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xfs: fix soft lockup via spinning in filestream ag selection loop

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The filestream AG selection loop uses pagf data to aid in AG
selection, which depends on pagf initialization. If the in-core
structure is not initialized, the caller invokes the AGF read path
to do so and carries on. If another task enters the loop and finds
a pagf init already in progress, the AGF read returns -EAGAIN and
the task continues the loop. This does not increment the current ag
index, however, which means the task spins on the current AGF buffer
until unlocked.

If the AGF read I/O submitted by the initial task happens to be
delayed for whatever reason, this results in soft lockup warnings
via the spinning task. This is reproduced by xfs/170. To avoid this
problem, fix the AGF trylock failure path to properly iterate to the
next AG. If a task iterates all AGs without making progress, the
trylock behavior is dropped in favor of blocking locks and thus a
soft lockup is no longer possible.

Fixes: f48e2df8a8 ("xfs: make xfs_*read_agf return EAGAIN to ALLOC_FLAG_TRYLOCK callers")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>
This commit is contained in:
Brian Foster 2022-04-26 13:34:54 +10:00 committed by Dave Chinner
parent 1a33850697
commit f650df7171
1 changed files with 4 additions and 3 deletions
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7
fs/xfs/xfs_filestream.c
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@ -128,11 +128,12 @@ xfs_filestream_pick_ag(
if (!pag->pagf_init) {
err = xfs_alloc_pagf_init(mp, NULL, ag, trylock);
if (err) {
xfs_perag_put(pag);
if (err != -EAGAIN)
if (err != -EAGAIN) {
xfs_perag_put(pag);
return err;
}
/* Couldn't lock the AGF, skip this AG. */
continue;
goto next_ag;
}
}