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NCCL RAS Race Segfault During Initialization

A race in the NCCL RAS subsystem segfaults during init when a RAS command runs before peer info is populated; fixed in NCCL 2.27.3.

Quick answer

A race in the NCCL RAS subsystem segfaults during init when a RAS command runs before peer info is populated.

Communication#nccl#ras#segfault#initialization#race-condition#2.27.3

What this failure is

NCCL RAS Race Segfault During Initialization is a Communication failure seen during ML training runs. A race in the NCCL RAS subsystem segfaults during init when a RAS command runs before peer info is populated; fixed in NCCL 2.27.3. Common tags: Nccl, Ras, Segfault, Initialization.

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Why it happens (the mechanism)

RAS report generation traversed peerInfo before bootstrapAllGather populated it. Race between RAS access and peer-info initialization. Unguarded read of uninitialized peer data. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.

What you'll observe

  • Segfault during NCCL init when RAS is queried concurrently
  • Only rank 0 crashes
  • Happens with the RAS feature (NCCL 2.26+)

Common symptoms and what they mean

SymptomWhy it happens
SIGSEGV at address 0x10 during NCCL initRAS report generation traversed peerInfo before bootstrapAllGather populated it
Backtrace in rasCollCommsInit()Race between RAS access and peer-info initialization
Crash when a RAS command runs during bootstrapUnguarded read of uninitialized peer data

Which systems are affected

  • NCCL 2.26.2 with RAS enabled
  • Statically linked libnccl.a builds
  • Multi-node InfiniBand jobs

How to confirm this is the problem

Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.

  • ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
  • ✓Verified signal present: SIGSEGV at address 0x10 during NCCL init
  • ✓Verified signal present: Backtrace in rasCollCommsInit()
  • ✓Verified signal present: Crash when a RAS command runs during bootstrap
  • ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.

The fix and the prevention pattern

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NCCL errors in context

NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.

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Related failures to investigate next

Nccl Cuda Failure often co-occurs with this failure because both share underlying assumptions about communication; if you fix this entry but still see drift, validate the related primary cause next. nccl cuda failure →
Nccl Bucket Size Mismatch often co-occurs with this failure because both share underlying assumptions about communication; if you fix this entry but still see drift, validate the related neighboring cause next. nccl bucket size mismatch →
Nccl Nvls Dualport Corruption often co-occurs with this failure because both share underlying assumptions about communication; if you fix this entry but still see drift, validate the related neighboring cause next. nccl nvls dualport corruption →

Root cause

  • RAS report generation traversed peerInfo before bootstrapAllGather populated it
  • Race between RAS access and peer-info initialization
  • Unguarded read of uninitialized peer data

The fix and how to prevent it

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References

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