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Here the following changes are introduced:
- A read barrier machine op is added to Cmm.
- The order in which a closure's fields are read and written is changed.
- Memory barriers are added to RTS code to ensure correctness on
out-or-order machines with weak memory ordering.
Cmm has a new CallishMachOp called MO_ReadBarrier. On weak memory machines, this
is lowered to an instruction that ensures memory reads that occur after said
instruction in program order are not performed before reads coming before said
instruction in program order. On machines with strong memory ordering properties
(e.g. X86, SPARC in TSO mode) no such instruction is necessary, so
MO_ReadBarrier is simply erased. However, such an instruction is necessary on
weakly ordered machines, e.g. ARM and PowerPC.
Weam memory ordering has consequences for how closures are observed and mutated.
For example, consider a closure that needs to be updated to an indirection. In
order for the indirection to be safe for concurrent observers to enter, said
observers must read the indirection's info table before they read the
indirectee. Furthermore, the entering observer makes assumptions about the
closure based on its info table contents, e.g. an INFO_TYPE of IND imples the
closure has an indirectee pointer that is safe to follow.
When a closure is updated with an indirection, both its info table and its
indirectee must be written. With weak memory ordering, these two writes can be
arbitrarily reordered, and perhaps even interleaved with other threads' reads
and writes (in the absence of memory barrier instructions). Consider this
example of a bad reordering:
- An updater writes to a closure's info table (INFO_TYPE is now IND).
- A concurrent observer branches upon reading the closure's INFO_TYPE as IND.
- A concurrent observer reads the closure's indirectee and enters it. (!!!)
- An updater writes the closure's indirectee.
Here the update to the indirectee comes too late and the concurrent observer has
jumped off into the abyss. Speculative execution can also cause us issues,
consider:
- An observer is about to case on a value in closure's info table.
- The observer speculatively reads one or more of closure's fields.
- An updater writes to closure's info table.
- The observer takes a branch based on the new info table value, but with the
old closure fields!
- The updater writes to the closure's other fields, but its too late.
Because of these effects, reads and writes to a closure's info table must be
ordered carefully with respect to reads and writes to the closure's other
fields, and memory barriers must be placed to ensure that reads and writes occur
in program order. Specifically, updates to a closure must follow the following
pattern:
- Update the closure's (non-info table) fields.
- Write barrier.
- Update the closure's info table.
Observing a closure's fields must follow the following pattern:
- Read the closure's info pointer.
- Read barrier.
- Read the closure's (non-info table) fields.
This patch updates RTS code to obey this pattern. This should fix long-standing
SMP bugs on ARM (specifically newer aarch64 microarchitectures supporting
out-of-order execution) and PowerPC. This fixes issue #15449.
Co-Authored-By: Ben Gamari <ben@well-typed.com>
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Long ago, the stable name table and stable pointer tables were one.
Now, they are separate, and have significantly different
implementations. I believe the time has come to finish the split
that began in #7674.
* Divide `rts/Stable` into `rts/StableName` and `rts/StablePtr`.
* Give each table its own mutex.
* Add FFI functions `hs_lock_stable_ptr_table` and
`hs_unlock_stable_ptr_table` and document them.
These are intended to replace the previously undocumented
`hs_lock_stable_tables` and `hs_lock_stable_tables`,
which are now documented as deprecated synonyms.
* Make `eqStableName#` use pointer equality instead of unnecessarily
comparing stable name table indices.
Reviewers: simonmar, bgamari, erikd
Reviewed By: bgamari
Subscribers: rwbarton, carter
GHC Trac Issues: #15555
Differential Revision: https://phabricator.haskell.org/D5084
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Our new CPP linter enforces this.
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This changes heap overflow to throw a HeapOverflow exception instead of
killing the process.
Test Plan: GHC CI
Reviewers: simonmar, austin, hvr, erikd, bgamari
Reviewed By: simonmar, bgamari
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D2790
GHC Trac Issues: #1791
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