opal/fifo: use atomics to set fifo head in opal_fifo_push #1470
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This commit changes the opal_fifo_push code to use opal_update_counted_pointer to set the head. This fixes a data race that occurs because the read of the fifo head in opal_fifo_pop requires two instructions. This combined with the non-atomic update in opal_fifo_push can lead to an ABA issue that puts the fifo in an inconsistant state. There are other ways this problem could be fixed. One way would be to introduce an opal_atomic_read_128 implementation. On x86_64 this would have to use the cmpxchg16b instruction. Since this instruction would have to be in the pop path (and always executed) it would be slower than the fix in this commit. Closes open-mpi#1460. Signed-off-by: Nathan Hjelm <[email protected]>
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I tried to run the old opal_fifo in a loop and it segfaulted during the 8th run. With @hjelmn's patch applied it has now ran over 140 times without a segfault. I keep it running for a few more hours |
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Mellanox jenkins failure looks unrelated. |
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:bot:retest: |
hjelmn
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Mar 18, 2016
opal/fifo: use atomics to set fifo head in opal_fifo_push
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This commit changes the opal_fifo_push code to use
opal_update_counted_pointer to set the head. This fixes a data race
that occurs because the read of the fifo head in opal_fifo_pop
requires two instructions. This combined with the non-atomic update in
opal_fifo_push can lead to an ABA issue that puts the fifo in an
inconsistant state.
There are other ways this problem could be fixed. One way would be to
introduce an opal_atomic_read_128 implementation. On x86_64 this would
have to use the cmpxchg16b instruction. Since this instruction would
have to be in the pop path (and always executed) it would be slower
than the fix in this commit.
Closes #1460.
Signed-off-by: Nathan Hjelm [email protected]