hang in call to zil_lwb_flush_wait_all, blocking txg_sync #14982
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Type: Defect
Incorrect behavior (e.g. crash, hang)
Comments
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@prakashsurya What exactly ZFS version (commit) are we talking about? |
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Good question, sorry for not including that.. looking at the most recent report, it was running this commit d816bc5 .. merged with our Delphix changes, of course, but I don't think we have much/any downstream changes in the ZIL code paths. |
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In what looks like another instance of this hang, the system was running an older Delphix release.. and the openzfs commit is a582d52 |
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The first looks to be from February 23, long before my recent ZIL commits from May, that I was worrying about. So hopefully not me. ;) But it may be a consequence, not the cause. It just tells that some of ZIL writes of flushes got stuck didn't complete and zil_sync() is waiting for them. It may be hardware, or who knows what else in software. |
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Right.. and since we don't have a kernel crash dump, I think it'll be difficult to make forward progress toward a root cause.. In both cases that I've looked at, the system was rebooted without collecting a crash dump, unfortunately. But yea, I agree, based on the dates, likely unrelated to any of those recent changes. |
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Here's the analysis from another instance of this hang: This thread holds the rangelock as reader: The txg is stuck here waiting for inflight ZIL I/O to complete: But why aren't the ZIL writes completing? Looking at other ZIL thread we see that this thread wants the rangelock as writer: This means that any threads which may want the rangelock as reader will now block giving priority to the writer. Here's one such thread: Unfortunately this thread is in the middle of issuing ZIL I/O which is why And there we have the deadlock:
This commit is made a change which now ties ZIL completion with Now consumers of |
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@grwilson Thank you for your analysis, but I think it is not @jxdking's 152d6fd, but mine f63811f. The problem is that postponing zl_get_data() call to move it out of zl_issuer_lock also moved it past zl_lwb_inflight increment in zil_lwb_write_close() under the lock. As result, the LWB is already counted as inflight, that blocks the TXG sync, but can't be issued before we get the data for it, which data we can not get due to range lock being held. I see two possible solutions: either use zfs_rangelock_tryenter() and report error if we can't get it, in which case ZIL code will simulate write error and fall back to txg_wait_synced(); or we could implement high-priority read range lock, similar to what spa_config_enter_mmp(), that would bypass writers. The first may create additional delays during extra syncs, while the second won't help if the range lock is originally held by writer. I am open for any better ideas. |
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In other words, there should be nothing that can block TXG commit after we've done next block allocation, else we either dead deadlock or a space leak in case of crash. I have chosen pretty big windmills to tilt this time. Need to think more. |
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I am concerned that syncing context is now waiting for open context I/Os which seems counterintuitive. |
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@amotin any further thoughts on how to address this? |
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@grwilson Yes, I think so. I have one more ZIL refactoring patch that just passed first basic tests yesterday. It supposed to fix all of those kinds of deadlocks by postponing block allocation until after all copies are done and so allowing unbound sleeps anywhere, since TXG sync no longer need to wait for it. I should publish it nearest days if all go well. |
amotin
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The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
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amotin
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The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
amotin
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The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
amotin
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Aug 21, 2023
The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
amotin
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Aug 22, 2023
The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
amotin
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The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
amotin
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Aug 24, 2023
The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
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The previous patch #14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue #14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Reviewed-by: Rob Norris <[email protected]> Reviewed-by: Mark Maybee <[email protected]> Reviewed-by: George Wilson <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes #15122
amotin
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The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Reviewed-by: Rob Norris <[email protected]> Reviewed-by: Mark Maybee <[email protected]> Reviewed-by: George Wilson <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15122
behlendorf
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The previous patch #14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue #14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Reviewed-by: Rob Norris <[email protected]> Reviewed-by: Mark Maybee <[email protected]> Reviewed-by: George Wilson <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes #15122
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The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Co-authored-by: Alexander Motin <[email protected]>
lundman
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Dec 12, 2023
The previous patch openzfs#14841 appeared to have significant flaw, causing deadlocks if zl_get_data callback got blocked waiting for TXG sync. I already handled some of such cases in the original patch, but issue openzfs#14982 shown cases that were impossible to solve in that design. This patch fixes the problem by postponing log blocks allocation till the very end, just before the zios issue, leaving nothing blocking after that point to cause deadlocks. Before that point though any sleeps are now allowed, not causing sync thread blockage. This require slightly more complicated lwb state machine to allocate blocks and issue zios in proper order. But with removal of special early issue workarounds the new code is much cleaner now, and should even be more efficient. Since this patch uses null zios between write, I've found that null zios do not wait for logical children ready status in zio_ready(), that makes parent write to proceed prematurely, producing incorrect log blocks. Added ZIO_CHILD_LOGICAL_BIT to zio_wait_for_children() fixes it. Reviewed-by: Rob Norris <[email protected]> Reviewed-by: Mark Maybee <[email protected]> Reviewed-by: George Wilson <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15122
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Is this an issue after #15122? |
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We've seen
zil_lwb_flush_wait_allhang on a few systems, the stack looks like this:As a result of
txg_syncbeing blocked, all TXGs halt, and thus all IO to this pool effectively stops.. we've resolved this via a reboot.. and we haven't yet been able to capture a kernel crash dump when this situation occurs.So far, this situation seems relatively rare.. only been reported 2 times.. but since it causes all IO to the pool to hang, it's relatively impactful when it does happen, as it requires an admin to detect the situation and manually reboot the system to recover.
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