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55 lines
2.4 KiB
Diff
55 lines
2.4 KiB
Diff
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commit de03266983ceb62e5365aac84fcd3b2fd4d16e6f
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Author: Phillip Lougher <phillip@squashfs.org.uk>
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Date: Thu Sep 18 01:28:11 2014 +0100
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mksquashfs: fix rare race in fragment waiting in filesystem finalisation
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Fix a rare race condition in fragment waiting when finalising the
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filesystem. This is a race condition that was initially fixed in 2009,
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but inadvertantly re-introduced in the latest release when the code
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was rewritten.
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Background:
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When finalising the filesystem, the main control thread needs to ensure
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all the in-flight fragments have been queued to the writer thread before
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asking the writer thread to finish, and then writing the metadata.
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It does this by waiting on the fragments_outstanding counter. Once this
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counter reaches 0, it synchronises with the writer thread, waiting until
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the writer thread reports no outstanding data to be written.
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However, the main thread can race with the fragment deflator thread(s)
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because the fragment deflator thread(s) decrement the fragments_outstanding
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counter and release the mutex before queueing the compressed fragment
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to the writer thread, i.e. the offending code is:
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fragments_outstanding --;
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pthread_mutex_unlock(&fragment_mutex);
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queue_put(to_writer, write_buffer);
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In extremely rare circumstances, the main thread may see the
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fragments_outstanding counter is zero before the fragment
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deflator sends the fragment buffer to the writer thread, and synchronise
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with the writer thread, and finalise before the fragment has been written.
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The fix is to ensure the fragment is queued to the writer thread
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before releasing the mutex.
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Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
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diff --git a/squashfs-tools/mksquashfs.c b/squashfs-tools/mksquashfs.c
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index 87b7d86..f1fcff1 100644
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--- a/squashfs-tools/mksquashfs.c
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+++ b/squashfs-tools/mksquashfs.c
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@@ -2445,8 +2445,8 @@ void *frag_deflator(void *arg)
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write_buffer->block = bytes;
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bytes += compressed_size;
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fragments_outstanding --;
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- pthread_mutex_unlock(&fragment_mutex);
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queue_put(to_writer, write_buffer);
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+ pthread_mutex_unlock(&fragment_mutex);
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TRACE("Writing fragment %lld, uncompressed size %d, "
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"compressed size %d\n", file_buffer->block,
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file_buffer->size, compressed_size);
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