issues either? It's just not > going to be supported to change away from that mode (or to that mode, > because of the change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING > check above) without adapting the code first. The checks above won't prevent NEW_MODE -> WRITE_BLOCKING. Of course, the cmpxchg() won't actually do anything as long as we still have BACKGROUND there as the expected old value. So in this case, QMP would probably return success, but we would stay in NEW_MODE. That's different from what I thought (I didn't really realise that we have a cmpxchg here and not just a xchg), but also not entirely right. Of course, all of this is hypothetical. I'm not aware of any desire to add a new copy mode. > Of course, if we want to allow switching from active to background mode, > the function needs to be adapted too. > > I wanted to make it more future-proof for the case where it might not be > the only place changing the value and based it on what Vladimir > suggested in the review of v2: > https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg03552.html As long as all of these places are GLOBAL_STATE_CODE(), we should be fine. If we get iothread code that changes it, too, I think your code becomes racy because the value could be changed by the iothread between the first check if we already have the new value and the actual change. Kevin
Am 23.10.23 um 14:59 schrieb Kevin Wolf: > Am 23.10.2023 um 13:37 hat Fiona Ebner geschrieben: >>>> + current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, >>>> + change_opts->copy_mode); >>>> + if (current != MIRROR_COPY_MODE_BACKGROUND) { >>>> + error_setg(errp, "Expected current copy mode '%s', got '%s'", >>>> + MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND), >>>> + MirrorCopyMode_str(current)); >>>> + } >>> >>> The error path is strange. We return an error, but the new mode is still >>> set. On the other hand, this is probably also the old mode unless >>> someone added a new value to the enum, so it didn't actually change. And >>> because this function is the only place that changes copy_mode and we're >>> holding the BQL, the case can't even happen and this could be an >>> assertion. >>> >> >> AFAIU and testing seem to confirm this, the new mode is only set when >> the current mode is MIRROR_COPY_MODE_BACKGROUND. The error is only set >> when the current mode is not MIRROR_COPY_MODE_BACKGROUND and thus when >> the mode wasn't changed. > > Yes, the new mode is only set when it was MIRROR_COPY_MODE_BACKGROUND, > that's the meaning of cmpxchg. > > And now that I checked the return value of qatomic_cmpxchg(), it's not > the actual value, but it returns the second parameter (the expected old > value). As this is a constant in our call, that's what we'll always get > back. So the whole check is pointless, even as an assertion. It's > trivially true, and I expect it's even obvious enough for the compiler > that it might just optimise it away. > From testing, I can see that it returns the current value, not the second parameter. I.e. if I am in MIRROR_COPY_MODE_WRITE_BLOCKING, it will return MIRROR_COPY_MODE_WRITE_BLOCKING. (Of course, I have to comment out the other check to reach the cmpxchg call while in that mode). > Just qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, > change_opts->copy_mode); without using the (constant) result should be > enough. > >> Adding a new copy mode shouldn't cause issues either? It's just not >> going to be supported to change away from that mode (or to that mode, >> because of the change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING >> check above) without adapting the code first. > > The checks above won't prevent NEW_MODE -> WRITE_BLOCKING. Of course, > the cmpxchg() won't actually do anything as long as we still have > BACKGROUND there as the expected old value. So in this case, QMP would > probably return success, but we would stay in NEW_MODE. > No, that's the whole point of the check. It would fail with the error, saying that it expected the current mode to be background and not the new mode. > That's different from what I thought (I didn't really realise that we > have a cmpxchg here and not just a xchg), but also not entirely right. > > Of course, all of this is hypothetical. I'm not aware of any desire to > add a new copy mode. > >> Of course, if we want to allow switching from active to background mode, >> the function needs to be adapted too. >> >> I wanted to make it more future-proof for the case where it might not be >> the only place changing the value and