an infinite loop in ext4 in 3.14

[Mikulas Patocka]

Hi

I hit a bug in ext4 - jbd2 was stuck in an infinite loop when remounting 
the root filesystem read-only during shutdown.

The filesystem is ext3, but it uses the ext4 driver with the following 
options: rw,relatime,discard,errors=remount-ro,data=ordered

The machine was stuck, but it was possible to obtain a stacktrace with 
Alt-SysRQ-P. I put several stacktraces here:

http://people.redhat.com/~mpatocka/crashes/ext4/

The stacktraces change in jbd2_log_do_checkpoint and 
jbd2_cleanup_journal_tail. jbd2_log_do_checkpoint doesn't call 
jbd2_cleanup_journal_tail from a loop, so the the probable location of the 
infinite loop was this piece of code in jbd2_journal_flush:

while (!err && journal->j_checkpoint_transactions != NULL) {

Mikulas
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