Ameer Hamza 4ce030e025 Fix snapshot automount race causing duplicate mounts and AVL tree panic
Multiple threads racing to automount the same snapshot can both spawn
mount helper processes that successfully complete, causing both parent
threads to attempt AVL tree registration and triggering a VERIFY()
panic in avl_add(). This occurs because the fsconfig/fsmount API lacks
the serialization provided by traditional mount() via lock_mount().

The fix adds a per-entry mutex (se_mtx) to zfs_snapentry_t that
serializes mount and unmount operations on the same snapshot. The first
mount thread creates a pending entry with se_spa=NULL and holds se_mtx
during the helper execution. Concurrent mounts find the pending entry
and return success without spawning duplicate helpers. Unmount waits on
se_mtx if a mount is pending, ensuring proper serialization. This allows
different snapshots to mount in parallel while preventing the AVL panic.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #17943
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OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community. This repository contains the code for running OpenZFS on Linux and FreeBSD.

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Official Resources

Installation

Full documentation for installing OpenZFS on your favorite operating system can be found at the Getting Started Page.

Contribute & Develop

We have a separate document with contribution guidelines.

We have a Code of Conduct.

Release

OpenZFS is released under a CDDL license. For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197

Supported Kernels

  • The META file contains the officially recognized supported Linux kernel versions.
  • Supported FreeBSD versions are any supported branches and releases starting from 13.0-RELEASE.
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