based it on what Vladimir >> suggested in the review of v2: >> https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg03552.html > > As long as all of these places are GLOBAL_STATE_CODE(), we should be > fine. If we get iothread code that changes it, too, I think your code > becomes racy because the value could be changed by the iothread between > the first check if we already have the new value and the actual change. > Right, but I think the only issue would be if the mode changes from MIRROR_COPY_MODE_BACKGROUND to MIRROR_COPY_MODE_WRITE_BLOCKING between the checks, because then the QMP call would fail with the error that the mode was not the expected MIRROR_COPY_MODE_BACKGROUND. But arguably, that is still correct. If we are already in the requested mode at the time of the first check, we're fine. Still, I'll add the GLOBAL_STATE_CODE() and a comment for the future :) Best Regards, Fiona
Am 23.10.2023 um 16:14 hat Fiona Ebner geschrieben: > Am 23.10.23 um 14:59 schrieb Kevin Wolf: > > Am 23.10.2023 um 13:37 hat Fiona Ebner geschrieben: > >>>> + current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, > >>>> + change_opts->copy_mode); > >>>> + if (current != MIRROR_COPY_MODE_BACKGROUND) { > >>>> + error_setg(errp, "Expected current copy mode '%s', got '%s'", > >>>> + MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND), > >>>> + MirrorCopyMode_str(current)); > >>>> + } > >>> > >>> The error path is strange. We return an error, but the new mode is still > >>> set. On the other hand, this is probably also the old mode unless > >>> someone added a new value to the enum, so it didn't actually change. And > >>> because this function is the only place that changes copy_mode and we're > >>> holding the BQL, the case can't even happen and this could be an > >>> assertion. > >>> > >> > >> AFAIU and testing seem to confirm this, the new mode is only set when > >> the current mode is MIRROR_COPY_MODE_BACKGROUND. The error is only set > >> when the current mode is not MIRROR_COPY_MODE_BACKGROUND and thus when > >> the mode wasn't changed. > > > > Yes, the new mode is only set when it was MIRROR_COPY_MODE_BACKGROUND, > > that's the meaning of cmpxchg. > > > > And now that I checked the return value of qatomic_cmpxchg(), it's not > > the actual value, but it returns the second parameter (the expected old > > value). As this is a constant in our call, that's what we'll always get > > back. So the whole check is pointless, even as an assertion. It's > > trivially true, and I expect it's even obvious enough for the compiler > > that it might just optimise it away. > > > > From testing, I can see that it returns the current value, not the > second parameter. I.e. if I am in MIRROR_COPY_MODE_WRITE_BLOCKING, it > will return MIRROR_COPY_MODE_WRITE_BLOCKING. (Of course, I have to > comment out the other check to reach the cmpxchg call while in that mode). You're right, I misread. Sorry for the noise. > > Just qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, > > change_opts->copy_mode); without using the (constant) result should be > > enough. > > > >> Adding a new copy mode shouldn't cause issues either? It's just not > >> going to be supported to change away from that mode (or to that mode, > >> because of the change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING > >> check above) without adapting the code first. > > > > The checks above won't prevent NEW_MODE -> WRITE_BLOCKING. Of course, > > the cmpxchg() won't actually do anything as long as we still have > > BACKGROUND there as the expected old value. So in this case, QMP would > > probably return success, but we would stay in NEW_MODE. > > > > No, that's the whole point of the check. It would fail with the error, > saying that it expected the current mode to be background and not the > new mode. Yes, this makes sense now. > > That's different from what I thought (I didn't really realise that we > > have a cmpxchg here and not just a xchg), but also not entirely right. > > > > Of course, all of this is hypothetical. I'm not aware of any desire to > > add a new copy mode. > > > >> Of course, if we want to allow switching from active to background mode, > >> the function needs to be adapted too. > >> > >> I wanted to make it more future-proof for the case where it might not be > >> the only place changing the value and based it on what Vladimir > >> suggested in the review of v2: > >> https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg03552.html > > > > As long as all of these places are GLOBAL_STATE_CODE(), we should