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Fix ZAP upgrade deadlock #4692

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Fix ZAP upgrade deadlock #4692

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10 changes: 10 additions & 0 deletions module/zfs/zap.c
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Original file line number Diff line number Diff line change
Expand Up @@ -129,10 +129,20 @@ zap_tryupgradedir(zap_t *zap, dmu_tx_t *tx)
{
if (RW_WRITE_HELD(&zap->zap_rwlock))
return (1);

#ifndef __linux__
/*
* Unsafe under Linux because the rw_tryupgrade() implementation must
* release the lock before attempting to upgrade. This can result in
* a lock inversion with the l_rwlock unless the leaf lock is dropped
* first and reaquired. Rather than restruct this code disable this
* minor optimization under Linux.
*/
if (rw_tryupgrade(&zap->zap_rwlock)) {
dmu_buf_will_dirty(zap->zap_dbuf, tx);
return (1);
}
#endif
return (0);
}

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