be > > fine. If we get iothread code that changes it, too, I think your code > > becomes racy because the value could be changed by the iothread between > > the first check if we already have the new value and the actual change. > > > > Right, but I think the only issue would be if the mode changes from > MIRROR_COPY_MODE_BACKGROUND to MIRROR_COPY_MODE_WRITE_BLOCKING between > the checks, because then the QMP call would fail with the error that the > mode was not the expected MIRROR_COPY_MODE_BACKGROUND. But arguably, > that is still correct. If we are already in the requested mode at the > time of the first check, we're fine. > > Still, I'll add the GLOBAL_STATE_CODE() and a comment for the future :) Thanks, sounds good. Kevin
diff --git a/block/mirror.c b/block/mirror.c index 8992c09172..889cce5414 100644 --- a/block/mirror.c +++ b/block/mirror.c @@ -1075,7 +1075,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp) */ job_transition_to_ready(&s->common.job); } - if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) { + if (qatomic_read(&s->copy_mode) != MIRROR_COPY_MODE_BACKGROUND) { s->actively_synced = true; } @@ -1246,6 +1246,32 @@ static bool commit_active_cancel(Job *job, bool force) return force || !job_is_ready(job); } +static void mirror_change(BlockJob *job, BlockJobChangeOptions *opts, + Error **errp) +{ + MirrorBlockJob *s = container_of(job, MirrorBlockJob, common); + BlockJobChangeOptionsMirror *change_opts = &opts->u.mirror; + MirrorCopyMode current; + + if (qatomic_read(&s->copy_mode) == change_opts->copy_mode) { + return; + } + + if (change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING) { + error_setg(errp, "Change to copy mode '%s' is not implemented", + MirrorCopyMode_str(change_opts->copy_mode)); + return; + } + + current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND, + change_opts->copy_mode); + if (current != MIRROR_COPY_MODE_BACKGROUND) { + error_setg(errp, "Expected current copy mode '%s', got '%s'", + MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND), + MirrorCopyMode_str(current)); + } +} + static const BlockJobDriver mirror_job_driver = { .job_driver = { .instance_size = sizeof(MirrorBlockJob), @@ -1260,6 +1286,7 @@ static const BlockJobDriver mirror_job_driver = { .cancel = mirror_cancel, }, .drained_poll = mirror_drained_poll, + .change = mirror_change, }; static const BlockJobDriver commit_active_job_driver = { @@ -1467,7 +1494,7 @@ static bool should_copy_to_target(MirrorBDSOpaque *s) { return s->job && s->job->ret >= 0 && !job_is_cancelled(&s->job->common.job) && - s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING; + qatomic_read(&s->job->copy_mode) == MIRROR_COPY_MODE_WRITE_BLOCKING; } static int coroutine_fn GRAPH_RDLOCK @@ -1812,7 +1839,7 @@ static BlockJob *mirror_start_job( s->is_none_mode = is_none_mode; s->backing_mode = backing_mode; s->zero_target = zero_target; - s->copy_mode = copy_mode; + qatomic_set(&s->copy_mode, copy_mode); s->base = base; s->base_overlay = bdrv_find_overlay(bs, base); s->granularity = granularity; diff --git a/qapi/block-core.json b/qapi/block-core.json index c6f31a9399..01427c259a 100644 --- a/qapi/block-core.json +++ b/qapi/block-core.json @@ -3044,6 +3044,17 @@ { 'command': 'block-job-finalize', 'data': { 'id': 'str' }, 'allow-preconfig': true } +## +# @BlockJobChangeOptionsMirror: +# +# @copy-mode: Switch to this copy mode. Currenlty, only the switch +# from 'background' to 'write-blocking' is implemented. +# +# Since: 8.2 +## +{ 'struct': 'BlockJobChangeOptionsMirror', + 'data': { 'copy-mode' : 'MirrorCopyMode' } } + ## # @BlockJobChangeOptions: # @@ -3058,7 +3069,7 @@ { 'union': 'BlockJobChangeOptions', 'base': { 'id': 'str', 'type': 'JobType' }, 'discriminator': 'type', - 'data': {} } + 'data': { 'mirror': 'BlockJobChangeOptionsMirror' } } ## # @block-job-change:
which allows switching the @copy-mode from 'background' to 'write-blocking'. This is useful for management applications, so they can start out in background mode to avoid limiting guest write speed and switch to active mode when certain criteria are fulfilled. In presence of an iothread, the copy_mode member is now shared between the iothread and the main thread, so turn accesses to it atomic. Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> --- Changes in v3: * turn accesses to copy_mode atomic and... * ...slightly adapt error handling in mirror_change as a consequence block/mirror.c | 33 ++++++++++++++++++++++++++++++--- qapi/block-core.json | 13 ++++++++++++- 2 files changed, 42 insertions(+), 4 deletions(-)